CA2340511A1 - A portable partial rebreathing circuit to set and stabilize end tidal and arterial pco2 despite varying levels of minute ventilation - Google Patents

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 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

This is a continuation in~art application of Prior Prioriy Canadian APPlication Serial Number 2,304,292 filed March 31~ 2000 from which Priori , is sought.
TITLE OF INVENTION
A portable partial rebreathing circuit to set and stabilize end tidal and arterial PCOZ 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
PhvsioloQv Venous blood returns to the heart from the muscles and organs partially depleted of oxygen (02) and a full complement of carbon dioxide (COZ). Blood from various parts of the body is mixed in the right side of the heart (resulting i n 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 COZ (PCOZ) in the mixed venous blood (PvCOz) and that in the alveolar PCO2. The COz diffuses into the alveoli from the mixed venous blood from the beginning of inspiration until an equilibrium is reached between the PvC02 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 C02 concentration between the capillaries and the alveoli; the PCOZ in this gas is the end-tidal PCOZ (PETCOZ ).
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 PCOZ (PaC02).
The arterial blood has a PCOz equal to the PCOZ at equilibrium between the capillaries and alveoli. With each breath some COZ is eliminated from the lung and fresh air containing little or no COZ (COZ concentration is assumed to be 0% is inhaled and dilutes the residual alveolar PC02, establishing a new gradient for C02 to diffuse out of the mixed venous blood into the alveoli. The flow of fresh gas i n and out of the lungs each minute, or minute ventilation (V), expressed in L/min, is that required to eliminate the COz brought to the lungs and maintain an equilibrium PCOZ (and PaCOz) of approximately 40 mmHg (in normal humans). When one produces more COZ (e.g., as a result of fever or exercise), more COZ 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 V contributes to elimination of CO2. Some V
goes to the air passages (trachea and major bronchi) and alveoli with little blood perfusing them, and thus contributes minimally to eliminating C02. This V 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 V that goes to well-perfused alveoli and participates in gas exchange is called the alveolar ventilation (VA) 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 (W098/41266), 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 COZ such that the concentration of C02 in the reserve gas of about 6%
has a partial pressure equal to the partial pressure of C02 in the mixed venous blood. At no time while using this method, will the patient re-breathe Qas 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 PCOZ 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 PCOZ 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 COZ 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% COZ concentration substantially having a PC02 equal to that of mixed venous blood or about 46 mm Hg.
Referring to our prior application (Fisher JA, Yesely A., Sasano H., Yoly~si G., Tester J.:
Improved rebreathing circuit for maintaining isoazpnia. Filal in Canada March 2000 and in the USA October 2000) filed by Joe Fisher therein 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 PCOZ substantially equal to it. As rebreathed gas contains anesthetics in anesthetized patients, the use of rebreathed gas to prevent the decrease in PCOz with increased ventilation instead of separately constituted reserve gas to prevent the decrease in PCOZ with increased ventilation, will not promote the enhancement of elimination of anesthetics.
There are applications for a circuit that maintains PCOz 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 prior application to Fisher (rebreathing) above 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 COZ 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 3 m. 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.
Our circuit would share some of the advantages set out in the prior application of Fisher et al (non rebreathing) such as, but not limited to a) raising PC02 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.

To simplify the circuit taught by Fisher (rebreathing circuit), 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 described by Fisher et al (non-rebreathing) the expired gas reservoir consists of a flexible bag 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.
Discussion of Prior Art Circuits Circuit previously described b;T Fisher et al (Non rebreathing~
Fisher (Non rebreathing) has previously described a circuit which when fresh gas flow is provided, maintains PC02 independent of minute ventilation by supplying the difference between fresh gas flow and minute ventilation from gas expired from a previous breath. Fisher's 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 PCOz 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.

Our present circuit exploits the same principle in maintaining PCOZ 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 COz in the expired gas reservoir.
It would therefore be advantageous to reap the benefits of controlling the PCOZ 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 PCOZ 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 PC02 can be prevented.
This heretofore was not contemplated in the prior art nor in the prior disclosure of Joseph Fisher et al (non-rebreathing) discussed above.
It is therefore a primary objective of this invention to provide a simplified method of controlling PC02 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 i n 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 - r7 -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
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 by Fisher et al (nonrebreathing) designed to keep the PCOz 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 i n 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 8 _ 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 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 5L 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 PCOz level of alveolar gas despite the increased minute ventilation.
When setting the fresh gas flow to maintain a desired PCOZ 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 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 by Fisher et al (rebreathing) 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 PC02 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 PCOZ and thus maintain the oxygen delivery to the brain i n 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 COz being eliminated from the blood. As this blood reaches the brain, the low PCOZ 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 PCOZ 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 PCOZ 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.
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 PCOZ and prevent the PCOZ 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.
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 PC02 and inducing changes in PCOz during MRI
2) Inducing and/or maintaining increased PCOZ

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 Oz therapy, especially in pregnant patients 4) Other applications where OZ therapy is desired and it is important to prevent the accompanying drop in PCOZ
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. W h a n 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.
The basic approach of preventing a decrease in PC02 with increased ventilation is similar as that taught by Fisher (W098/41266). In brief, only breathing the fresh gas contributes to alveolar ventilation (VA) which establishes the gradient for COZ 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 (PvC02), the less the effect on COZ elimination.
Fisher (W098/41266) expressed the relationship of alveolar ventilation, minute ventilation (V) and PCOZ of rebreathed gas as follows:
VA = FGF + (V - FGF) (PvCOz - PCOZ 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 PCOZ 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 n o gas exchange has occurred. The PCOZ 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 PC02 closest to that of mixed venous blood and is termed 'alveolar gas'.

Gas exhaled between these 2 periods has a PCOZ intermediate between the two concentrations. The equation cited above explains why rebreathing alveolar gas would be the most effective in maintaining the PCOz at a constant level when minute ventilation increases.
Accordingly, in our 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.
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 PC02 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. W
h a n 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 i n 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 o n 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 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 PC02 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.
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 (2)

1. A method of establishing a constant flow of fresh gas in the form of atmospheric air in a breathing circuit, said flow being forced as a result of breathing efforts of a patient but being independent of the extent of ventilation, said flow being delivered into the breathing circuit designed to keep the PCO2 substantially constant by providing expired gas, and preferably alveolar gas, to be inhaled when the minute ventilation exceeds the flow of fresh gas and having 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.

2. A re-breathing circuit comprising 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 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 5L 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 PCO2 level of alveolar gas despite the increased minute ventilation.