WO2002072185A1

WO2002072185A1 - Rebreathing circuit

Info

Publication number: WO2002072185A1
Application number: PCT/CA2002/000338
Authority: WO
Inventors: Joseph A. Fisher; Steve Iscoe; Hiroshi Sasano; Alex Stenzler; Alex Vesely
Original assignee: Fisher Joseph A; Steve Iscoe; Hiroshi Sasano; Alex Stenzler; Alex Vesely
Priority date: 2001-03-12
Filing date: 2002-03-12
Publication date: 2002-09-19
Also published as: EP1370319A1

Abstract

An improved breathing circuit to be used to assist patients who are or run the risk of suffering the effects of high altitude sickness, or who have suffered a cardiac arrest, or who have suffered from an interruption of blood flow to an organ or region of an organ and are at a risk of suffering oxidative injury on restoration of blood perfusion as would occur with a stroke or heart attack or resuscitation of the newborn.

Description

REBREATHING CIRCUIT

A portable partial rebreathing circuit to set and stabilize end tidal and arterial PCO2 despite varying levels of minute ventilation

FIELD OF INVENTION

The purpose of this invention is to provide a portable breathing circuit that provides ambient air to breathe unless the minute ventilation exceeds the rate of ambient air entry into the circuit and further if minute ventilation does exceed the rate of ambient air entry into the circuit then the difference between minute ventilation and the rate of ambient air entry into the circuit is composed of rebreathed alveolar gas in preference to dead space gas. All gas entering the circuit is breathed before exiting the circuit.

BACKGROUND OF THE INVENTION

Physiology Nenous blood returns to the heart from the muscles and organs partially depleted of oxygen (0₂) and a full complement of carbon dioxide (C0₂). Blood from various parts of the body is mixed in the right side of the heart (resulting in the formation of mixed venous blood) and pumped to the lungs. In the lungs the blood vessels break up into a net of small vessels surrounding tiny lung sacs (alveoli). The vessels surrounding the alveoli provide a large surface area for the exchange of gases by diffusion along their concentration gradients. A concentration gradient exists between the partial pressure of C0₂ (PC0₂) in the mixed venous blood (PvC0₂) and that in the alveolar PC0₂. The C0₂ diffuses into the alveoli from the mixed venous blood from the beginning of inspiration until an equilibrium is reached between the PvC0 and the alveolar PC0₂ at some time during the breath. When the subject exhales, the first gas that is exhaled comes from the trachea and major bronchi which do not allow gas exchange and therefore will have a gas composition similar to that of inhaled gas. The gas at the end of exhalation is considered to have come from the alveoli and reflects the equilibrium CO2 concentration between the capillaries and the alveoli; the PC0₂ in this gas is the end-tidal PC0₂ (PETC0₂ ).

When the blood passes the alveoli and is pumped by the left side of the heart to the arteries in the rest of the body it is known as the arterial PC0₂ (PaC0₂). The arterial blood has a PC0₂ equal to the PC0₂ at equilibrium between the capillaries and alveoli. With each breath some C0₂ is eliminated from the lung and fresh air containing little or no C0₂ (CO2 concentration is assumed, to be 0% is inhaled and dilutes the residual alveolar PC0_2/ establishing a new gradient for C0₂ to diffuse out of the mixed venous blood into the alveoli. The flow of fresh gas in and out of the lungs each minute, or minute ventilation (N), expressed in L/min, is that required to eliminate the C0₂ brought to the lungs and maintain an equilibrium PC0₂ (and PaC0₂) of approximately 40 mmHg (in normal humans). When one produces more C0₂ (e.g., as a result of fever or exercise), more C0 is produced and carried to the lungs. If C02 production is normal, the PaC02 falls, if one increases one's ventilation (hyperventilation); conversely, if C02 production remains normal, the PaC02 rises if the ventilation falls (hypoventilation).

It is important to note that not all N contributes to elimination of C0₂. Some N goes to the air passages (trachea and major bronchi) and alveoli with little blood perfusing them, and thus contributes minimally to eliminating C0₂. This N is termed "dead space" ventilation and gas in the lung that has not participated in gas exchange with the blood is called "dead space" gas. That portion of N that goes to well-perfused alveoli and participates in gas exchange is called the alveolar ventilation (NA) and exhaled gas that has participated in gas exchange in the alveoli is termed "alveolar gas". Referring to the PCT Application No. W098/41266 filed by Joe Fisher, there is taught a method of accelerating the resuscitation of a patient having been anaesthetized by providing the patient with a source of fresh gas and a source of reserve gas (see below). When the patient breathes at a rate such that his ventilation is less than or equal to the fresh gas flowing into the circuit, all of the inhaled gas is made up of fresh gas. When the patient's minute ventilation exceeds the fresh gas flow, the inhaled gas is made up of all of the fresh gas and the additional gas is provided by "reserve gas" consisting of fresh gas plus CO2 such that the concentration of C0₂ in the reserve gas of about 6% has a partial pressure equal to the partial pressure of CO2 in the mixed venous blood. At no time while using this method, will the patient re-breathe gas containing anaesthetic. In order to accelerate the resuscitation of the patient, a source of fresh gas is provided for normal levels of minute ventilation, typically 5 L per minute and a supply of reserve gas is provided for levels of ventilation above 5 L per minute wherein the source of reserve gas includes approximately 6% carbon dioxide having a PCO2 level substantially equal to that of mixed venous blood. It has been found that this method and various circuits and processes for implementing the method are advantageous not only for resuscitating individuals from surgery, but also to deal with carbon monoxide poisoning or the like as taught in the application. By allowing increased ventilation yet maintaining the PCO2 level substantially equal to that prior to normal ventilation, it has been found that in utilizing the method, maximum benefits of gas elimination are achieved without changing the C0₂ levels in the patient. However, one limitation is that a source of reserve gas and its delivery apparatus must be supplied to pursue the method and that the reserve gas must be at about 6% C0₂ concentration substantially having a PCO2 equal to that of mixed venous blood or about 46 mm Hg.

It would therefore be advantageous to reap the benefits of controlling the PCO2 at a constant level and not having to incur the expense and inconvenience of supplying fresh gas. Furthermore the compact nature of the invention would make its use practical outdoors, during physical activity and in remote environments. It has been determined that people living at high altitude such as mountaineers, miners, astronomical observatory personnel would benefit from preventing the PCO2 level falling excessively as a result of the involuntary tendency to hyperventilate "while they are at high altitude. It has also been determined that resuscitation of newborns with air has demonstrable advantages over resuscitation with oxygen if excessive decrease in PCO2 can be prevented. This heretofore was not contemplated in the prior art nor in the priority disclosure of Joseph Fisher, et al discussed herein.

It is therefore a primary objective of this invention to provide a simplified method of controlling PCO2 at a predetermined desired level without the need of gas from another source flowing into the circuit under pressure.

It is a further objective of this invention to provide a simplified, compact and more effective method of storing the expired gas while preventing dilution with atmospheric air such that alveolar portion of the expired gas is rebreathed in preference to dead space gas.

It is a further object of this invention to provide an improved breathing circuit to be used to assist patients who are or run the risk of suffering the effects of high altitude sickness, or who have suffered a cardiac arrest, or who have suffered from an interruption of blood flow to an organ or region of an organ and are at risk of suffering oxidative injury on restoration of blood perfusion as would occur with a stroke or heart attack or resuscitation of the newborn.

It is a further object of this invention to provide methods of treatment using the said circuit and the use of the said circuit to assist patients who are or run the risk of suffering the effects of high altitude sickness, or who have suffered a cardiac arrest, stroke, or heart attack. Further and other objects of the invention will become apparent to those skilled in the art when considering the following summary of the invention and the more detailed description of the preferred embodiments illustrated therein.

SUMMARY OF THE INVENTION

To simplify the circuit we have now discovered that the source of fresh gas, usually consisting of pressurized gas or mechanical gas pump, can be replaced by a passive system wherein the act of inhaling by the subject results in a constant sub- atmospheric pressure inside the circuit, independent of the extent of breathing or the size of the breaths, providing the pressure gradient driving atmospheric air into the circuit. The opening into the circuit from the atmosphere consists of tubing whose length and diameter provides for a particular flow of ambient air into the circuit for a given pressure gradient. As long as the minute ventilation is equal to or exceeds the flow of ambient air into the circuit, the pressure gradient, and hence the flow of ambient air into the circuit will remain constant. To further improve the portability of the circuit we have determined that the expired gas reservoir consists of a flexible bag, preferably of approximately 3 L capacity, containing a tubular structure at the point of gas entry and a tubular structure at the point of gas exit.

Referring to prior Application No. 2,304,292 from which priority is claimed there is described a method of simplifying the circuit taught by Fisher (W098/41266); the reserve gas can be replaced by previously exhaled gas. The gas at the end of exhalation has substantially equilibrated with mixed venous gas and thus has a PCO2 substantially equal to it. As rebreathed gas contains anesthetics in anesthetized patients, the use of rebreathed gas to prevent the decrease in PCO2 with increased ventilation instead of separately constituted reserve gas to prevent the decrease in PCO2 with increased ventilation, will not promote the enhancement of elimination of anesthetics. There are applications for a circuit that maintains PC0₂ constant despite increased ventilation which are not invalidated by using exhaled gas as the reserved gas such as the control of brain blood flow during magnetic imaging scanning or after a stoke, and maintain placental and fetal brain blood flow during labor of pregnancy. The portability of the circuit taught by the priority application to Fisher, et al above mentioned is limited by the requirement for a source of gas flow to provide the fresh gas flow and for a long tubular structure to provide a reservoir for expired gas, keeping the last expired gas (alveolar gas) available to be the first gas rebreathed and being sufficiently long to prevent atmospheric air from diffusing in and diluting the expired CO2 concentrations. For example, while climbing at high altitude it would be very difficult to carry oxygen tanks and a long tubular expired gas reservoir required to prevent dilution of expired gas with atmospheric air, typically about 3m. Another example of such a difficulty would be when preventing hyperventilation while ventilating with air in the course of resuscitating newborns and adults in out-of-hospital settings. It would be advantageous to eliminate the requirement for gas tanks or gas pumps in the circuit which then becomes less expensive and easier to transport.

The present inventive circuit would share some of the advantages set out in the prior application of such as, but not limited to: a) raising PCO2 i) during pregnancy to improve placental and fetal brain blood flow, ii) to prevent shivering, iii) to increase tissue perfusion, and iv) protect tissue from oxidative damage after a period of severe hypoxia or ischemia by permitting resuscitation with normal atmospheric oxygen concentrations and meeting tissue oxygen demand through C0₂-mediated increased tissue blood flow.

Canadian Patent Application No. 2,304,292 from which priority is claimed has previously described a circuit which when fresh gas flow is provided, maintains PCO2 independent of minute ventilation by supplying the difference between fresh gas flow and minute ventilation from gas expired from a previous breath. The circuit contains a fresh gas reservoir bag whose relaxed position is collapsed then fills passively with fresh gas when and only when fresh gas is forced into the circuit under pressure. Fresh gas is forced into the circuit at a constant rate independent of the phase of breathing. The expired gas reservoir consists of a long tube open to atmosphere. When a volume of expired gas is rebreathed, an equal volume of outside air enters the tube and mixes with expired gas. As this will dilute the expired gas and decrease the effectiveness of the circuit in maintaining a constant PC0₂ with increased minute ventilation, the tubular expired gas reservoir must be as long as possible to separate the expired alveolar gas from expired gas diluted by atmospheric air.

The present circuit exploits the same principle in maintaining PCO2 constant; however it replaces the fresh gas reservoir bag with a substantially flexible container which is actively collapsed by the inspiratory effort of the patient during inspiration and passively expands during expiration drawing into itself and the circuit atmospheric air through a port provided for that purpose. The expiratory reservoir is provided with a flexible bag so that the volume of expired gas rebreathed is displaced by collapse of the bag rather than entrainment of atmospheric air, thus preventing the dilution of CO2 in the expired gas reservoir.

According to a primary aspect of the invention there is provided a method of establishing a constant flow of fresh gas in the form of atmospheric air, the flow of which is forced as a result of breathing efforts by the patient but independent of the extent of ventilation. This flow is delivered into a breathing circuit such as that taught in the priority application designed to keep the PCO2 constant by providing expired gas to be inhaled when the minute ventilation exceeds the flow of fresh gas. Furthermore there is provided a compact expired gas reservoir capable of organizing exhaled gas so as to be preferentially inhaled during re-breathing when necessary by providing alveolar gas for re-breathing in preference to dead space gas.

The preferred circuit in effecting the above-mentioned method includes a breathing port for inhaling and exhaling gas, a bifurcated conduit adjacent said port, preferably being substantially Y-shaped, and including a first and second conduit branch, said first conduit branch including an atmospheric air inlet the flow through which is controlled by a resistance for example that being provided by a length of tubing, and a check valve disposed proximate the port, said check valve allowing the passage of inhaled atmospheric air to the port but closing during exhalation, said second conduit including a check valve which allows passage of exhaled gas through said check valve but prevents flow back to the breathing port once the gas passes through the check valve, said first conduit branch having located proximate the terminus thereof, an atmospheric air aspirator (AAA) consisting of a collapsible container tending to recoil to an open position, said second conduit branch having located proximate the terminus thereof, an exhaled gas reservoir, preferably being a thin walled flexible bag approximately 3 L in capacity containing a tube extending into the bag through which gas enters the bag and containing a second tube extending into the bag through which gas exits the bag, said terminus of said first and second conduit branches having extending there between an interconnecting conduit and having a check valve located therein, wherein when minute ventilation for the patient is equal to the rate of atmospheric air aspirated into the circuit, for example 5 L per minute, atmospheric air enters the breathing port from the first conduit branch at a predetermined rate and preferably 5 L per minute and is exhaled through the second conduit branch at a rate of preferably 5 L per minute, wherein the exhaled gas travels down to the exhaled gas reservoir, wherein when it is desirable for the minute ventilation to exceed the fresh gas flow, for example 5 L per minute, the patient will inhale expired gas retained in the expired gas reservoir which will pass through the check valve in the interconnecting conduit at a rate making up the shortfall of the atmospheric air flow of for example 5 L per minute, wherein the shortfall differential is made up of rebreathed gas, thereby preventing a change in the PC0₂ level of alveolar gas despite the increased minute ventilation.

When setting the fresh gas flow to maintain a desired PCO2 it is important that the atmospheric air aspirator be allowed to first be depleted of gas until it just empties at the end of the inhalation cycle. In this way once it is desired to increase the minute ventilation, the increased breathing effort required to do so will further decrease the sub-atmospheric pressure in the first conduit, being the inspiratory limb, and open the check valve in the interconnecting conduit to allow further breathing of gas beyond the level of ventilation supplied by the volume of atmospheric air aspirated into the circuit during the entire breathing cycle.

The uses for this particular circuit are those described in the priority application and in addition this circuit is particularly useful for maintaining isocapnia when atmospheric air is a suitable form of fresh gas and it is inconvenient or impossible to access a source of compressed gas or air pump to provide the fresh gas flow. During mountain climbing or working at high altitude some people tend to increase their minute ventilation to an extent greater than that required to optimize the alveolar oxygen concentration. This will result in an excessive decrease in PCO2 which will in turn result in an excessive decrease in blood flow and hence oxygen delivery to the brain. By using the above-mentioned circuit at high altitude a limit can be put on the extent of decrease in PCO2 and thus maintain the oxygen delivery to the brain in the optimal range.

During resuscitation of an asphyxiated newborn or an adult that has suffered a cardiac arrest the blood flow through the lungs is markedly slowed during resuscitation attempts. Even normal rates of ventilation may result in excessive CO2 being eliminated from the blood. As this blood reaches the brain, the low PCO2 may constrict the blood vessels and limit the potential blood flow to the already ischemic brain. By attaching the circuit to the gas inlet port of a resuscitation bag and diverting all expired gas to expired gas reservoir the decrease in PCO2 would be limited by the flow of atmospheric air aspirated into the circuit and be otherwise independent of the minute ventilation.

According to yet another aspect of the invention, there is provided a method of enhancing the results of a diagnostic procedure or medical treatment comprising the steps of: providing a circuit that does not require a source of forced gas flow which is capable of organizing exhaled gas so as to provide to the patient preferential rebreathing of alveolar gas in preference to dead space gas, (for example the circuit described above) when the patient is ventilating at a rate greater than the rate of atmospheric air aspirated, and when inducing hypercapnia is desired, by decreasing the rate of aspirated atmospheric air and passively provide a corresponding increase in rebreathed gas so as to prevent the PCO2 level of arterial blood from dropping despite increases in minute ventilation, continuing inducing hypercapnia until such time as the diagnostic or medical therapeutic procedure is completed, wherein the results of said diagnostic or medical procedure are enhanced by carrying out the method in relation to the results of the procedure had the method not been carried out. Examples of such procedures would be MRI or preventing spasm of brain vessels after brain hemorrhage, radiation treatments or the like. This method may be realized by utilizing the embodiment of the circuits described herein in the summary of the invention.

According to yet another aspect of the invention there is provided a method of treating or assisting a patient, preferably human, during a traumatic event characterized by hyperventilation comprising the steps of: providing a circuit that does not require a source of forced gas flow which alveolar ventilation is equal to the rate of atmospheric air aspirated and increases in alveolar ventilation with increases in minute ventilation is prevented by a circuit (for example the preferred circuit described above) which is capable of organizing exhaled gas so as to provide to the patient preferential rebreathing alveolar gas in preference to dead space gas following ventilating the patient at a rate of normal minute ventilation, preferably approximately 5L per minute, and when desired inducing hypercapnea so as to increase arterial PC0₂ and prevent the PCO2 level of arterial blood from subsequently dropping below that achieved as^r a result of decreasing the fresh gas flow, continuing maintaining normocapnia despite the ventilation at an increased rate until such time as the traumatic event and concomitant hyperventilation is completed, wherein the effects of hyperventilation experienced during the traumatic event are minimized for example the mother during labour becoming light headed or the baby during the delivery also being effected with the oxygen delivery to its brain being decreased as a result of contraction of the blood vessels in the placenta and fetal brain. This method may be realized by utilizing the embodiment of the circuits described herein in the summary of the invention.

A list of circumstances in which the method enhancing the diagnostic procedure results or the experience of the traumatic event are listed below.

Applications of this method and circuit

1) Maintenance of constant PCO2 and inducing changes in PCO2 during MRI;

2) Inducing and /or maintaining increased PCO2; a) to prevent or treat shivering and tremors during labor, post-anesthesia, hypothermia, and certain other pathological states; b) to treat fetal distress due to asphyxia; c) to induce cerebral vasodilatation, prevent cerebral vasospasm, and provide cerebral protection following subarachnoid hemorrhage cerebral trauma and other pathological states; d) to increase tissue perfusion in tissues containing cancerous cells to increase their sensitivity to ionizing radiation and delivery of chemotherapeutic agents; e) to aid in radiodiagnostic procedures by providing contrast between tissues with normal and abnormal vascular response; f) protection of various organs such as the lung, kidney and brain during states of multi-organ failure.

3) Prevention of hypocapnia with O2 therapy, especially in pregnant patients.

4) Other applications where O2 therapy is desired and it is important to prevent the accompanying drop in PCO2. When minute ventilation is greater than or equal to the rate of atmospheric air aspirated, the above-mentioned preferred circuit ensures that the patient receives all the atmospheric air aspirated into the circuit independent of the pattern of breathing since atmospheric air alone enters the fresh gas reservoir, and exhaled gas enters its own separate reservoir and all the aspirated air is delivered to the patient during inhalation before any rebreathed exhaled gas. The atmospheric air aspirator is large enough not to fill to capacity during a prolonged exhalation when the total minute ventilation exceeds the rate of atmospheric air aspiration ensuring that under these circumstances atmospheric air continues to enter the circuit uninterrupted during exhalation. The preferred circuit prevents rebreathing at a minute ventilation equal to the rate of air being aspirated into the atmospheric air aspirator because the check valve in the interconnecting conduit does not open to allow rebreathing of previously exhaled gas unless a sub-atmospheric pressure less than that generated by the recoil of the aspirator exists on the inspiratory side of the conduit of the circuit. The circuit provides that after the check valve opens, alveolar gas is rebreathed in preference to dead space gas because the interconnecting conduit is located such that exhaled alveolar gas contained in the tube conducting the expired gas into the expiratory reservoir bag will be closest to it and dead space gas will be mixed with other exhaled gases in the reservoir bag. The exhaled gas reservoir is preferably sized at about 3 L which is well in excess of the volume of an individual's breath. When the patient inhales gas from the reservoir bag, the reservoir bag collapses to displace the volume of gas extracted from the bag, minimizing the volume of atmospheric air entering the bag.

According to another aspect of the invention there is provided a method of

providing constant PCO2 to a patient wearing a breathing port having an inspiratory

side and an expiratory side, the method comprising:

aspirating atmospheric air from the inspiratory side to a patient when the

patient inhales through the inspiratory side; accumulating the gas exhaled by the patient in an expiratory gas reservoir

connected to the expiratory side, through which the patient exhales; and

allowing the gas exhaled by the patient and stored in the expiratory gas

reservoir to flow into the inspiratory side to mix with the aspirated atmospheric air

when a minute ventilation of the patient exceeds the atmospheric air aspirated to the

inspiratory side.

According to another aspect of the invention there is provided an isocapnia circuit,

comprising:

a breathing port, through which a subject inhales and exhales;

an inspiratory port, communicating to the breathing port with an inspiratory

valve that allows air flowing to the breathing port and prevents air flowing from the

breathing port to the inspiratory port, the inspiratory port having an atmospheric air

aspirator to aspirate the atmospheric air therein;

an expiratory port, communicating to the breathing port with an expiratory

valve that allows air flowing from the breathing port to the expiratory port and

prevents air flowing to the breathing port, the expiratory port having an expiratory

gas reservoir to store gas exhaled by the subject flowing across the expiratory valve;

and

a bypass conduit, communicating the inspiratory and expiratory ports with a

bypass valve, the bypass valve allows a one-way flow of air from the expiratory port

to the inspiratory port with a pressure differential applied thereto. Preferably, the atmospheric air aspirator further comprises:

a first end plate, where the inspiratory port opens to;

a collapsible plicate tube;

a second end rigid plate, with the collapsible tube accommodated between

the first end plate and the second end rigid plate;

an inspiratory port nozzle located between the inspiratory valve and the first

end plate, where the inspiratory port opens to the atmospheric air;

a first tube, attached to the inspiratory port nozzle;

an end plate nozzle, from which the collapsible tube opens to the atmospheric

air;

a second tube, attached to the end plate nozzle; and

a protuberance, attached on the first end plate and pointing at the end plate

nozzle to close the opening of the collapsible plicate tube while collapsed.

According to another aspect of the invention the atmospheric air aspirator further

comprises a spring to recoil the collapsible plicate tube.

Preferably, the expiratory gas reservoir further comprises:

an alveolar gas reservoir, connecting the expiratory port and the expiratory

gas reservoir, around one end of which the expiratory gas reservoir being sealed, the

alveolar gas reservoir has the other end extending into the expiratory the gas

reservoir; and an exhaust tubing, from which the gas within the expiratory gas reservoir

exhausts.

According to another aspect of the invention the alveolar gas reservoir has a

diameter larger than that of the exhaust tubing.

Preferably, the expiratory gas reservoir has a capacity in excess of a volume of a

user's breath.

comprising:

a breathing port, through which a subject exhales and inhales;

a bifurcated conduit adjacent and connected to the breathing port, including a

first conduit branch and a second conduit branch, the first conduit branch further

including:

an atmospheric air inlet; and

an inspiratory check valve, located between the breathing port and the

atmospheric air inlet, wherein the inspiratory and expiratory check valves are both

one-way passage valves, and the second conduit branch further including an

expiratory check valve;

an atmospheric air aspirator connected to the first conduit branch, having a

collapsible container formed to recoil to an open position; a flexible expiratory gas reservoir, having an entrance tubing through which

the flexible expiratory gas reservoir is connected to the second conduit, and an exit

tubing open to the atmospheric air; and

a bypass conduit communicating between the first and the second conduit

branches, having a one-way check valve therein.

According to another aspect of the invention there is provided a method of

obtaining and sustaining substantially constant PCO2 in a subject using a re¬

breathing apparatus having a one way inspiratory port in communication with an

automatic air aspirator to provide fresh gas to the inspiratory port, and a one way

expiratory port, said ports being interconnected by a one way valve responding to

predetermined sub-atmospheric pressure to draw expired gas into the inspiratory

port,

the method comprising:

providing fresh gas to the inspiratory port to a patient at a controlled rate

when the patient inhales;

allowing the subject to exhale,

accumulating the gas exhaled by the patient in an expiratory gas reservoir

connected to the expiratory port, through which the patient exhales; said expiratory

gas reservoir providing for alveolar gas to be preferentially rebreathed in preference

to dead space gas when the predetermined sub-atmospheric pressure is realized;

allowing the gas exhaled by the patient and sorted in the expiratory gas

reservoir to flow into the inspiratory port to mix with fresh gas when the minute ventilation of the patient exceeds the fresh gas flow provided to the inspiratory side

of the apparatus;

wherein said method establishes a substantially constant PCO2 level for the

subject independent of minute ventilation.

comprising:

a breathing port through which a subject inhales and exhales;

a one way inspiratory limb, communicating with the breathing port, the

inspiratory limb also in communication with an atmospheric air aspirator to provide

fresh gas to the inspiratory port at a controlled rate,

a one way expiratory limb, communicating with the breathing port, the

expiratory port also in communication with an expiratory gas reservoir to store gas

exhaled by the subject and to provide for alveolar gas to be preferentially rebreathed

in preference to dead space gas; and

the inspiratory and expiratory ports being interconnected by a one way

bypass valve, the bypass valve allowing a one-way flow of gas from the expiratory

gas reservoir to the inspiratory port when a predetermined pressure differential is

applied thereto,

wherein the PC0₂ level of the subject will remain substantially constant

independent of the extent of ventilation of the subject.

Preferably, the atmospheric air aspirator further comprises: a first end plate, where the inspiratory port opens to;

a collapsible plicate tube;

a second end rigid plate, with the collapsible tube accommodated between

the first end plate and the second end rigid plate;

an inspiratory port nozzle located between the inspiratory valve and the first

end plate, where the inspiratory port opens to the atmospheric air;

a first tube, attached to the inspiratory port nozzle;

an end plate nozzle, from which the collapsible tube opens to the atmospheric

air;

a second tube, attached to the end plate nozzle; and

a protuberance, attached on the first end plate and pointing at the end plate

nozzle to close the opening of the collapsible plicate tube while collapsed.

Preferably, the atmospheric air aspirator further comprises a spring to recoil the

collapsible plicate tube.

Preferably, the expiratory gas reservoir further comprises:

an alveolar gas chamber, connecting and sealing the expiratory port and the

expiratory gas reservoir proximate an inlet thereto, and around one end of which

the expiratory gas reservoir is sealed and extends into the expiratory the gas

reservoir; and

an exhaust tube, from which the gas within the expiratory gas reservoir

exhausts. According to another aspect of the invention the alveolar gas reservoir has a

diameter larger than that of the exhaust tubing.

According to another aspect of the invention the expiratory gas reservoir has a

capacity in excess of the volume of a subject's breath.

According to another aspect of the invention there is provided a method of an

isocapnia circuit, comprising:

a breathing port, through which a subject exhales and inhales;

a bifurcated conduit adjacent and connected to the breathing port, including a

including:

a fresh gas inlet; and

a one way inspiratory check valve, located between the breathing port and

the fresh gas inlet,

the second conduit branch further including a one way expiratory check

valve located between the breathing port and an exhaust outlet;

an atmospheric air aspirator connected to the first conduit branch, having a

collapsible container formed to recoil to an open position;

a flexible expiratory gas reservoir, having an entrance tubing through which

tubing open to the atmospheric air; and a bypass conduit communicating between the first and the second conduit

branches, having a one-way check valve therein, and responding to a predetermined

pressure, to draw expired gas into the first branch to maintain a substantially

constant PC0₂ level in the subject independent of minute ventilation.

Preferably, said atmospheric air aspirator may further comprise a second port for

fresh gas entry. This may prove useful for other uses of the circuit described herein,

such as high altitude applications and the like.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 illustrates schematically the nature of the breathing circuit not dependent on an external source of fresh gas flow and components enabling the PCO2 to remain constant despite increase in minute ventilation.

Figures 2 and 3 are charts of our data resulting from utilizing the method and circuit of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to the figures the patient breathes through one port of a Y-piece (1). The other 2 arms of the Y-piece contain 1-way valves. The inspiratory limb of the Y-piece contains a one-way valve, the inspiratory valve (2) which directs gas to flow towards the patient when the patient makes an inspiratory effort but during exhalation acts as a check valve preventing flow in the opposite direction. The other limb of the Y- piece, the expiratory limb, contains a one-way valve, the expiratory valve (3), positioned such that it allows gas to exit the Y-piece when the patient exhales but acts as a check valve preventing flow towards the patient when the patient inhales. Immediately distal to the expiratory limb of the Y-piece is attached large bore tubing termed the 'alveolar gas reservoir' (4), contained in a pliable bag of about 3 L in volume whose proximal end is sealed around the proximal end of the alveolar gas reservoir (4) said bag termed 'expiratory reservoir bag' (5). The expiratory reservoir bag (5) contains a second length of tubing termed 'exhaust tubing' (6) with a smaller diameter than the alveolar gas reservoir preferably at its distal end where expired gas exits to atmosphere (7) and is situated such that most of the tubing is contained within said bag (5) and with said bag sealed to the circumference of the tube at its distal end. The alveolar reservoir tube (4) is preferably about 35 mm in diameter, and its length is such that the total volume of the tubing is about or greater than 0.3 L when it is being used for an average (70 Kg) adult. The expiratory gas reservoir bag (5) has preferably a capacity of about 3 L. The exhaust tubing (6) has a diameter of preferably less than 15 mm at its distal end.

The inspiratory port opens into a cylindrical container composed of a rigid proximal end plate (8), a collapsible plicated tube (9) extending distally from the circumference of the proximal plate (8) and a rigid plate sealing the distal end of the collapsible plicated tube (10). When not in use, the tube is kept open by the force of gravity on the distal plate (10) and/or by the force of a spring (11) and/or by intrinsic recoil of the plicated tubing. The inspiratory port is open to atmosphere by means of a nozzle (12) to which a length of tubing (13) is attached. The distal end plate is open to a nozzle (15) to which a length of tubing (16) is attached. The proximal end plate contains a protuberance (16) pointing into the tube that is aligned with the internal opening of the distal end plate nozzle (14). The combined proximal end plate (8), plicated tubing (9), distal end plate (10) spring (11), inspiratory port nozzle (12), tubing attached to inspiratory port nozzle (13), distal end plate nozzle (14), tubing attached to distal end plate nozzle (15), proximal end plate protuberance (16) are in aggregate referred to as the 'atmospheric air aspirator' (AAA). A bypass conduit (17) connects the expiratory limb and the inspiratory limb. The opening of the conduit to the expiratory limb is preferably as close as possible to the expiratory one-way valve. This conduit contains a one-way valve (18) allowing flow from the expiratory to the inspiratory limb. The conduit's one-way valve requires an opening pressure differential across the valve slightly greater than the pressure difference between the inspiratory limb pressure and atmospheric pressure that is sufficient to collapse the plicated tube. In this way, during inspiration, atmospheric air contained in the atmospheric air aspirator and the air being continuously aspirated into the inspiratory limb is preferentially drawn from the inspiratory manifold.

Circuit Function

Assuming initially a version of the circuit without the spring (11), nozzle on the distal end plate (14), or internally directed protuberance (16). When the subject begins to breathe, each inspiration is drawn initially from the atmospheric air aspirator, collapsing the plicated tubing (9) and approximating the distal end plate (10) to the proximal end plate (8). As long as the tubing is partially collapsed, there is a constant sub-atmospheric pressure in the inspiratory limb of the circuit. Said sub-atmospheric pressure creates a pressure gradient drawing atmospheric air into the inspiratory limb of the circuit through the nozzle (12) and tubing (13). When the subject's minute ventilation is equal to or less than the intended flow of atmospheric air into the aspirator, only atmospheric air is breathed. During exhalation atmospheric air accumulates in the atmospheric air aspirator. During inhalation inspired gas consists of the contents of the atmospheric air aspirator and the atmospheric air flowing into the inspiratory limb through the nozzle. When minute ventilation exceeds the net flow of atmospheric air into the circuit, on each breath, air is breathed until the atmospheric air aspirator is collapsed. Additional inspiratory efforts result in an additional decrease in gas pressure on the inspiratory side of the circuit. When this pressure differential across the bypass conduit's valve exceeds its opening pressure, the one-way valve opens and exhaled gas is drawn back from the expired gas reservoir into the inspiratory limb of the Y-piece and hence to the patient. To the extent that the opening pressure of the bypass valve is close to the pressure generated by the recoil of the atmospheric air aspirator, there will be little change in the flow of atmospheric air into the circuit during inspiration after the atmospheric air aspirator has collapsed. The last gas to be exhaled during the previous breath, termed 'alveolar gas' is retained in the alveolar gas reservoir (4) and is the first gas to be drawn back into the inspiratory limb of the circuit and inhaled (rebreathed) by the subject. After several breaths, the rest of the gas in the expiratory gas reservoir (5) contains mixed expired gas. The mixed expired gas from the expired gas reservoir replaces the gas drawn from the alveolar gas reservoir and provides the balance of the inspired volume required to meet the inspiratory effort of the patient. The greater restriction in the diameter of the second tube (6) than in the alveolar gas reservoir (4) results in the gas being drawn into the alveolar gas reservoir being displaced by the collapse of the expired gas reservoir bag in preference to drawing air from the ambient atmosphere. The second tube in the expiratory bag (6) provides a rout for exhaust of expired gas and acts as a reservoir for that volume of atmospheric air that diffuses into said expiratory gas reservoir bag through the distal opening, tending to keep such atmospheric air separate from the mixed expired gas contained in the expired gas reservoir.

During exhalation and all of inhalation until the collapse of the atmospheric gas aspirator, the flow of atmospheric air into the circuit will remain constant. However, after the atmospheric air aspirator collapses the pressure gradient will increase. The effect of the increase in total flow will depend on the difference between the opening pressure of the bypass valve (18) and the recoil pressure of the atmospheric air aspirator times the fraction of the respiratory cycle when the atmospheric air aspirator is collapsed. If the fraction of the respiratory cycle when the atmospheric air aspirator is collapsed is great, as when there is a very great excess minute ventilation above the rate of atmospheric air aspiration, the atmospheric air aspirator can be modified adding a second port for air entry at, for example, the distal end plate (14) such that the total flow from the two ports provides the desired total flow of air into the circuit under the recoil pressure of the atmospheric air aspirator. When the atmospheric air aspirator collapses on inspiration the second port (14) is occluded by the protuberance (16), the remaining port (12) providing a greater resistance to air flow to offset the greater pressure gradient being that gradient required to open the bypass valve (18).

The embodiment described above assumes that the force of gravity acting on the distal plate provides the recoil pressure to open the atmospheric air aspirator. The disadvantage to this configuration is that the distal end plate must be fairly heavy to generate the sub-atmospheric pressure. This may be too heavy to be supported by attachment to a face mask strapped to the face. Furthermore movement such as walking or running or spasmodic inhalation will cause variations in the pressure inside the atmospheric air aspirator and hence variation in flow of air into the atmospheric air aspirator. In such cases it is better to minimize the mass of the distal endplate and use a different type of motive force to provide recoil symbolized by the spring (11).

The basic approach of preventing a decrease in PCO2 with increased ventilation is similar as that taught in prior Application No. W098/41266. In brief, only breathing the fresh gas contributes to alveolar ventilation (NA) which establishes the gradient for C0₂ elimination. All gas breathed in excess of the fresh gas entering the circuit, or the fresh gas flow, is rebreathed gas. Fisher (W098/41266) has in his prior application taught that the closer the partial pressure of C0₂ in the inhaled gas to that of mixed venous blood (PVCO2), the less the effect on C0₂ elimination. Fisher (W098/41266) expressed the relationship of alveolar ventilation, minute ventilation (N) and PCO2 of rebreathed gas as follows:

NA = FGF + (N - FGF) (PvC0₂ - PCO2 of exhaled gas)/PvC0₂

(Where FGF stands for the fresh gas flow, and other terms as described previously. With respect to this circuit, the fresh gas flow is equivalent to the rate of atmospheric air aspirated into the atmospheric air aspirator.) It is clear from this equation that as the PCO2 of the exhaled gas approaches that of the mixed venous blood, the alveolar ventilation is determined only by the fresh gas flow and not the minute ventilation.

As one exhales, the first gas to exit the mouth comes from the trachea where no gas exchange has occurred. The PCO2 of this gas is identical to that of the inhaled gas and is termed 'dead space gas'. The last gas to exit the mouth originates from the alveoli and has had the most time to equilibrate with mixed venous blood, has a PCO2 closest to that of mixed venous blood and is termed 'alveolar gas'. Gas exhaled between these 2 periods has a PCO2 intermediate between the two concentrations. The equation cited above explains why rebreathing alveolar gas would be the most effective in maintaining the PCO2 at a constant level when minute ventilation increases.

Accordingly, in the present circuit,

1. All of the fresh gas, in the form or atmospheric air, is inhaled by the subject and contributes to alveolar ventilation when minute ventilation is equal to or exceeds the rate of atmospheric air aspirated into the AAA. 2. The 'alveolar gas' is preferentially rebreathed when minute ventilation exceeds the fresh gas flow.

The advantages of the present circuit and method in terms of its operation and portability are clearly evident from our data as charted in Figures 2 and 3 wherein levels of minute ventilation, expired gas flow and airway PCO2 are compared.

Preferably the circuit as described above is installed in a case to render it fully portable. The case may include the appropriate number of capped ports to allow proper set up and use of the circuit.

'.I While the foregoing provides a detailed description of a preferred embodiment of the invention, it is to be understood that this description is illustrative only of the principles of the invention and not limitative.

Furthermore, as many changes can be made to the invention without departing from the scope of the invention; it is intended that all material contained herein be interpreted as illustrative of the invention and not in a limiting sense.

Claims

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

1. A method of providing constant PCO2 to a patient wearing a breathing port

having an inspiratory side and an expiratory side, the method comprising:

aspirating atmospheric air from the inspiratory side to a patient when the

patient inhales through the inspiratory side;

accumulating the gas exhaled by the patient in an expiratory gas reservoir

connected to the expiratory side, through which the patient exhales; and

allowing the gas exhaled by the patient and stored in the expiratory gas

inspiratory side.

2. An isocapnia circuit, comprising:

a breathing port, through which a subject inhales and exhales;

an inspiratory port, communicating to the breathing port with an inspiratory

aspirator to aspirate the atmospheric air therein;

an expiratory port, communicating to the breathing port with an expiratory

prevents air flowing to the breathing port, the expiratory port having an expiratory gas reservoir to store gas exhaled by the subject flowing across the expiratory valve;

and

a bypass conduit, communicating the inspiratory and expiratory ports with a

to the inspiratory port with a pressure differential applied thereto.

3. The isocapnia circuit as claimed in claim 2, wherein the atmospheric air

aspirator further comprises:

a first end plate, where the inspiratory port opens to;

a collapsible plicate tube;

a second end rigid plate, with the collapsible tube accommodated between the

first end plate and the second end rigid plate;

an inspiratory port nozzle located between the inspiratory valve and the first

end plate, where the inspiratory port opens to the atmospheric air;

a first tube, attached to the inspiratory port nozzle;

an end plate nozzle, from which the collapsible tube opens to the atmospheric

air;

a second tube, attached to the end plate nozzle; and

a protuberance, attached on the first end plate and pointing at the end plate

nozzle to close the opening of the collapsible plicate tube while collapsed.

4. The isocapnia circuit as claimed in claim 4, wherein the atmospheric air

aspirator further comprises a spring to recoil the collapsible plicate tube.

5. The isocapnia circuit as claimed in claim 2, wherein the expiratory gas

reservoir further comprises:

an alveolar gas reservoir, connecting the expiratory port and the expiratory

alveolar gas reservoir has the other end extending into the expiratory the gas

reservoir; and

an exhaust tubing, from which the gas within the expiratory gas reservoir

exhausts.

6. The isocapnia circuit as claimed in claim 6, wherein the alveolar gas reservoir

has a diameter larger than that of the exhaust tubing.

7. The isocapnia circuit as claimed in claim 2, wherein the expiratory gas

reservoir has a capacity in excess of a volume of a user's breath.

8. An isocapnia circuit, comprising:

a breathing port, through which a subject exhales and inhales;

a bifurcated conduit adjacent and connected to the breathing port, including a

including:

an atmospheric air inlet; and an inspiratory check valve, located between the breathing port and the

one-way passage valves, and the second conduit branch further including an

expiratory check valve;

an atmospheric air aspirator connected to the first conduit branch, having a

collapsible container formed to recoil to an open position;

a flexible expiratory gas reservoir, having an entrance tubing through which

tubing open to the atmospheric air; and

a bypass conduit communicating between the first and the second conduit

branches, having a one-way check valve therein.

9. A method of obtaining and sustaining substantially constant PCO2 in a subject

using a re-breathing apparatus having a one way inspiratory port in communication

with an automatic air aspirator to provide fresh gas to the inspiratory port, and a one

way expiratory port, said ports being interconnected by a one way valve responding

to predetermined sub-atmospheric pressure to draw expired gas into the inspiratory

port,

the method comprising:

providing fresh gas to the inspiratory port to a patient at a controlled rate

when the patient inhales;

allowing the subject to exhale, accumulating the gas exhaled by the patient in an expiratory gas reservoir

to dead space gas when the predetermined sub-atmospheric pressure is realized;

allowing the gas exhaled by the patient and sorted in the expiratory gas

reservoir to flow into the inspiratory port to mix with fresh gas when the minute

ventilation of the patient exceeds the fresh gas flow provided to the inspiratory side

of the apparatus;

wherein said method establishes a substantially constant PCO2 level for the

subject independent of minute ventilation.

10. An isocapnia circuit, comprising:

a breathing port through which a subject inhales and exhales;

a one way inspiratory limb, communicating with the breathing port, the

fresh gas to the inspiratory port at a controlled rate,

a one way expiratory limb, communicating with the breathing port, the

in preference to dead space gas; and

the inspiratory and expiratory ports being interconnected by a one way

bypass valve, the bypass valve allowing a one-way flow of gas from the expiratory gas reservoir to the inspiratory port when a predetermined pressure differential is

applied thereto,

wherein the PCO2 level of the subject will remain substantially constant

independent of the extent of ventilation of the subject.

11. The isocapnia circuit as claimed in claim 10, wherein the atmospheric air

aspirator further comprises:

a first end plate, where the inspiratory port opens to;

a collapsible plicate tube;

a second end rigid plate, with the collapsible tube accommodated between the

first end plate and the second end rigid plate;

an inspiratory port nozzle located between the inspiratory valve and the first

end plate, where the inspiratory port opens to the atmospheric air;

a first tube, attached to the inspiratory port nozzle;

an end plate nozzle, from which the collapsible tube opens to the atmospheric

air;

a second tube, attached to the end plate nozzle; and

a protuberance, attached on the first end plate and pointing at the end plate

nozzle to close the opening of the collapsible plicate tube while collapsed.

12. The isocapnia circuit as claimed in claim 11, wherein the atmospheric air

aspirator further comprises a spring to recoil the collapsible plicate tube.

13. The isocapnia circuit as claimed in claim 10, wherein the expiratory gas

reservoir further comprises:

an alveolar gas chamber, connecting and sealing the expiratory port and the

expiratory gas reservoir proximate an inlet thereto, and around one end of which the

expiratory gas reservoir is sealed and extends into the expiratory the gas reservoir;

and

an exhaust tube, from which the gas within the expiratory gas reservoir

exhausts.

14. The isocapnia circuit as claimed in claim 13, wherein the alveolar gas

reservoir has a diameter larger than that of the exhaust tubing.

15. The isocapnia circuit as claimed in claim 11, wherein the expiratory gas

reservoir has a capacity in excess of the volume of a subject's breath.

16. An isocapnia circuit, comprising:

a breathing port, through which a subject exhales and inhales;

a bifurcated conduit adjacent and connected to the breathing port, including a

including:

a fresh gas inlet; and

a one way inspiratory check valve, located between the breathing port and the

fresh gas inlet, the second conduit branch further including a one way expiratory check valve

located between the breathing port and an exhaust outlet;

an atmospheric air aspirator connected to the first conduit branch, having a

collapsible container formed to recoil to an open position;

a flexible expiratory gas reservoir, having an entrance tubing through which

tubing open to the atmospheric air; and

a bypass conduit communicating between the first and the second conduit

pressure, to draw expired gas into the first branch to maintain a substantially

constant PCO2 level in the subject independent of minute ventilation.

17. The circuit of any previous claim wherein said atmospheric air aspirator

further comprises a second port for fresh gas entry.

18. A method of enhancing the results of a diagnostic procedure or medical treatment comprising the steps of:

providing a circuit that does not require a source of forced gas flow which is

capable of organizing exhaled gas so as to provide to the patient preferential

rebreathing of alveolar gas in preference to dead space gas, (for example the circuit

described above) when the patient is ventilating at a rate greater than the rate of

atmospheric air aspirated, and when inducing hypercapnia is desired, by decreasing

the rate of aspirated atmospheric air and passively provide a corresponding increase in rebreathed gas so as to prevent the PCO2 level of arterial blood from dropping

despite increases in minute ventilation, continuing inducing hypercapnia until such

time as the diagnostic or medical therapeutic procedure is completed, wherein the

results of said diagnostic or medical procedure are enhanced by carrying out the

method in relation to the results of the procedure had the method not been carried

out.

19. A method of treating or assisting a patient, preferably human, during a traumatic event characterized by hyperventilation comprising the steps of:

providing a circuit that does not require a source of forced gas flow which

alveolar ventilation is equal to the rate of atmospheric air aspirated and increases in

alveolar ventilation with increases in minute ventilation is prevented by a circuit (for

example the preferred circuit described above) which is capable of organizing

exhaled gas so as to provide to the patient preferential rebreathing alveolar gas in

preference to dead space gas following ventilating the patient at a rate of normal

minute ventilation, preferably approximately 5L per minute, and when desired

inducing hypercapnea so as to increase arterial PCO2 and prevent the PCO2 level of

arterial blood from subsequently dropping below that achieved as a result of

decreasing the fresh gas flow, continuing maintaining normocapnia despite the

ventilation at an increased rate until such time as the traumatic event and

concomitant hyperventilation is completed, wherein the effects of hyperventilation

experienced during the traumatic event are minimized for example the mother

during labour becoming light headed or the baby during the delivery also being effected with the oxygen delivery to its brain being decreased as a result of

contraction of the blood vessels in the placenta and fetal brain.

20. The method of claim 18 utilizing the circuit of any previous claim.

21. The method of claim 19 utilizing the circuit of any previous claim.