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FLOW CONTROL SYSTEM FOR MEDICAL
VENTILATOR
BACKGROUND OF THE INVENTION
This invention relates generally to the field of medical ventilators. More particularly, it relates to a system for controlling the flow of inspiratory gas in a volumecycled, pressure-limited ventilator.
Volume-cycled, pressure-limited ventilators (commonly called "volume" ventilators), have become wellestablished for life support and respiratory therapy, particularly for adults. While the volume ventilator has assumed numerous forms, in general it operates by providing a predetermined volume of respiratory gas (air or oxygen-enriched air, typically with added water vapor) to the patient during the inspiratory phase of each breathing cycle. Specifically, the volume ventilator delivers the gas in accordance with a predetermined flow rate function, wherein the delivered flow rate is integrated over time until a predetermined volume is delivered.
A typical volume ventilator includes a flow control valve that is electronically or pneumatically actuated to produce an instantaneous flow rate throughout the inspiratory phase that corresponds with a preselected flow rate function, as set by the operator. An example of such a flow control system is disclosed in U.S. Pat. No. 4,527,557—DeVries et al., assigned to the assignee of the invention disclosed and claimed herein. The system disclosed in the DeVries et al. patent comprises a flow control valve actuated by a stepper motor that is controlled by a control signal generated by a microcomputer. The microcomputer generates the control signal by comparing an instantaneous flow rate signal produced by a flow transducer with a flow rate value required by a flow rate function stored in the microcomputer's memory. Other volume ventilator systems using a signal from a flow sensor to actuate a flow control valve are disclosed in the following U.S. Pat. Nos.: 3,923,056—Bingmann et al.; 3,961,627—Ernst et al.; 3,972,327—Ernst et al.; and 4,928,684—Breitenfelder et al.
The system disclosed in the above-mentioned DeVries et al. patent exemplifies the use of a real time flow rate-indicative signal, generated by a flow transducer, as a feedback signal, wherein the instantaneously sensed flow rate is the parameter whose value is compared with the stored nominal value to generate the stepper motor control signal. The use of a closed-loop feedback system, including a stepper motor under the command of a microprocessor, to operate the flow control valve allows the system to achieve a relatively high degree of precision over a wide range of flow rates, with the ability to accommodate a wide variety of flow rate patterns.
While the system described above has provided highly satisfactory levels of performance, there has been a desire to improve responsiveness and reliability beyond the limitations inherent in state-of-the-art flow transducers. For example, instead of sensing flow rate directly, a value for the instantaneous flow rate can be calculated by measuring the pressure drop across a flow orifice of known area. State-of-the-art pressure transducers can achieve high levels of accuracy and reliability, and a variety of means can be used to determine the size of a variable orifice, either directly or indirectly, with precision. With state-of-the-art high speed micro
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processors, a highly precise value for the instantaneous flow rate can be obtained in real time or near real time.
The general method of measuring a fluid flow rate as a function of the sensed pressure differential across an
5 orifice is well-known in the fluid metering art, as exemplified by the following U.S. Pat. Nos.: 3,055,389— Brunner; Re. 29,383—Gallatin et al.; and 4,277,832—Wong. This general method has also been employed to measure air flow in air conditioning and
10 room ventilation ducts, as shown in U.S. Pat. No. 4,026,321 Kahoe et al. and U.S. Pat. No. 4,796,651—Ginn et al.
Medical ventilators have likewise employed flow control systems in which an instantaneous flow rate value is calculated from a measured value for pressure and a known or measured value for flow orifice size. Examples of such ventilators are disclosed in U.S. Pat. No. 4,637,385—Rusz and U.S. Pat. No.
20 4,883,051—Westenkow et al.
As far as is known, despite the desire for ever-increasing reliability, responsiveness, and flexibility in flow control capability in volume ventilators, the prior art has not contemplated the precision actuation of a venti
25 lator flow control valve using a stepper motor under the command of a microprocessor, wherein the microprocessor transmits a correction or control signal to the stepper motor in response to signals indicative of (a) the varying differential pressure across a variable flow ori
30 fice in the valve, and (b) the effective flow area of the valve orifice.
SUMMARY OF THE INVENTION
Broadly, the present invention is a system for control
35 ling the flow of inspiratory gas in a volume ventilator, comprising a variable-orifice flow control valve actuated by a stepper motor under the command of a microprocessor, wherein the microprocessor receives signals representing (a) gas pressure upstream from the valve,
40 (b) gas pressure downstream from the valve, (c) gas temperature downstream from the valve, and (d) effective flow orifice area. The microprocessor calculates a measured instantaneous actual flow rate value as a function of these parameters, and then logic circuitry com
4^ pares the calculated actual value with a desired value based on the controls set by the operator. The resultant flow signal is used to generate a control valve position signal, in accordance with a predetermined relationship,
5Q that is used to control the stepper motor so as to drive the valve mechanism to a position wherein the orifice yields an instantaneous flow rate substantially equal to the desired flow rate value. Additional feedback loops are used to improve accuracy and response times.
5J In effect, the system uses both the effective flow orifice area and the differential pressure across the orifice as active feedback control parameters, whereby the differential pressure across the orifice is periodically detected, and the resulting representative signal is used
60 in a feedback loop, along with a signal representing the effective orifice area, to adjust the orifice area to provide the nominal instantaneous flow rate value.
In a specific preferred embodiment, the flow control valve comprises a housing with a flow orifice between
65 an inlet and an outlet. The orifice defines a seat for a valve element that is lifted off of the seat by a camdriven pushrod. The cam, in turn, is rotated by the shaft of the stepper motor. The effective flow area of the
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orifice is proportional to the distance between the valve element and the seat.
The microprocessor receives the upstream gas pressure signal from a first pressure transducer located upstream from the valve. The downstream gas pressure 5 signal is received from a second pressure transducer located downstream from the valve. The downstream pressure sensed by the second pressure transducer may advantageously be the airway proximal pressure of the patient. A thermistor provides the downstream gas 10 temperature signal received by the microprocessor.
An electrically erasable programmable read only memory (EEPROM) stores a look-up table of values representing the effective orifice area as a function of values indicating the angular position of the stepper 15 motor shaft. Optical sensors operatively associated with the stepper motor shaft generate signals representing the extreme open and closed positions of the valve element as reference points for a logic circuit that provides a signal indicative of the angular position of the shaft. ^° The microprocessor addresses the EEPROM with the shaft position signal to obtain the corresponding value for the effective orifice area. The microprocessor then computes the actual volumetric flow rate from the val- 2J ues it receives for upstream pressure, downstream pressure, downstream temperature, and effective orifice area, using a formula stored in its memory. The calculated flow rate, which is corrected for standard temperature and atmospheric pressure, is thus the instanta- 3Q neous flow rate actually delivered to the patient through the flow control valve.
The calculated delivered flow rate value is compared, in a logic circuit, with a desired flow rate value that is derived from the settings of parametric controls by the 35 operator. The comparison yields a difference signal that is converted to a valve position signal, in accordance with a predetermined relationship. This valve position signal is supplied as a control signal to control circuitry for the stepper motor, whereby the stepper motor is 40 actuated to drive the valve element to a position that provides a flow rate substantially equal to the desired flow rate.
The present invention offers significant advantages over prior art ventilator flow control systems. For ex- 45 ample, the use of gas pressure and valve position sensors to obtain a calculated flow rate value, and the use of differential pressure and valve position as feedback parameters, result in greater reliability in flow rate control than has heretofore been possible in systems that 50 measure flow rate directly, due to limitations in the reliability of flow rate sensors. Moreover, this precise control is obtainable throughout a wide range of flow rates, and for a wide variety of flow rate patterns. Thus, the system is adaptable for use in numerous different 55 applications and in all commonly-used ventilation modalities. The mechanical aspects of the system are relatively simple, for improved reliability, and, as will be seen, is easily calibrated. Moreover, the use of predetermined relationships between valve orifice area stepper 60 motor position, and between stepper motor position and flow rate, coupled with the multiple feedback control mechanism, allows the invention to achieve very rapid response times for gross adjustment of the flow rate, while providing for fine adjustment of the flow rate to 65 a high degree of accuracy.
These and other advantages will be more fully appreciated from the detailed description that follows.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional schematic diagram of a volume ventilator incorporating a flow control system in accordance with the present invention;
FIG. 2 is a functional schematic representation of the electro-pneumatic interface unit employed in the ventilator of FIG. 1;
FIG. 3 is a semi-schematic representation of the flow control valve, the stepper motor that actuates the valve, and the circuitry that controls the motor, as used in the present invention; and
FIG. 4 is a schematic flow chart diagram showing the operation of the electronic control system microcomputer used to control the flow of inspiratory gas in the ventilator of FIG. 1.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
A volume ventilator incorporating a preferred embodiment of the present invention is illustrated schematically in FIG. 1.
The ventilator receives air and oxygen from pressurized supplies. The air and oxygen are filtered and regulated to 18 psig by conventional means, well-known in the art, illustrated schematically by a functional box 10, labeled "Inlet Pneumatics System". A description of a typical embodiment of the inlet pneumatics system is found in U.S. Pat. No. 4,527,557—DeVries et al., assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. The inlet pneumatics system 10 includes an air pressure tap 12 and an oxygen pressure tap 14, which lead, respectively, to an air pressure port 16 and an oxygen pressure port 18 of an electro-pneumatic interface (EPI) unit 20, to be described below.
From the inlet pneumatics system 10, the air and oxygen are fed to a blender 22, preferably of a type similar to that disclosed and claimed in U.S. Pat. No. 4,602,653—Ruiz-Vela et al., assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. The proportion of the oxygen in the blended gas may be varied between 21% and 100% by the blender 22, based on the oxygen percentage set on the control panel of the ventilator. An electronic control system (ECS) 23 converts the mechanical selection of the blend by the operator into an electrical signal, by conventional means, which signal is fed to the EPI unit, where it is conditioned and fed to the control mechanism for the blender 22. The air/oxygen mixture (which may now be referred to as "inspiratory gas") is then fed into an accumulator 24.
The inlet pneumatics system 10 also has a drive gas outlet 26, through which drive gas is provided for the operation of several ventilator subsystems, as will be discussed below. The drive gas is conducted through a shut-off system 28, described more fully below, which has a low pressure outlet 29 and a high pressure outlet 30. Drive gas from the low pressure outlet 29 is regulated to about 2 psig, while gas from the high pressure outlet 30 remains at approximately 18 psig.
From the accumulator 24, the major portion of the inspiratory gas is directed to a flow control valve 32, which will be described in detail below. A small portion of the accumulator outflow may optionally be tapped for direction to a conventional nebulizer system 34, which is controlled by a signal received from the EPI unit 20. The tapped flow from the nebulizer system 34 is
directed to a nebulizer port 36. The major portion of the accumulator outflow is directed through the flow control valve 20, then through a subambient/overpressure relief ("SOPR") valve 38, to a patient outlet 40. From the patient outlet 40, the inspiratory gas is conducted to 5 the patient through a conventional patient circuit (not shown), after passing through a bacterial filter (not shown) and an optional humidifier (not shown).
The SOPR valve 38 performs a dual function: (1) It allows the patient to breathe ambient air in the event of 10 ventilator failure; and (2) it acts as a relief valve to limit the maximum pressure delivered by the ventilator. The SOPR valve 38 has an inspiratory gas inlet port 41 that receives inspiratory gas from the flow control valve 32, and, in normal operation, passes the gas through to the patient outlet 40. The SOPR valve 38 has a first reference pressure inlet 42 that receives 2 psig drive gas pressure from the low pressure outlet 29 of the shut-off system 28. The 2 psig pressure acts as a reference pressure on one side of a spring-biased diaphragm (not shown), the other side of which is exposed to the gas flow received from the flow control valve. The diaphragm's pressure ratio is selected such that when the inspiratory gas pressure received from the flow control valve 32 exceeds about 175 cmH20, the diaphragm opens an internal valve passage (not shown) from the inspiratory gas inlet port 41 to an overboard exhaust port 44, thereby relieving the excess pressure.
Should the pressure at the first reference pressure inlet 42 fall to near zero (indicating a ventilator failure, due to, for example, loss of power or a failure of the inlet pneumatics system 10), the diaphragm opens under the force of its biasing spring (not shown), opening the above-mentioned internal passage to provide direct communication between the patient outlet 40 and the overboard exhaust port 44, thereby allowing the patient to inhale ambient air.
As mentioned above, the shut-off system 28 provides the 2 psig reference pressure to the SOPR valve 38 through the low pressure outlet 29. This reference pressure is also directed to a second reference pressure inlet port 46 in a PEEP/exhalation control subsystem 48. ("PEEP" is an acronym for Positive End Expiratory Pressure.) The PEEP/exhalation control subsystem 48 controls the operation of an exhalation valve 50, as will 45 be described below. The 18 psig drive gas from the high pressure outlet 30 of the shut-off system is delivered to a high pressure inlet port 52 in the PEEP/exhalation control subsystem 48.
The shut-off system 28, which is under the control of 50 the EPI unit 20, includes a solenoid valve (not shown), having an inlet that receives drive gas from the inlet pneumatics system 10, and an outlet that communicates substantially directly with the high pressure outlet 30, and through a low pressure regulator (not shown) with the low pressure outlet 29. During normal operation of the ventilator, the solenoid is energized, passing 18 psig drive gas through to the high pressure outlet 30 and to the low pressure regulator, the latter dropping the pressure to 2 psig for passage through the low pressure outlet 29.
If ventilator power is lost, the solenoid valve in the shut-off system 28 closes, cutting off drive gas to the PEEP/exhalation control subsystem 48, and thus depressurizing the exhalation valve 50. With no drive 65 pressure applied to it, the exhalation valve 50 remains fully open, allowing the patient to exhale unimpeded. Closure of the shut-off system 28 also opens the passage
in the SOPR valve 38 between the patient outlet 40 and the overboard exhaust port 44, as described above, thereby allowing unimpeded inhalation of ambient air.
The PEEP/exhalation control subsystem 48 and the exhalation valve 50 are similar to those disclosed in the above-mentioned U.S. Pat. No. 4,527,557—DeVries et al., and thus only a brief description is needed here. The purpose of these components is twofold: (1) to close off the expiratory limb of the patient circuit during inhalation and open it during the exhalation; and (2) to maintain a selected PEEP level during exhalation. The exhalation valve 50, which is controlled by a signal from the EPI unit 20, also acts as a check valve, preventing inhalation through the expiratory leg of the patient circuit.
The PEEP/exhalation control subsystem 48 includes a variable-orifice needle valve (not shown), the upstream side of which receives 18 psig drive gas from the high pressure outlet 30 of the shut-off system 30. A jet pump (not shown), downstream from the needle valve, creates a static pressure that is applied to the ventilator side of the exhalation valve 50 to establish the PEEP. The level of PEEP is selected by varying the orifice of the needle valve by means of a PEEP adjustment control 54 on the control panel of the ventilator.
During exhalation, the patient's exhaled breath is conducted from the exhalation valve 50 to an exhalation outlet 56 through an exhalation flow transducer 58, preferably of the "hot wire" type. The exhalation flow transducer 58 generates an analog electrical output signal that is conditioned (i.e., digitized, amplified, and temperature-compensated) by associated flow transducer circuitry 59, before being fed into the EPI unit 20, which, in turn, produces an output signal directed to the ECS 23, which includes means (not shown) for integrating the signal over time to generate a visual display (not shown) of the exhalation volume.
The EPI unit 20 is best understood with reference to FIG. 2. One major purpose of the EPI unit is to provide a clear interface between the ECS 23 and the pneumatic components of the ventilator. From the manual controls of the ECS 23, the EPI unit 20 receives signals representing the operator-selected values for the operational parameters, such as flow rate, oxygen concentration, and valve states. These signals are communicated to the EPI unit 20 every 10 milliseconds. Based on these signals, the EPI unit 20 commands the various valves (e.g., the blender 22, the flow control valve 32, and the PEEP/exhalation control subsystem 48) to assume their correct positions or states.
Another major function of the EPI unit 20 is to receive pneumatic and electrical signals representing pressures, temperatures, and flow rates within the ventilator, and to process these signals for use in controlling the ventilator, either automatically (i.e., by servo-control mechanisms), or manually (i.e., through the ECS 23).
As shown in FIG. 2, the EPI unit 20 includes several pneumatic input ports. Among these are the previously mentioned air pressure port 16 and the oxygen pressure port 18. Also included are a machine pressure port 60, a proximal pressure port 62, a PEEP port 64, and flow control valve pressure port 66. The machine pressure port 60 receives a static pressure from a machine pressure tap 68 at the downstream side of the SOPR valve 38. The proximal pressure port 62 receives a static pressure from a proximal pressure tap 70 on the ventilator side of the patient connector (not shown) that couples the patient to the patient circuit. The PEEP port 64
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receives the PEEP from a PEEP tap 72 in the PEEP/exhalation control subsystem 48. The flow control valve pressure port 66 receives a static pressure from a flow control valve pressure tap 74 located on the upstream side of the flow control valve 32. 5
The air pressure port 16, the oxygen pressure port 18, the machine pressure port 60, the proximal pressure port 62, and the flow control valve pressure port 66 each communicates with an associated miniature gage pressure transducer 80. The proximal pressure port 62 10 also communicates with one side of a miniature differential pressure transducer 82, the other side of which is in communication with the PEEP port 64. The transducers 80, 82 are preferably of a type marketed as the "16PC Series" by the Micro Switch Division of Honey- 15 well, Inc. Transducers of this type are sensitive enough for the purposes of this invention, with a suitable pressure range, and yet are small enough for mounting on a printed circuit board 84 with the other electronic components, described below, of the EPI unit. 20
Each of the gage transducers 80 produces an analog electrical output signal representing the value of the pressure applied to that transducer's associated port. The differential transducer 82 produces an analog electrical output signal representing the value of PEEP 25 minus the proximal pressure; this differential signal may be termed "pdelta", and usually has a negative value. The analog signals are fed into an analog-to-digital converter (ADC) 86, which includes suitable amplification circuitry (not shown). After the signals are digi- 30 tized and amplified, they are fed into a microcomputer 88, which also receives signals from the ECS 23, the blender 22 and the exhalation flow transducer circuitry 59. Because the transducers 80 and 82 are mounted directly on the board 84, the transmission distances of 35 their analog output signals to the ADC 86 are minimized, thereby rendering them less susceptible to noise (e.g., EMI) from external sources. Calibration factors for the pressure transducers 80, 82 are advantageously stored in an EEPROM 90 that is electronically coupled 40 to the microcomputer 88.
By way of specific example, the microcomputer 88 may advantageously comprise a microcontroller (such as a Motorola 68HC05 microcontroller, or a substantial equivalent thereof) and a 4 MHz crystal, with the fol- 45 lowing on-chip functions: 176 bytes of random access memory (RAM), 7584 bytes of Read Only Memory (ROM), three 8-bit I/O ports, a serial peripheral interface port (for communication with the ADC 86 and the exhalation flow transducer circuitry 59), a serial com- 50 munication interface port (for communicating with the ECS 23), and an 8-bit timer.
Digital output signals from the microcomputer 88 are used to control the blender 22, the nebulizer system 34, the shut-off system 28, the PEEP/exhalation control 55 subsystem 48, and the flow control valve 32, the control of the flow control valve being described in detail below.
The signals from the above-described pressure transducers have the following significance: 60
The air and oxygen pressure signals indicate proper operation of the inlet pneumatic system 10. An air or oxygen pressure below a predetermined threshold indicates a gas failure, as a result of which the microcomputer 88 is signaled to trigger an alarm (not shown) and 65 to actuate the shut-off system 28, as described above, through a solenoid control circuit 92, of a type that is well-known in the art.
The machine pressure signal represents the inspiratory gas pressure in the ventilator downstream from the flow control valve 32. The proximal pressure signal represents the pressure on the ventilator side of the patient connector. This signal is used, as will be explained below, to calculate the flow rate of inspiratory gas delivered to the patient through the flow control valve 32. It may also be used as an input signal to actuate a proximal pressure display, such as a gage 94 (FIG. 1) or an external pressure monitor (not shown). The flow control valve pressure signal represents the inspiratory gas pressure on the upstream side of the flow control valve 32. This signal is used, along with the proximal pressure signal, to calculate the flow rate through the flow control valve 32, as will be explained below.
The Pdelta signal represents the difference between the PEEP and the proximal pressure. This signal is used as a feedback control signal for controlling the flow control valve 32 when the ventilator is in the pressure control, pressure support, or pressure augmentation mode, as will be explained below.
The flow control valve 32, illustrated in FIG. 3, comprises a housing 100 having a gas inlet 102 that receives inspiratory gas from the accumulator 24, and a gas outlet 104 that delivers the gas to the SOPR valve 38. Between the inlet 102 and the outlet 104 is a valve seat 106 defining a valve orifice 108, the effective flow area of which is varied by a poppet valve element 110, preferably spherical in form, as shown. The position of the valve element 110 relative to the valve seat 106, and thus the effective flow area of the valve orifice 108, is controlled by a rod 112 that is journaled, by suitable bearings (not shown), for axial movement within the valve housing 100. One end of the rod 112 engages the valve element 110, while the other end is engaged by an asymmetrical cam 114 that is mounted on a shaft 116 of a stepper motor 118. The rod 112 is advantageously maintained in engagement with the cam 114 by a coil spring 120.
In terms of its overall mechanical structure, the flow control valve 32 is similar to the flow control valve disclosed in the above-mentioned U.S. Pat. No. 4,527,557—DeVries, except that in the prior art valve, the valve element seats against the downstream side of the flow orifice, while in the flow control valve 32 employed in the present invention, the valve element 110 seats against the upstream side of the orifice 108. In addition, the flow control valve 32 of the present invention uses the spring 120 to assure that the rod 112 follows the cam 114, while the prior art device uses gas pressure for this purpose. It should be noted that a flow control valve of the type disclosed in the DeVries patent can be used in a flow control system in accordance with the present invention, without departing from the spirit and scope of the invention.
The valve housing 100 contains temperature sensing means, such as a thermistor 122, downstream from the valve orifice 108. The thermistor 122 generates an analog electrical signal representing the temperature of the inspiratory gas, and this signal is then inputted to the EPI unit 20, and, more specifically, to the ADC 86.
The stepper motor 118 is a unipolar, four-phase motor, with a resolution of at least about 0.9 degrees of rotation per half step, and an effective rotational speed of approximately 1100 half steps per second. Through the shaft 116, the cam 114 and the rod 112, the motor 118 drives the valve element 110 between a fully closed
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