CA1215155A

CA1215155A - Method and apparatus for detection of breathing gas interruptions

Info

Publication number: CA1215155A
Application number: CA000432242A
Authority: CA
Inventors: James A. Mcewen
Original assignee: SALTWATER WEST RESEARCH Ltd
Current assignee: SALTWATER WEST RESEARCH Ltd
Priority date: 1982-07-15
Filing date: 1983-07-12
Publication date: 1986-12-09
Also published as: AU1644883A; ZA834774B; US4550726A; JPS602259A; EP0099743A2; EP0099743A3

Abstract

METHOD AND APPARATUS FOR DETECTION OF
BREATHING GAS INTERRUPTIONS

Abstract of the Invention A method for detecting an interruption in the supply of breathing gas to a patient, and apparatus adapted to carry out the method by:
(a) during a reference time period, sensing the pressure in the patient's breathing gas;
(b) storing, in memory apparatus, reference breathing information derived from the pressure sensed during the reference time period, (c) after the reference time period, sensing the pressure in the patient's breathing gas;
(d) comparing active breathing information derived from the pressure sensed after the reference time period with the reference breathing information; and, (e) producing an alarm signal upon detection, during the comparing step, of predetermined variations between the reference breathing information and the active breathing information.

Description

METHOD ANN A RATS F R DETECTION OF
BREATHING GAS INrrERRUPTIONS

Field of the Invention This application pertains to a method and apparatus for detecting an interruption in the supply of breathing gas to a patient. In the art, the terms "patient-circuit monitor", "disconnect monitor", "breathing-circuit monitor" and 'flow pressure alarm" are 10 interchanc3eably used for such apparatus.
sack round of the Invention "Ventilators are medical devices or deliver-in a breathing gas to a patient Typically, variables such as the patient's breathing rate or frequency, volt 15 use of breathing, and inspiratory flow rate may be con-trolled by the ventilator operator. Usually, ventilate ours employed in hospital critical care units provide a supply of air enriched with oxygen for inspiration by the patient, and may conventionally include controls for 20 either assisting or controiliny breathing, exhaled volt use indicators, aroma systems, positive en exploratory }rouser valve, rouser indicators yes concentration monitors, flow indicators, and heated humidifiers for warming and humidifying the breathing yes. Ventilators 25 intended err use with anesthetized patients are usually much simpler to operate than ventilators intended for use in critical care units, but require the anesthetist -to add specific ancillary devices and accessories to the "patient breathing circuit conn~ctiny the ventilator to the patient, as warranted by factors such as the physic-logical status of the patient, the nature of the surge-eel procedure, thy anesthetic technique employed etch Typically, in both anesthetic and critical care applications, the "patient breathing circuit" in-eludes hoses which connect the ventilator and ancillary devices and accessories to an endotracheal tube inserted into the patient's trachea to permit breathing gas to pass through the trachea into the patient's lungs.
The following discussion pertains to ventila-ions and patient breathing circuits intended for ayes-Thetis placation, wince it is believed that these are the more demanding and crucial applications, but it should be understood thaw the discussion applies gene-rally to applications in which ventilators are employed in critical care units.
it sometimes happens that the tubing used to convey breathing gas to the anesthetized patient become disconnected, or blocked, or develops leaks, or that the endotracheal tube become dislodged; any of which may elite in an interruption in the supply of breathing gas to the patient. Interruption ox the patient's breathing aye supply or a relatively hire period may have err-out cons~uenc~3.q including hy~c~x;La, cardiac arrest or even death. Since an anesthetized patient is often in-tensional paralyzed with curare-like drums, and since the attending anesthetist's view of the patient may be largely obscured by the ~u~yical drapes, the positioning of the patient, the nature of the surgical procedures, other equipment, etc., it it often not possible for the anesthetist to observe the patient and patient breathing circuit to visually detect hazardous condo-lions. Recent studies have shown that even very expert fenced anesthetists may not be able to recognize swamp-toys of an interruption in the patient's supply of breathing gas until after the interruption has gone us detected for a period of time sufficient to result in hypoxia, bradycardia or cardiac arrest.
"Disconnect monitors" (also known in the art as "patient-circuit monitors", "breathing-circuit monk-ions" or "low pressure alarms", but hereinafter collect-lively referred to a "diaconnec~ monitors") are Davis for monitoring a patient breathing gas supply and for producing alarm signals upon detection of symptoms of a disconnection or blockage in the patient breathing air-cult. Prior art disconnect monitors have utilized pressure sensors, gas volume sensors or gas flow sensors to monitor the pressure, volume or f low characteristics of the breathing gas supplied to a patient. Typically, such prior art disconnect monitors produce alarm signals upon detection of breathing gas pressures, volumes or flow rates which fail to meet operator defined criteria.
However, such prior art disconnect monitors have proved incapable of reliably detecting certain types of disco-sections or blockages in the patient breathing circuit.
For example, the tubing which supplies breathing gas to the patient may become disconnected, and the open end of the tubing may be partially or totally blocked if it rests against a surgical drape, pillow or bed sheet.
Such partial or total blockage of the tubing has been found to result in a back pressure within the tubing sufficient to maintain the breathing yes pressure in the vicinity of a disconnect monitor pressure sensor within acceptable limits. Accordingly, some prior art disco-neat monitors have failed to produce an alarm signal even though the patient's breathing gas supply from the patient breathing circuit is completely cut of.

Some prior art disconnect monitors have also proved incapable of reliably detecting interruptions in the patient's breathing gas supply caused by partial extubation of the endotracheal tube or kinking of the breathing gas tubing, either of which may also result in back pressures sufficient to maintain the breathing gas pressure in the vicinity of the disconnect monitor pressure sensor within acceptable limits and thereby inhibit production of an alarm signal even though the patient's breathing gas supply is partially or completely cut off.
Summary of the Invention To overcome these problems, and in an effort to maximize the probability of reliably detecting an interruption in a patient's breathing gas supply, the present invention provides a method of analyzing pressure variations during successive cycles of inhalation and exhalation of breathing gas by the patient to develop a reference breathing waveform representative of "normal" pressure fluctuations in the patient breathing circuit over a representative "normal"
inhalation and exhalation breathing cycle of the patient during a reference time period initiated by an operator. The reference time period ends after a reference breathing waveform has been developed. The pressure in the patient's active or ongoing inhalation and exhalation breathing cycles is then monitored to develop active breathing waveforms representative of pressure fluctuations in each active breathing cycle. Each active breathing waveform and an alarm signal is produced upon detection of any one of a wide range of predetermined variations between the reference breathing waveform and the active breathing waveform. Apparatus adapted to carry out the foregoing method is provided. The apparatus incorporates an optional gas sensor capable of reliably deriving gas pressure information from an oxygen analyzer. The derived gas pressure information may be compared with gas pressure information derived I`

~21~

from a conventional gas pressure sensor so as to crosscheck gas pressure information and detect a malfunction of either the optional gas sensor or the conventional gas pressure sensor, or their associated signal processing circuitry.
The invention is believed capable of more reliably detecting interruptions in a patient's breathing gas supply than are prior art disconnect monitors because, unlike prior art disconnect monitors which, for example, simply monitor pressure cycles above an operator defined lower limit, the present invention monitors a broad range of parameters which characterize the breathing cycle waveform.
The invention provides a method of detecting an interruption in the supply of breathing gas to a patient, comprising steps of:
(a) during a reference time period initiated by an operator, sensing the pressure in the patient's breathing gas (b) storing, in memory apparatus, reference breathing information derived from the pressure sensed during the reference time period in respect of a normal breathing cycle of the patient;
(c) after the reference time period, sensing the pressure in the patient's breathing gas;
(d) comparing active breathing information derived from the pressure sensed after the reference time period in respect of an active breathing cycle of the patient with the reference breathing information; and, (e) producing an alarm signal upon detection, during the comparing step, of predetermined variations between the reference breathing information and the active breathing information.
Preferably, before the stoning step, a referential comparison is made between:

(a breathing information derived from the prey-surf in thy patient's wreathing gas during a first referential Bryan cycle of the patient; and (b) breathing information derived from the pros-sure in the patient breathing gas during a second referential breathing cycle of the patient which follows the first referential breathing cycle;
and the storing stop is taken only if the breathing in-formation derived from the rouser sensed during said first referential brPa~hing cycle differs, by no more than a selected amount, from the breathing information derived from the pressure sensed during the second referential breathing cycle; otherwise, the referential comparing step is repeated The breathing information may include such parameters as the average breathing gas pressure ensued during the breathing cycle in respect of which the breathing information is derived; the maximum breathing gas pressure sensed during the breathing cycle in rest poet of which the breathing information is derived; the minimum breathing gas pressure sensed during the breath in cycle in respect of which the breathing information is derived; ratio of the time, during the breathing cycle in respect of which the breathing information is derived, the patient inspires breathing gas, to the time, during the breathing cycle in respect of which the breathing information is derived, the patient expires breathing gas the period of the breathing cycle in rest poet of which the breathing information is derived.
The alarm signal is produced upon detection of any of the following conditions (a) detection of an active breathing cycle hiving a breathing gas pressure in excess of about 45 cmH20;
(b) detection of an active breathing cycle having a breathing 9~5 pressure less than about -8 cmH20:
(c) detection, for at least 15 seconds during an active breathing cycle, of breathing gay pros-surges less than the greater of:
I (i) 5 cmH2O; and, (ii) the average breathing yes pressure of said representative normal breathing cycle;
(d) detection of an active breathing cycle having a period longer than about 30 seconds;
(en detection, during the reference time period, of Zen succe~ive reverential breathing cycles, each having an average pressure which differs, by more than about ten percent, from the average pressure of the immediately lot-lowing referential breathing cycle;
(f) detection of an active breathing cycle having a maximum breathing was pressure which dip-lens, by more than about I percent from ills maximum breathing yes pressure of the repro-tentative normal breathing cycle;
(g) detection of an active breathing cycle having a minimum breathing gas pressure which dip-lens, by more than about 17 percent, from the minimum breathing gas pressure of the repro-tentative normal breathing cycle;
(ho detection of an active breathing cycle having a period which differs, by more than about 17 percent, from the period of the representative normal breathing cycle;

~2~5~
.... I

(i) detection of an active breathing cycle having an average breathing gas pressure which differs, by more than about 17 percent, from the average breathing gas pressure of the representative normal breathing cycle;
(j) detection of an active breathing cycle during which the ratio of the time the patient inspires breathing gas, to the time the patient expires breathing gas, is less than about 0.2 or more than about 4.0;
I detection of an active breathing cycle during which the ratio of the time the patient inspires breathing gas to the time the patient expires breathing gas differs, by more than about 25 percent, from the ratio of time the patient inspires breathing gas during the representative normal breathing cycle.
The invention also provides apparatus for detecting an interruption in the supply of breathing gas to a patient, comprising pressure sensing means for sensing pressure in the patient's breathing gas and for producing an output signal representative thereof; memory apparatus for storing reference breathing information derived from the output signal during a reference time period initiated by an operator; signal comparison means for comparing:
(a) active breathing information derived from the output signal after the reference time period;
with (b) the reference breathing information; and, alarm means for producing an alarm signal upon detection, by the comparison means, of a pro-determined variation between the active I, i bruiting information and the reference breathing information.
By _ lion of the Dry Figure 1 it a block diagram showing the con-figuration of a typical patient breathing circuit Figures PA through YE are graph in which patient breathing circuit pressure in Shea it plotted as the ordinate versus time as the abscissa.
Figure 3 is a block diagram of an apparatus for detecting an interruption in the supply of breathing gas to a patient, according to the preferred embodiment Figure 4 is a pictorial representation of a control/di~play panel for the apparatus of Figure I
Figure 5 is a electronic circuit schematic diagram for the microprocessor and related circuitry which controls the apparatus of the referred ~mbodi-mint, but doe not include circuit for the optional modified gay monitor and its associated signal process-in mean.
I Figure PA is a block diagram of the optional motif ted yaw monitor and its associated signal process-in means. Figure 6B is a pictorial representation of an integrated pressure and yes sensor which may be used with the optional motif ted gas monitor. Figure 6C is an electronic circuit schematic diagram for the circuitry of the signal processing means associated with the optional modified gas monitor Description of the Preferred Embodiment I
Figure 1 it a block diagram showing the con f figuration of a typical patient breathing circuit including a gas-powerod ventilator 10 for dependably delivering a desired volume of gas to a patient at a specified rate, a carbon dioxide absorber I for remove in carbon dioxide from that: portion of gas exhaled key the patient which may be intentionally directed back to the patient for inhalation with fresh gas, anesthetic machine 14 for supplying a desired mixture of fresh gas-en in sufficient quantity, a gas monitor 15 for provide 5 in an indication of the concentration of a specificyas, a Spiro meter 16 for providing an indication of the violin of gas passing through a portion of the patient breathing circuit, a pressure gauge 20 for providing an indication of the instantaneous gas pressure, and post-live end expiatory prowar PUP") valve 21 for pro-venting a return to zero pressure at the end of exhale anion.
Hoses 22 facilitate the flow of breathing gas to and from endotracheal tube connector 27, and through out the patient breathing circuit. One-way valves 24 and 26 facilitate the flow of gas in the desired direct lion in the patient breathing circuit. Although Figure 1 shows one typical pun breathing circuit, a wide variety of other con figurations may be employed for specific applications for example, a coaxial or "Boolean patient breathing circuit which does not require valve 24 and 26 and carbon dioxide absorber 12 it widely used.
In operation, a patient breathing circuit such as the one shown in Figure 1 it typically assembled for a specific patient and for a specific surgical procedure by an anesthetist. the control on anesthetic machine 14 are jet to supply a desired mixture of gases, typic gaily nitrous oxide, oxygen and in some cases a volatile anesthetic agent, at a sufficient rate of flow. The control on ventilator 10 are set to supply the patient via the patient breathing circuit, endotracheal tube connector I and endotracheal tube snot shown in Figure 1) with gas at a specified "minute volume", ire. at a specified number of breathing cycles or artificial it breaths per minute and a a specified volume per cycle.
Ventilator lo begins to supply the specified volume to the patient breathing circuit at the beginning of a breathing cycle (typically by compressing a bellows within the ventilator). An equivalent volume of gas, consisting of fresh gas supplied by anesthetic machine 14 together with some gas from ventilator lo which has passed through carbon dioxide absorber 12~ passes through one way valve 26 and hose 22 Jo the patient lo through endotracheal tube connector 27. When ventilator lo has pushed the specified volume of gas into the patient breathing circuit and has returned to it normal state during the expiatory phase of the breathing cycle, expired gas from the patient passes through endow tracheal tube connector 27, hose 229 PEEP valve 21, gas monitor 15, Spiro meter I pressure gauge 20 and one-way valve 24. Some of the expired gas remain in ventilator lo until the next breathing cycle, some is drawn off through ventilator 10 to a scavenging system (not shown which safely removes it from the operating room, and some pastes into carbon dioxide absorber 12. The above described cycle is then repeated at the set rote.
The graphs of Figure 2 were obtained by mews-using pressure yin cm~l2n~ at the inlet port of carbon dioxide absorber 12 of a patient breathing circuit con-figured according to Figure l.
Figure PA shows a normal patient breathing circuit pressure waveform comprising a series of "breathing cycles" Of, C2, C3, etch, each breath-I in cycle comprising an in~piratory phase "I" and anexpiratory phase "E" as shown in Figure PA with refer-once to breathing cycle COO
Prior art disconnect monitors typically enable the operator to define an expected minimum peak breath-in circuit pressure. The minimum peak breathing air-I
I

cult pressure it usually defined to be some pressure less than the peak pressure which the operator expect will be generated in the patient breathing circuit, erg, some pro sure between 2-15 cm~20. For example, with reference to Figure PA, the minimum peak breathing pros-sure might be defined by the operator to coincide with the broken line marked "mix" which represents a peak pressure of 5 cmH20, In operation/ such typical prior art disconnect monitors monitor the pressure in the patient breathing circuit which normally continually cycles by rising above, and then falling below, the defined minimum peak breathing circuit pressure at frequency defined by the ventilator rate. Normally, between four and thirty breathing cycles per minute are completed, depending on the punts aye, physiologic status, etc., and the anesthetic technique, Prior art disconnect monitors of this sort produce alarm signals if the pressure in the patient wreathing circuit fails to rise above the operator-defined minimum peak breath-in circuit pressure within a specified period of time Typically this period of time in some prior art disco-neat monitors is fixed at about 15 seconds, and in other prior art disconnect monitors can be set by the opera-ion, e.g. to some time between 2-30 seconds.
Figure 2B shows a patient breathing circuit pressure waveform obtained by completely disconnecting the breathing gas tubing from the endotracheal tube (which would completely cut off the patient's breathing Casey supplied from the patient breathing circuit), and I leaving the end of thy gay tubing free of obstructions.
As Figure 2B shows, the patient breathing circuit pros-sure remains approximately constant at 0 cmH20 under these conditions. Prior art disconnect monitors which operate by detecting continued cycling of thy breathing gas pressure above an operator defined minimum peak pressure can usually reliably detect a failure of }he sort shown in Figure 2B.
Figure 2C shows a patient breathing circuit pressure waveform obtained under conditions similar to those used to produce Figure 2B, but in which the end of the breathing gas tubing was completely obstructed. In practice such a condition might occur due to partial extubation of the endotracheal tube, or due to disco-section of the endotracheal tube connector accompanied 10 by kinking of a breathing hose or complete occlusion of the aperture of the breathing hose by a drape, pillow, sheet, or similar materials Some prior art disconnect monitors of the sort described above have proved Inca-able of reliably detecting a failure of the sort shown in Figure 2C because the patient breathing circuit pros-sure continues to cycle above the minimum peak breathing circuit pressure typically defined by the operator, due to the back pressure caused by the obstructed tubing, despite the fact that the patient's breathing gas supply from the patient breathing circuit is completely cut off.
Figure ED shows a patient breathing circuit pressure waveform obtains under conditions similar to those used to produce Figure 2B, but in which the end of US the disconnected breathing gas tubing was about 50~
obstructed with a surgical drape. A study ox Dyson-sections in patient breathing circuits has revealed that a majority of such disconnections have occurred at the endotracheal tube connector. When this happens, it is quit possible for the breathing gas tubing to fall against and become partially obstructed by a surgical drape, pillow or bed sheet as contemplated by Figure ED.
A prior art disconnect monitor capable of having an operator defined minimum peak broaching circuit prowar of about 2 cmH~0 or lest could fail to reliably detect the condition depicted in Figure ED.
Figure YE show a patient breathing circuit pressure wavefo~n obtained under conditions similar to those used to produce Figure 2B, but in which the end of the disconnected breathing gas tubing way about 75% ox-structsd with a surgical drape. Figure YE is considered to be representative of a patient breathing circuit pressure waveform for an incident which ultimately no-suited in a patient's death. A prior art type disco-neat monitor of the type described above which was em plowed during this incident failed to produce an alarm signal, despite the complete cutoff of the patient's breathing gas supply from the patient breathing circuit The monitor in question was examined ir~nediately after the incident and was found to be functioning according to the manufacturer's specifications. An examination of Figure YE suggests that the failure likely occurred because the patient breathing circuit pressure continued to cycle above the minimum peak breathing circuit pros-sure defined by the operator due to the back pressure caused by the partially obstructed tubing, despite the fact that the patients breathing gas supply was come pletely cut off.
I II. Introduction to the Preferred Embodiment:
The present invention represents an attempt to overcome some obvious deficiencies of prior art disco-neat monitor, including a failure to trigger an alarm despite a complete cutoff of the patient's breathing gay 30 supply from the patient breathing circuit. Other deli-shanties of prior art disconnect monitors revealed by the inventors investigations include zither a complete inability to dote or an inability to consistently and reliably detect failures such as: disconnections in US the patient breathing circuit other than at the endow I

tracheal tube connector, associated with entrainment of air into the patient breathing circuit; disconnections associated with the use of positive end expiatory pressure ("PEEP") valves; kinks in the breathing circuit tubing leading to hazardously high pressures a the endotracheal tube; partial extubation (i.e. partial dislodging) of the endotracheal tube such that the aperture of the tube is located, and is partially occluded, inside the patient's mouth; and complete extubation with the endotracheal tube connected to a coaxial breathing hose having high flow resistance The present invention attempts to maximize the probability of reliably detecting any of the above noted failures of interruptions in the patient breathing gas supply.
In operation, a reference time period is initiated by an operator. After initiation of the referenced time period, apparatus of the preferred embodiment begins by sampling the pressure in the patient breathing circuit every one-tenth of a second, and averaging the sampled pressure values over a period of about ten seconds. The average value Ox the pressure so determined is then used to detect the beginning of the next breathing cycle, which is defined as the "first referential breathing suckle labeled C1, in Figure PA.
The average pressure, maximum pressure, minimum pressure, inspiration time to expiration time ratio (hereinafter called the I/E ratio), and total period of the first referential breathing cycle C1, are determined by sampling the breathing circuit of pressure every one tenth of a second during first referential breathing cycle C1. The values of average pressure, maximum pressure, minimum pressure, I/E ratio and total period so determined are then compared to like values determined in similar fashion by sampling the breathing circuit pressure during a second referential breathing cycle C2, which immediately follows first referential breathing Cycle Of.
If all values determined in respect of sea-I I

on referential breathing cycle C2 are within about ten percent of corresponding values determined in rest poet of first referential breathing cycle Of, the two breathing cycles are deemed to be sufficiently similar, and the values from both cycles are averaged together, thereby defining a "reference breathing cycle" which is deemed to represerSt the patient's "normal" breathing condition. The averaged reference breathing cycle in-formation is stored in a memory device for later use a hereinafter described. If the values determined in rest poet of referential breathing cycle C2 are not suffix ciently #imllar to the corrs~pondlng values ~s~ermined J in respect of referential breathing cycle I a third referential breathing cycle C3 which immediately lot-lows second referential breathing cycle C2 is sampled and the values so determined are compared to those determined in respect of second referential breathing cycle C2. This process is repeated until two suffix ciently similar consecutive referential breathing cycles are found so that a "reruns breathing cycle" can be defined.
After the reference breathing cycle is define Ed the apparatus continues to sample the pressure in the patient breathing circuit every one-tenth of a suckled during each "active" or ongoing breathiness cycle of the patient. At the end of each active breathing cycle, the values of average pressure, maximum pressure, minimum pressure, I/E ratio, and total period for that active breathing cycle are determined and compared to the corresponding values previously derived in respect of the reference breathing cycle. If all of the active breathing cycle value are sufficierstly similar (as hereinafter explained under the heading "Alarms's) to the corresponding values previously derived in respect of the reference breathing cycle, and provided certain other criteria are met (also hereinafter explained under the heading "Alarms) the active breathing cycle it deemed to be sufficiently similar Jo the reference bra thin cycle, and therefore sufficiently similar to 5 the patient's normal breathing condition that there should be/cause for alarm. Otherwise, audible and Vim vat alarms are triggered to alert the operating room personnel to a problem which may require their alien-lion.
Because the patient's physiological status may change during a surgical procedure and because eating of specific parameters of the anesthetic ventilator end other related device connected to the patients Brie-in circuit may deliberately by changed during the our-lo Cal procedure in a manner such that the patients breathing pattern may also change, the operator may wish to redefine the reference breathing cycle by sampling and averaging successive similar breathing cycles as described above. This radix the probability of con-I tinting false alarms that might otherwise be triggered.
In the preferred embodiment, an optional electronic signal proportional Jo pressure in the patient breathing circuit it obtained by modifying (as herein-aster described) the output of a conventional gas Mooney ion such as an oxygen analyzer which is normally include Ed in the patient breathing circuit, so that signals proportional to both the concentration of the gas and to changes in the partial pressure of the Casey can be de-roved, sampled and analyzed. These signals are analyzed Jo to check the function and calibration of the primary pressure tensing means at startup during referential breathing cycles, and during active breathing suckle, Additionally, the output signal from the primary pros-sure ~ensirlg means may ye used to check the function and calibration of the yaw monitor at start-up, during referential breathing cycles and during active breathing cycles as hereinafter described.
Figure 3 is a block diagram which illustrates the operation of the preferred embodiment of the present invention. A pressure sensing means 50 such as an Alec-ironic pressure transducer is coupled via hose 52 into the patient breathing circuit, preferably a endotrach-cat tube connector 27. If connection at endotracheal tube connector 27 is inconvenient, then hose 52 may be lo connected into the patient breathing circuit near the inspiratory valve of carbon dioxide absorber lo, or near the oxygen analyzer (if used), or at another convenient position in the particular configuration of breathing circuit employed, preferably close to endotracheal tube connector 27. Pressure sensing means 50 senses the breathing gas pressure in the patient breathing circuit and produces an output signal representative of that pressure. This output signal is digitized by combined multiplexer and analog to digital converter 51. The same output signal is also amplified by output amplifier 53 so that it may be connected to a recording or display means (not shown).
Memory apparatus such as random access memory 54 is provided for storing the reference breathing in-formation. Memory apparatus such as read-only memory 55 is provided for storing the computer program which con-trots the operation of microprocessor 56. A signal come prison means including microprocessor 56 is provided for comparing each active breathing cycle with the rev-erroneous breathing information stored in memory apparatus. An alarm means, such as display 58 (preferably an alphanumeric Doyle it ~rovidad for producing an alarm signal, such as a visible message, upon detection, by microprocessor 56, of any of the alarm conditions discussed hereinafter under the heading "Alarms". The alarm means may also include an audibly alarm 60 for producing an audible alarm signal.
user control panel I is provided to enable temporary suppression of the audible alarm, and to enable the operator to initiate a reference time period in order to first define the reference breathing information, or to redefine the reference breathing information by causing microprocessor 56 to replace the information stored in memory apparatus 54 with information defining a new reference breathing cycle distinct from an previously defined reference breathing cycle.
As mentioned above, an optional electronic signal proportional to changes in the pressure in the patient breathing circuit may be derived by modifying the circuitry of a sensing means such as conventional gas monitor 15 so that a first output signal representative of a time-varying component of the breathing gas partial pressure is obtained, to be filtered and amplified by signal processing means 65, and separated by signal processing means 65 into a second output signal representative of a time-varying component of the breathing gas partial pressure, which time invariant component varies in proportion to the gas concentration.
The first and second output signals are connected, via external signal ports 66 and 68, to multiplexer and analog to digital converter 51. Digital samples of these signals are employed as hereinafter described to check the function and calibration of pressure sensing means 50, and to detect alarm conditions associated with referential and active breathing cycles. In similar manner, a third output signal produced by pressure sensing means 50 may be used to check the function and calibration of gas monitor 15 and signal processing means 65.
The apparatus of the preferred embodiment is powered by a rechartable battery operated power supply 64, except for signal processing means 65 which is in-corporate into, and powered by, the power supply of gas monitor 15.
The preferred embodiment will firs be dew-cried from the point of view of a typical user such as an operating room nurse or technician. A technical desk Croatian of the construction and operation of the pry furred embodiment will when be provided, followed by a discussion of the software programming for the micro-processor used in the preferred embodiment.
lo III. Opera on by Typical User:
(a) Self-Test Mode of Operation.
Figure 4 shows a control/display panel for the apparatus of the preferred embodiment. The apparatus is activated by moving an "on/off" switch (not shown in lo Figure 4) from the "off" position to the "on" position, although an automatic pre~s-lre sensitive wish connect-Ed to anesthesia ventilator lo could be employed instead of an on/off switch. The apparatus automatically enters a "self-test" mode of operation which is indicated by scrolling the message "~0*0*0*0*0*" across display SUB, so that the operator can verify that all the emanates of alpha-numeric display 58 are functioning. Audible alarm 60 is also activated so that the operator can verify that it is working.
During the "self-te~tl' mode of operation, the apparatus samples the signals present at external signal ports 66 and 68 of signal processor 65 figure 3) and analyzes those signals to determine whether or not a suitably modified gas analyzer has been incorporated into the patient breathing circuit. After two seconds the apparatus terminates the "self-test" mode, silences audibly alarm So, displays the message text "READY" on display 58 and enters the "normal" mode of operation, If the apparatus determines that a suitably modified gay analyzer has not teen incorporated in the patient I

breathing circuit, then the message text "NO 02 ANALYZER" is displayed on display 58 or two seconds. The apparatus then enters the "wait" sub-mode of the "normal" operation.
(b) Normal Mode of Operation:
The normal mode of operation is divided into four sub-modes: a 'Wait'' mode during which thy apparatus waits for the operator to activate the learning mode and thereby initiate a reference time period; a learning" mode during which the reference breathing information is defined and stored; an "active" mode during which the patient's active breathing pressure cycles are sampled and compared with the stored reference breathing information; and a "calibrate"
mode which allows the operator to verify the accuracy of pressure sensing means 50, test the condition of battery operated power supply 64, and test the condition of the signals at ports 66 and 68.
The "learning" mode is manually activated by depressing switch 70 after normal" breathing conditions have been established in the patient breathing circuit.
This action initiates a reference time period. Once the reference breathing information has been successfully defined, or "learned", the apparatus automatically ends the reference time period, exits the "learning" mode and enters the "active" mode.
If the apparatus has previously determined that a suitably modified gas monitor has been incorporated into the patient breathing circuit, then, during the learning mode of operation the apparatus also averages the signal proportional lo changes in pressure that is transmitted from gas analyzer 15 via external port 68, over the same ten-second interval during which the apparatus averages the signal from pressure sensing means 50 as described previously. Samples are taken at one tenth second intervals. When averaging is complete a reference us breathing cycle based on the signal proper-I

tonal to the prosier in the patient breathing circuit as detected by the modified gay analyzer, is established in a manner exactly similar to that used to drove thy reference breathing cycle from the signals produced by pressure sensing means 50.
When both reference breathing cycles have bean established, the apparatus performs a "cross check of pressure sensing means 50 and gas analyzer 15. If the difference between the maximum pressure and minimum pressure which in par defines the reference cycle detected by pressure sensing means 50 is sufficiently similar to the difference between the maximum and mini-mum pressure which in part defines the reference cycles detected by gas analyzer 15, after adjustments to compensate for the different methods of detection, then the apparatus proceeds into the "active" mode. However, if the quantities described differ by more than about ten percent, it is assumed that either pressure sensing means 50 or modified gas analyzer 15 with signal pro-cussing meals ho it not operating properly, and the message "CHECK SENSORS" is displayed on display 5B, In the "active" mode, the pressure in the patient breathing circuit is sensed every one-tenth of a second and, at the end of each active breathing cycle, the sampled pressure values are used to define the char-acteristics of the active breathing cycle. The active breathing cycle information is compared against certain absolute criteria (hereinafter described under the head-in "Alarms") and against certain relative criteria (also hereinafter described under the heading "Alarms").
Audible alarm 60 is sounded and text messages art disk played on display 58 if any of the absolute or relative criteria are not met. Otherwise, display 58 displays either the symbol " " indicating the inspiratory phase of an active breathing cycle, or the symbol "_" indicate in the expiatory phase of an active breathing cycle.
Alto in the active mode, the values of maximum pressure, minimum pres~ural average pressure, I/E ratio and period of the active breathing cycle waveform as detected by modified gas analyzer 15 are compared to similar values defining the reference breathing cycle as previously determined by the modified gas analyzer, and provided certain critsrla are met (as hereinafter ox-planned under the heading "Alarms") the active brea~hingcycle is deemed to be sufficiently similar to the patient's "normal" breathing condition that where should be no cause for alarm In addition, the value of the signal sensed at external signal input 66 it sampled, and provided certain criteria are met (also described under the heading 'alarms) the concentration of the selected gas in the patient breathing circuit is deemed to be acceptable and hence no cause for alarm. other-wise, audible and visual alarms are triggered to alert the operating room personnel to a problem which may no-quite their attention.
Switch 70 may be used during the "active"
mode, to cause microprocessor 56 to replace or update the stored reference breathing cycle information with information defining a new reference breathing condo-lion, provided that alarm conditions has been detected.
Switch 70 thus enables the operator to adapt the appear-tusk to changes in the settings of the ventilator and other related devices, and to changes in the pushiness physiological status (which may involve changes in the patients breathing condition), but will not allow the apparatus to "learn" new reference breathing information in the presence of an alarm condition.
Alarm suppression means comprising witch 72 is yrovlded for temporarily suppressing the audible alarm signal produced by audible alarm I If switch 72 is depressed, the audible alarm is suppressed for 30 seconds only to ensure that alarms awry no noted, The audible alarm is permanently suppressed only when the condition which triggered production of the alarm has been corrected Switches 70 and switch 72 together enable the operator to forte microprocessor 56 to replace or update reference breathing cycle information even in the pros-once of alarm conditions, as may be required, for exam-pie, if a change in the patient's physiological status has resulted in a change in the patient's breathing con-diction, which may in turn cause an alarm condition. To cause microprocessor 56 to "learn" new breathing cycle information in the presence of an alarm condition, the operator mutt activate switch 72 and keep it activated while momentarily activating switch 70 twice. This no-quirement for deliberate and simultaneous activation of two switches is intended to prevent accidental "learn in" during abnormal or alarm conditions, while also permitting "learning" following an anticipated change in patient or equipment.
Switch 70 and switch 72 additionally enable the operator to enter the "calibration" mode. To enter the calibration mode the operator must activate switch 72 and keep it activated while activating switch 70 three times.
The "calibration" mode of operation is divided into four sub-modes: pressure calibration, battery level testing, "auxiliary signal 1 testing" and "axle-Mary inlay 2 testing". when the calibration mode is entered, the apparatus assumes the "pressure caliber-lion" mode of operation. In this mode, the pressure in the patient breathing circuit is sampled every one-tenth of a second, and the resulting value is displayed on display 58. This allows the operator to verify the calibration of pressure sensing means 50 with respect to a known standard, If the apparatus is in the pressure caliber-lion mode, then the "battery level testing" mode of operation may be entered by momentarily activating switch 70. Thy apparatus Snow samples the battery voltage every one-tenth of a equine and displays thy resulting value on display 58.
If the apparatus is in the battery level testing mode, then the "auxiliary signal 1 testing" and "auxiliary signal 2 testing" modes may be entered by momentarily activating switch 70 once or twice. The auxiliary signal testing modes allow the operator to test the condition of signals at external inputs 66 and 68 by sampling the appropriate signal every one-tenth second and displaying the result on display 58.
Activation of witch 70 a fourth time causes the apparatus to revert to the "pressure calibration sub-mode.
The "calibration" mode of operation is terming axed when the operator activates switch 72 and keep it activated while activating switch 70 three times.
IV. Alarms:
I Any one of the conditions hereinflft~r describe 1 nay Rowley in yro~uc~lon it on pull lark nil by audible alarm 60 end in the display, van display 58, ox a message text defining the condition which triggered the alarm signal. In the preferred embodiment, display on 58 is an eight-character alpha-numeric display comprise inch two Litronix DO 141~ intelligent alpha-numeric disk plays. Messages comprised of fewer than nine characters are displayed "as is" on display 58. Longer messages are scrolled across display 58 twice in rotating "bill-- I -board" fashion. Audible alarm 60 is preferably a SON ALERT audible alarm.
The message text "MALFUNCTION" is displayed on display 58 upon detection by the software of an in-ability of the apparatus to complete any of its program-mod tasks in a riven period of time. Such an event would be indicative of a major failure within the apparatus.
The message text "DEAD BATTERY DIM OFF" it in displayed on display 58 if the voltage ox battery 64 falls below 9.6 volts, which is considered too low to power the apparatus for dependably monitoring pressure in the patient breathing circuit.
The message text "LOW BATTERY" is displayed on display 58 once every 12.8 seconds upon the detection, at any time, of a voltage at battery 64 less than Lowe volts. This serves as a warning to the operator of a potential failure of battery 64.
The message text IT PROSIER" is displayed on display 58 upon detection, during any active breath-in cycle, of a breathing gas pressure in excess of about 45 cmH20. The message text "NEGATIVE PRESSURE"
is displayed on display 58 upon detection, during any active breathing cycle, of a breathing yes pressure lower than about -8 cmH200 These limits represent the maximum and minimum breathinc3 gas pressure which may occur in a patient breathing circuit without endangering the patient.
The message text "LOW PRESSURE FOR lo SEIKO" it displayed on display 58 upon detection, for at least 15 seconds, of breathing circuit pressures which are less than the greater of either 5 cmH20, or, the average breathing gas pressure of the representative normal reference breathing cycle. This condition could occur, for example, if part of the patient circuit had become disconnected, or if the patient '5 breathing gas supply was interrupted for gore reason.
The message text "PERIOD > 30 SEC" is display-Ed on display 58 upon d0tec~ion of any active breathing cycle which lasts longer than 30 seconds. A 30-second breathing cycle is considered to be the longest breath-in cycle that would normally be encountered in a pa-tint breathing circuit, and may be indicative of a change in the patient's physiologic status, or of an interruption or blockage in the patient's breathing gas supply, either of which may require attention The mes5ag~ text "I/E > 4.0" or "I/E < .2"
are displayed, respectively on display 58 upon dejection of any active breathing cycle during which the ratio of the time thy patient inspires breathing gas to the time the patient expires breathing gas is greater than about 4.0 or less than about 0.2. These conditions represent the range of values for the ratio of inspiration time to expiration time normally found in patient breathing circuits. Inspiration time to expiration time ratios outside these limits may indicate conditions requiring attention.
In the presence of external signals such as those from modified gas analyzer 15, which in the pro-furred embodiment is a Critikon Oxychek~ oxygen analyzer modified as hereinafter described, the message text "CHECK I SENSOR" is displayed on display 58 upon the detection, at any time r of an oxygen concentration less than ablate 21 percent. This could indicate either a malfunction of the gas analyzer or a low level of oxygen in the patient breathing circuit, swather ox which would require attention.
The message text "INCONSISTENT SWISS" is disk played on display 58 upon detection, during the learning mode, of ten successive referential breathing cycles in which the values of the parameters defining a given breathing cycle differ by more than about ten percent from the values of the parameters defining the referent trial breathing cycle immediately preceding the given cycle This may be indicative of conditions such as an erratic patient breathing cycle, or an intermittent blockage or interruption of the patients breathing gas supply, either of which would require attention.
During the "learning" mode, the message text "CHECK SENSORS" is displayed on display 58 upon detect lion Of external signals at ports 66 and 68 if the "peak to peak" output of pressure sensing means 50 differ, by more than about 15 percent, from the peak to peak prey sure detected by modified gas analyzer 15 during a given breathing cycle after adjustments to compensate for the different methods of pressure detection. This may be indicative of a miscalibration or malfunction of either pressure sensing means 50 or gas analyzer 15 and signal processing means 65, either of which may require alien-lion.
The message text "CHECK 02 SONORA' is display-Ed on display 58 if the output signals from a modified gas analyzer are connected to port 66 and 68, and if there is a change of more than about ten percent in the concentration of oxygen in the patient breathing circuit at the be~innincJ and end of a breathing cycle. This could be a result of conditions such as a malfunction of yes analyzer 15, or a change in the concentration of oxygen in the patient breathing circuit, either of which may require attention.
Additional alarms are produced if any of the values of average pressure, maximum pressure, minimum pressure, I/E ratio, or total period defining the cur rent breathing cycle differ from the corresponding values defining the reference breathing cycle Each of I

these conditions may be indicative of the hazards such as disconnection of part of the patient breathing air-cult, interruption of the patient's breathing gas, blockage of the patient breathing circuit, or a change in the physiological status of the patient, any of which may require attention. The conditions under which these alarms are produced, and the message texts asocial with them, are described below.
, The message texts "MAX PRESSURE > REFERENCE"
or "MAX PRESSURE < REFERENCE" are displayed, as appear pretty, on display 58 upon dejection of a maximum prey-sure in any active breathing cycle which differs, by more than about 13 percent, from the maximum pressure of the reference breathing cycle.
The message texts "MIX PRESSURE > REFERENCE"
or "MIX PRESSURE < REFERENCE" are displayed, as appear-private, on display 58 upon detection of a minimum pros-sure in any active breathing cycle which differ, by more than about 17 percent, from the minimum pressure of the reference breathing cycle.
The message texts "PERIOD REFERENCE" or "PERIOD < REVERENCE" are displayed, as appropriate, on display 58 upon detection of any active breathing cycle having a period which differs by more than about 17 per-cent, prom the period of the reference breathing cycle.
The mesa texts "AVERAGE PRE~XURE > REFERENCE" or "AVERAGE PRESSURE REFERENCE" are displayed, as appropriate, on display 58 upon detection of any active breathing cycle having an average pressure which differ, by more than about 17 percent, from the average pressure of the reference breathing cycle.
The message text "I/E > REFERENCE" or "I/E <
REFERENCE" are di~playedg a purport, on display 58 upon detection of any active breathing cycle during which the X/E ratio differs by more than about I per-cent from the same ratio for the reference breathing cycle .
V. Construction and Technical Operation:
lay Pressure Sensing Means:
In the preferred embodiment, pressure sensing means So is a Motorola MPX80~D pressure transducer 120 shown in Figure SAY together with amplifying and signal conditioning circuitry. The inlet to transducer 120 it connected directly Jo hose 52, which transmit the pressure in the patient wreathing circuit Jo a prey sure sensitive element within transducer 120. Transduce or 120 produces a output voltage in the range of 0 to 5 volts which corresponds to gauze pressures of -38 to ~813 cmH20. In the preferred embodiment, the pressure lo range used is -12 to +52 cmH20, corresponding to the range of pressure that might conceivably occur in a patient breathing circuit. This pressure range cores-ponds to a voltage range of 0.51 to 1.03 volts. A dual low power operational amplifier comprised of a National LM358 integrated circuit 112 shirts and scales the out-put of pressure transducer 120 by removing the 0~51 volt offset and by amplifying the pressure transducer output by a factor of 9.6 for presentation to analog to digital converter 114 (Figure S). The amplification factor is jet to I in order Jo produce a signal that rinks over thy entire 0 to S volt Input rarl~Je of the analog to digital converter, for pressures in the range of -12 to ~52 cmH20.
In the preferred embodiment, analog to digital converter 114 is an eight-bit, 16-channel National ADC0816 integrated circuit. A voltage reference comprise in a National WRAP integrated circuit 116 provides a standard reference for analog to ~igltal converter 114, as well as a standard reference for pressure transducer l20.

(b) Microprocessor 118 which monitors the patient breathing circuit pressure, triggers the alarms drives thy displays, eta, is a National Semiconductor MSC800 CMOS microprocessor. Two Intel 2732 OK x 8-bit elect ironically programmable read only memory PROM into-grated circuits 122 and 124 store the logic program (hereinafter described under the heading "Software") which defines the sequence of operations by which micro-a processor functions. Two National ~514 lo x 4-bit static random access memory ("ROME integrated circuits 126 and 128 serve as a Croatia pad" memory in which volatile data is stored.
A Natlon2l1 NSC~10 integrated circuit I/O
lo device 130 provides two I/O ports, a timer and an add-tonal 256 x 8 bits of static RAM
A 30-line system Gus 124 is used to pass in-formation between microprocessor 118 and the various electronic devices with which it must co~nunicate. Data and address information is passed in eight Kit multi-flexed format on lines AD through AD. Lines A to Aye are used to pass an additional eight bits of addressing information. The IO/M line and address lines Aye through Aye are used by National 74PC138 decoder into-grated circuit 134 to identify which of Proms 122 or 124, Rams 126 or 128, I/O device 130, analog to digital converter 114 or displays 136 or 138 are being addressed by microprocessor 118.
Address latch 140, an Intel 82PC12 eight bit on latch intec3rated circuit, demultiL)lexes the address in-~ormatlon van lines Do through Aye, using timing inform-anion provided by microprocessor 118 on the ALE line.
Demultiplexed address information is required for proper operation of Proms 122 and 124, and Rams 126 and 128.

I
- I -A 2.00 MHz quartz crystal 142 serves as a matter clock for microprocessor lo as well as a liming standard for the timer included in I/O device 130. The ILK line of bus 132 carries a frequency reference de-roved from crystal 142 to I/O device 130 and to analog to digital converter 114.
The remaining lines of buy ~32 carry timing information to the various electronic devices connected to the bus The reset line provides a signal to no et I/O device 130 when the power is turned on. The ROD
line it used by microprocessor 118 to indicate that it expects to read information from lines AD through AD.
The We 1 ire is used by microprocessor 118 to indicate that it expects to write data to lines AD
through AD.
The non-maskable interrupt (NMI) line of buy 132 carries a signal from the timer section of I/O
device 130 to microprocessor 118. This signal force microproce~or lift to execute, elect time inter-vets, an "interrupt service routine" called "trap", (hereinafter described under the heading "Swifter which is stored in Proms 122 and 124.
The input/output device addresses are defined as follows:
L I N E DEVICE
. .. . . .
Aye Aye Aye 0 0 0 Proms 122 and 124 0 0 1 I/O Device 130 0 1 0 Rams 126 and 128 0 1 l Analog to digital converter 114 1 0 0 Displays 136 and 138 Thy scaled and amplified output of pressure transducer 1.20 is presented to analog to digital con-venter 114 at its input terminal In (input 1). Channel 0 of analog to digital converter 114 is connected through a resistor divider network to battery 64 so that the battery voltage can be monitored Inputs IN and IN are connected to external signal input jacks 144 and 146 respectively. Microprocessor 11~ is programmed to apply appropriate signals at the start and address latch enable terminals of analog to digital converter 114 to cause it to convert thy pressure transducer, battery, or external input signals from analog to digital form. The converted eight-blt digital result is passed from analog to digital converter 114 to microprocessor 118 on lines AD through AD.
IT device 130 generates a signal for trigger-in audible alarm 60, and also reads in information remeasure control panel 62. These signals are conveyed as follows on lines PH0~PB7:

LINE DIRECTION _ FUNCTION
PB0 input end of conversion from analog Jo digital converter Pal input mode switch 70 PHI input alarm suppress switch I
PB5 output audible alarm 60 PB2,3 PB6,7 Displays 136 and 138 are two Litronix DL1416 intelliy~nt alphanumeric displays Roy display de-code information presented to them on lines AD through AD of system bus 132 to provide a complete set of alpha-numeric characters address lines A and A are used to select which of the four characters within each display is to be written to by microprocessor 118~
Alpha-numeric information it passed by microprocessor I 118 in standard seven bit ASCII format.

- I -Integrated clrcui~s 148, 150 and 152 comprise ivy respectively a National 74PC00 quad 2-input RAND
gate, a National 74~C02 quad 2-input NOR gate and a National 74PC04 hex inventor, provide miscellaneous logic functions needed to filial decode addressing inform motion venerated by microprocessor 118 for presentation to the various electronic devices connected to bus 132.
(c) External Pressure Sensing Means:
.. . .
In the preferred embodiment, gas monitor 15 it comprised of a modified Critikon Oxychek~ oxygen anal lousier having a Clark-type polarographic cell. Figure PA
shows a block diagram of the modified oxygen analyzer including signal processing means 65, which has been designed for a breathing gas containing 21 percent ox-gun and for a specific inspiratory flow pattern. Figure I shows how oxycJen tenor 99 of (was monitor 15 and pressure sensor 12Q of pressure sensing means 50 may be conveniently combined as an integrated sensor. Figure 6C illustrates the circuit of signal processing means 65. Signal processing means 65 contains band pass filter 90 designed to pass frequencies in the range of 0.05 to 1.0 Ho, Lopez filter 92 designed to pass frequencies less than 0.3 Ho, amplifier 94 designed to amplify the output of band pass filter 90 by a factor ranting from 500 to 2000 time, and amplifier 96 designed to amplify the output of Lopez filter 92 by a factor of five times, Also included in signal processing means 65 is a reference round amplifier 97 disowned to provide an ad-just able DC offset to the signals from amplifiers 94 and 96.
As shown in Figure PA gas monitor 15 in the preferred embodiment consi~ts/an oxygen sensor 99 and sensor processing means 98. Similarly, pressure sensing means So in the preferred embodiment consists of press sure sensor 120 and processing means 112. Sensors and sensor I may be an integrated sensor 101 as shown in Figure 6B. In order Jo fabricate integrated sensor 101, a hole must be drilled trough sensor head 105, and a notch must be made in washer 107 which is situated by-tweet sensor head 105 and sensor element 109. The hole and notch provide a passage 103 whereby pressure sensor 120 communicates with the pressurized gas in the patient breathing circuit at the normal location of the oxygen , sensor 99~ The integrated sensor 101 simplifies connect lions to the patient breathing circuit, assures that pressure dependent signals from both sensors 99 and 120 are obtained from the same location in the patient wreathing circuit and facilitates the comparative anal-skis of signals derived from both sensors by MicroPro censor 118.
Figure 6C shows the detailed circuitry of 8i9-net processing means 65 which, in the preferred embody-mint, is physically located on a circuit board inside the gas monitor 15. Band pass filter 90 consists of three sections of a National Semiconductor L~3~4 quad operational amplifier. The first section 160 is used as a preamplifier with a fixed gain of two. The second section 162 is used as a whops filter. The third section 164 is used as a Lopez filter. There is a combined gain of two in the two filter sections.
Amplifier 94 keenest of one section of a National Semiconductor LM324 quad operational amplifier, and is connected Jo provide a variable amplification factor ranging from 125 to 500 times. The output of amplifier 94 is connected to external input 66.
Lopez fitter 92 consists of two sections 168 and 170 of a national Semiconductor LM324 quad opera-tonal amplifier. The firs section 168 acts as a preamplifier for the Lopez filter consisting of a resistor and capacitor network. The second section 170 act a voltage reference for removing any offset that might be prevent at the input of amplifier 1680 Reference amplifier 97 comprises one section of a National Semiconductor ~M324 quad operational amplifier 1720 A reference voltage taken from Test Point 7 (TP7) of sensor professing means 98 provides a standard for this amplifier.
The signal from Test Point 3 (TP3) of signal , processing means 98 it presented to the inputs of both bandpas~ jilter I and Lopez filter 92. It is from this signal thaw pressure and yes concentration infamy-lion detected by gas monitor 15 are derived. Electrical power to operate circuitry 65 is also derived from sign net processing means 98 in gas monitor.
VI. Software Appendix "A" to this specification is a 31 pave source code listing for a computer program develop-Ed or the preferred embodiment. The computer program is written in the "C" progeamllling language. The program was cross-complled from the "C" prograll~ing language into 8080 machine language code acceptable to the National Semiconductor NSC800 microprocessor integrated circuit used in the preferred embodiment. This micro-processor has as its instruction set a superjet of the instructions for the well known 8080 microprocessor integrated circuit and so the cro~-compiled code could be run on the SKYE microprocessor.
Although it is believed that the computer pro gram listing, together with the comments embedded Thor in, should enable those skilled in the art to understand the operation of the computer program, a brief overview of the operation of the computer program is hereinafter provided.
Execution of the computer program begin with the routine named " main) t- . The "main" routine culls a subroutine named 'initials" which starts the timer section of I/0 device 130 and causes it to generate a signal every one-tenth of a second. This signal used to cause microprocessor 118 to execute the subroutine "trap)" every one~enth of a second, regardless of the present state of the program The routine "initial-issue)" also urns on audible alarm 60 and displays the message "*0*0*0*0*" as part of the self-test procedure.
upon completion of the "inlays)" routine, the program returns to the routine "main)" and enters an endless loop within which all further routines are coordinated. A the beginning of each cycle of the loop, the condition of external inputs 66 and 68 are checked to see if external signals have been applied.
If no such signals are present, the program displays the message "No 02 ANALYSERn. This terminates the self-testing function.
During the execution of all routines cordon-axed by the routine "main', and hence during the entire time the program is running, the timer within It device 130 which was programmed and started in the routine in-itialize causes the microprocessor Jo execute the inter-rut ~ervlce routine "Tripoli" every one-tcnth ox a eke-on. The routine trap causes a new value of pressure a detected by pressure transducer 120 to be read each time the routine is executed, as well as reading the battery volt and the conditions ox hot external input signals. In addition, the routine "trap" detects the operation of switches 70 and 72, and operates several timer used by other routines in the program. Before program control it returned to the interrupted routine, "trap" executes the subroutine "check)".
The subroutine "check" compares the latest reading of patient breathing circuit pressure, battery voltage, and the status of external signals with some absolute criteria. If any of these criteria are not met, the routine "check" causes an alarm. Program control it then returned Jo the interrupted routine.
Then the self-test procedure is couplets, the reptilian "main" display eke Moe "EDDY" and wait for the operator to activate the "mode" switch 70. If an excessive period of time passes without the operator responding, thy message "PRESS STORE NEW DATA WHEN
READY" is displayed.
When mode switch 70 has been activated by the operator (as detected by the routine Itrap")l the rout tine "main" executes the routine "Lorraine".
The routine "learn" begins by eve ray g i no the pressure readings detected by "trap" for ten seconds.
This average pressure it stored in the variable design noted ref.cump. If an external signal is present, the pressure signal is averaged and stored in the variable oref.cump. Throughout the program, variables preceded with the letter "o" refer to information derived from external signal input.
When an average pressure is obtained, the rout tine "learn" execute the routine "read cycle". "Read-cycle' uses the average pressure stored in the variable ref.cump. to detect the beginning of either the expire-tory or insplratory phase ox the patient breathlngcycle. The routine thin continues to use pressure values read by the routine "trap" to detect the maximum pressure, minimum pressure, average pressure, inspire-lion tome and expiration time that characterizes the breathing cycle. Before returning to the calling rout tine, "read cycle" calculates the total period and IRE
ratio of the breathing cycle. If the IRE ratio is greats than or loss than certain limit, "readcycle~l displays appropriate warning midges and activate the audible alarm.

- 3g -When program control returns to the routine 'learn" from the routine "read cycle", the values of maximum pressure, minimum prosier average pressure, I/E ratio and total period are stored in the variable structures rev and ore The routine "read cycle" it executed again, and the resulting breathing cycle char-acteristics are compared to those stored in the rev and ore structures by the routine "compare".
The routine "compare" uses the routine "norm alive" to calculate the percent difference between the current breathing cycle data (stored in the structures cur and our) and the reference data stored in the rev and ore tractor, If all the comparisons fall within the percentage limit passed to the routine "coy-lo pare" by the calling routine, control is returned dir-easily to the calling routine. If any of the comparisons fail, a message is displayed and the audible alarm is sounded.
When program control is returned to the rout tine 'learn" from the routine "compare", the routine "learn" will, if all comparisons made by the routine compare caused no alarm conditions, average the values stored in the rev and ore structures with those in the cur and our structures to define a "reference breath-in cycle". If execution of the routine compare resulted in the generation of an alarm condition, the routine "read cycle" is called again to obtain a third referential breathing cycle. Further cycles are obtain-Ed until the routine "compare" is executed without the veneration of an alarm condition, or until an exce~ive number ox breathing cycle have been obtained without any two consecutive cycles matching with the con~tralnt~
of the routine "compare", at which time an appropriate message is displayed and the audible alarm is sounded.

When the routine learn is completed, the routine "main" execute the routine "Taoist The routine "test" inunediat~ly enquiry a loop that it not exiled until the operator presses mode switch 70 to cause the S microprocessor to "Lorraine a new reference breathing cycle. within the loop, the routine "test' simply calls "read cycle" and "compare" in succession, hence causing alarms to be activated whenever there is an unacceptable difference between the reference information and the current information determined by the writer "read-cycle". The routine "test" also checks to see if the operator ha activated the "calibrate" mode of opera-lion, and if so, calls the routine "calibrate".
The routine "calibrate" takes the values of pressure determined by the routine trap and display them on the alpha numeric display The routine "trays-late" is used to convert the binary numbers generated by analog to digital converter 114 into ASCII code for presentation to displays 136 and 138. "Calibrate" also checks the status of the mode switch 70 to determine if the operator wishes to proceed to the next sub-mode of the calibration mode. Each time the mode switch it activated, the routine calibrate begins displaying another ox the signals sensed in the routine trap. When the mode switch is activated a fourth time, the first sub-mode of the calibrate mode, displaying pressure information, is activated again. When the calibration mode is terminated by the operator, program control returns to the calling routine.
If the routine test it terminated by the open-atop through activation of the mode witch 70, program control it returned to the routine "main", which begins executing the loop again Tarrytown with the self text to determine if external signal sources have been connect US Ed In addition to? the routines already described there are four routines what are used by several other routine Jo execute particular functions. The routine "alarm is used to determine which of several simultan-eons alarm should be given priority, The routine"billb" causes the current message to be displayed, and provide a rotating billboard effect for messages that are too tony for the eiyht~ch~racter dl~pl~y~ The rout tine "adcmn~' is used to start the analog Jo digital con-venter in order to obtain a new value for the selected input signal. The last routine, "sense", is used to cause a new value of pressure to be read and added to a running total pressure for averaging purposes The rout tine sense also stores the maximum and minimum pressure values encountered during successive executions of the routine.
As will be apparent Jo those skilled in the anti in light of the foregoing disclosure, many alter lions and motif cations are possible in the practice of this invention without departing from the scope or spirit thereof. For example, the circuitry and software associated with the external pressure tensing means could be modified to accommodate breathing gas contain-in different percentage of oxygen and various ln~pira-25 Tory flow pattern Ann example, midlife ic~qtic~nsmay be my which would ~a.lLitat~3 use ox the invention with high frequency ventilator. ~ccordinyly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for detecting an interruption in the supply of breathing gas to a patient, comprising the steps of during a predetermined time period within at least one breathing cycle of a patient initiated solely an operator and significant of a reference time period during which a normal breathing cycle of the patient occurs, sensing the pressure in the patient's breathing gas;
deriving from the pressure sensed during the reference time period, reference breathing information including a reference breathing cycle waveform parameter, storing, in memory apparatus, the reference breathing information including said reference breathing cycle waveform parameter derived from the pressure sensed during said reference time period;
after said reference time period, sensing the pressure in the patient's breathing gas and deriving from the pressure sensed after said reference time period, active breathing information including an active breathing cycle waveform parameter;
comparing the active breathing information including said active breathing cycle waveform parameter derived from the pressure sensed after said reference time period with said reference breathing information including said reference breathing cycle waveform parameter; and, producing an alarm signal upon detection, during said comparing step, of a predetermined variation between the breathing cycle waveform parameters of said reference breathing information and said active breathing information, respectively.

2. A method as defined in claim 1, further comprising:
before said storing step, referentially comparing:
the reference breathing information derived from the pressure in the patient's breathing gas during a first referential breathing cycle of the patient; and, second reference breathing information derived from the pressure in the patient's breathing gas during a second referential breathing cycle of the patient which follows said first referential breathing cycle;
proceeding to said storing step if said breathing information derived from said pressure sensed during said first referential breathing cycle differs, by no more than a selected amount, from said breathing information derived from said pressure sensed during second referential breathing cycle; and, repeating said referential comparing step if said reference breathing information derived from said pressure sensed during said first referential breathing cycle differs, by more than a selected amount, from said second reference breathing information derived from said pressure sensed during said second referential breathing cycle.

3. A method as defined in claim 1, wherein said breathing information includes the average breathing gas pressure sensed during the breathing cycle in respect of which said breathing information is derived.

4. A method as defined in claim 1, wherein said breathing information includes the maximum breathing gas pressure sensed during the breathing cycle in respect of which said breathing information is derived.

5. A method as defined in claim 1, wherein said breathing information includes the minimum breathing gas pressure sensed during the breathing cycle in respect of which said breathing information is derived.

6. A method as defined in claim 1, wherein said breathing information includes the ratio of the time, during the breathing cycle in respect of which said breathing information is derived, the patient inspires breathing gas, to the time, during the breathing cycle in respect of which said breathing information is derived, the patient expires breathing gas.

7. A method as defined in claim 1, wherein said breathing information includes the period of the breathing cycle in respect of which said breathing information is derived.

8. A method as defined in claim 3, further comprising producing said alarm signal upon detection, for at least 15 seconds during an active breathing cycle, of breathing gas pressures less than the greater of:
(a) 5 cm H2O; and, (b) the average breathing gas pressure of said representative normal breathing cycle.

9. A method as defined in claim 7, further comprising producing said alarm signal upon detection of an active breathing cycle having a period longer than about 30 seconds.

10. A method as defined in claim 1, further comprising producing said alarm signal upon detection, during said reference time period, of ten successive referential breathing cycles, each having an average pressure which differs, by more than about ten percent, from the average pressure of the immediately following referential breathing cycle.

11. Apparatus for detecting an interruption in the supply of breathing gas to a patient, said apparatus comprising:
pressure sensing means for sensing the pressure in the patient's breathing gas during a breathing cycle of a patient and for producing an output signal representative thereof;
means for deriving breathing information including a reference breathing cycle waveform parameter from said output signal during a predetermined time period within at least one of said breathing cycles significant of a reference time period during which a normal breathing cycle of a patient occurs;
memory means for storing said reference breathing information including said reference breathing cycle waveform parameter derived from said output signal during said reverence time period;
means solely actuable by an operator for initiating the reference time period to cause storage of said reference breathing information including said reference breathing cycle waveform parameter;
means for deriving active breathing information including an active breathing cycle waveform parameter from said output signal after said reference time period;
signal comparison means for comparing;
active breathing information including said active breathing cycle waveform parameter derived from said output signal after said reference time period; with said reference breathing information including said reference breathing cycle waveform parameter derived from said output signal; and, alarm means for producing an alarm signal upon detection, by said comparison means, of a predetermined variation between said breathing cycle waveform parameters of said active breathing information and said reference breathing information.

12. Apparatus as defined in claim 11, wherein said pressure sensing means is an electronic pressure transducer.

13. Apparatus as defined in claim 11, wherein said alarm comprises an audible alarm for producing an audible alarm signal.

14. Apparatus as defined in claim 11 or 13, wherein said alarm means further comprises display means for displaying a visible message representative of said predetermined variation, upon detection thereof.

15. Apparatus as defined in claim 11, further comprising status indicator means for indicating an absence of detection, by said comparison means, of said predetermined variation.

16. Apparatus as defined in claim 13, further comprising alarm suppression means for suppressing said audible alarm signal.

17. Apparatus as defined in claim 16, wherein said alarm suppression means suppresses said audible alarm signal for no more than about 30 seconds.

18. Apparatus as defined in claim 11, further comprising control means for enabling replacement of said reference breathing information with revised reference breathing information derived from said output signal.

19. Apparatus as defined in claim 18, wherein storage of reference breathing information is prevented while said alarm signal is produced.