Abstract:
A re-breathing apparatus including a novel dual chamber reservoir connected to a ventilator circuit by means of a diverting adapter. The dual chamber reservoir utilizes two gas chambers which can be operated in reciprocating fashion. To accomplish re-breathing, expired gas is drawn into one chamber, while gas is ejected from the other chamber. The expired gas is then ejected into the breathing circuit as the patient inspires, while a charge of fresh gas is drawn into the other chamber. The diverting adaptor minimizes mixing of gases being drawn into and ejected from the two chambers. An advantage of the inventive method is that the total volume of gases in the system can be kept constant throughout re-breathing. The preferred embodiment of the inventive reservoir is a one-piece, blow-molded plastic, bellows-like structure which can be manufactured simply and inexpensively for one-time (disposable) use. Actuation of the system may be performed under microprocessor control.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to non-invasive approaches for determining cardiac output in patients, specifically to partial re-breathing techniques for determining cardiac output in patients, and most particularly to devices for storing and subsequently re-introducing into the ventilator circuit a volume of expired air in order to accomplish re-breathing, as well as ventilator circuits so equipped. 
     2. Statement of the Art 
     It is desirable, or even essential, to determine or monitor the cardiac output of a patient in many medical and surgical procedures. Invasive techniques well known and used in the art employ the use of catheters inserted at certain arterial points (e.g., femoral artery, jugular vein, etc.) to monitor blood temperature and pressure in order to determine cardiac output of the patient. Although capable of producing reasonably accurate results, the invasive nature of such procedures, with the attendant trauma and risk of infection, has demonstrated an unreasonably high potential for morbidity and mortality consequences. 
     Adolph Fick&#39;s measurement of cardiac output, first proposed in 1870, has served as the standard by which all other means of determining cardiac output have been evaluated since that date. Fick&#39;s well-known equation, written for CO 2 , is:        Q   =       V     CO   2         (       C     V     CO   2         -     C     A     CO   2           )                              
     where Q is cardiac output, V CO2  is the amount of CO 2  excreted by the lungs and C A     CO2    and C V     CO2    are the arterial and venous CO 2  concentrations, respectively. Notably, the CO 2  Fick Equation usually presumes an invasive method (i.e., catheterization) of determining cardiac output because the arterial and mixed venous blood must be sampled in order to determine arterial and venous CO 2  concentrations. 
     It has previously been shown, however, that non-invasive techniques may be used for determining cardiac output while still using principles embodied in the Fick Equation. That is, expired CO 2  (“pCO 2 ”) levels can be monitored to estimate arterial CO 2  concentrations and a varied form of the Fick Equation can be applied to evaluate observed changes in pCO 2  to estimate cardiac output. One use of the Fick Equation to determine cardiac output in non-invasive procedures requires the comparison of a “standard” ventilation event to a sudden change in ventilation which causes a change in expired CO 2  values and a change in excreted volume of CO 2 . One commonly practiced means of providing a sudden change in effective ventilation is to cause the ventilated patient to re-breath a specified amount of previously exhaled air. This technique has commonly been called “re-breathing.” 
     Prior methods of re-breathing have used the partial pressure of end-tidal CO 2  to approximate arterial CO 2  while the lungs act as a tonometer to measure venous CO 2 . Such an approach to re-breathing has not proven to be satisfactory for determining cardiac output because the patient is required to breath directly into and from a closed volume in order to produce the necessary effect. However, it is usually very difficult for sedated or unconscious patients to actively participate in inhaling and exhaling into a bag. The work of some researchers has demonstrated that the Fick Equation could be further modified to eliminate the need to directly calculate venous P CO     2    (P VCO     2   ) by assuming that the P VCO     2    does not change within the time period of the perturbation- an assumption that could be made by employing the partial re-breathing method. (See, Capek et al., “Noninvasive Measurement of Cardiac Output Using Partial CO 2  Rebreathing”, IEEE Transactions On Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-661.) 
     Known partial re-breathing methods are advantageous over invasive measuring techniques because they 1) are non-invasive, 2) use the accepted Fick principle of calculation, 3) are easily automated, 4) require no patient cooperation and 5) allow cardiac output to be calculated from commonly monitored clinical signals. Thus, non-invasive cardiac output techniques are rapidly gaining favor. However, most known re-breathing circuits for storing expired air and then delivering it to the patient for partial re-breathing cause an increase in the volume and resistance of the respiratory path, which may complicate the operation of the ventilator. In addition, many conventional re-breathing circuits provide only a fixed re-breathing volume, which may not be optimum, or even suitable, for patients of various sizes and respiratory capacities. Finally, conventional re-breathing circuits frequently include components which are of complex and relatively expensive construction, making them contamination-prone, difficult to sterilize, and too expensive to be used as disposable units. 
     It would be advantageous to provide a re-breathing circuit which accomplishes re-breathing with little or no change to the respiratory path volume or air flow resistance, to minimize or eliminate interference with the ventilator function and reduce the load “seen” by the patient. It would also be advantageous to provide a re-breathing circuit in which re-breathing volumes can be varied as needed. It would be desirable for the re-breathing circuit to be usable with state-of-the art ventilator circuits and monitors without modification thereto. In many cases it is desirable for a re-breathing circuit to be made up of relatively simple and inexpensive, easy to fabricate, one-use (disposable) components. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a novel re-breathing apparatus suitable for use with a mechanically ventilated patient. The re-breathing apparatus includes a dual-chamber gas reservoir, each chamber of which is connected via tubing to a diverting adapter which allows flow of air to and from a respiration circuit. The total volume of the reservoir is fixed and the two chambers operate in reciprocal fashion, with one expanding to draw in air while the other is contracting to expel air. Accordingly, the total volume of gases in the reservoir, as well as in the respiration circuit as whole, can remain unchanged during operation of the apparatus in a re-breathing mode. The invented re-breathing apparatus thus has the advantage that it is entirely “invisible” in terms of operation of the respiration circuit, because there is no change in either the air volume or resistance to flow of the circuit (a factor which is of particular concern when a ventilator is used in the breathing circuit). A method of operating the re-breathing apparatus, the re-breathing apparatus and its components, and a respiration circuit including the inventive re-breathing apparatus are encompassed by the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a ventilator circuit which incorporates the inventive re-breathing circuit; 
     FIG. 2A shows the configuration of the preferred embodiment of the dual-chamber reservoir during normal ventilation without re-breathing; 
     FIG. 2B depicts the expansion of chamber  102  and compression of chamber  101  of the dual-chamber reservoir, to store expired gases and eject fresh gases during the last expiratory breath prior to the start of re-breathing; 
     FIG. 2C shows the configuration of the dual-chamber reservoir at the end of the expiratory phase immediately prior to the start of re-breathing; 
     FIG. 2D shows the injection of expired gases stored in chamber  102  of the reservoir into the circuit during re-breathing, while fresh gas is simultaneously drawn into chamber  1 ; 
     FIG. 3A illustrates the preferred embodiment of dual-chamber reservoir in cross section; 
     FIG. 3B is an end view of the embodiment of the invention shown in FIG. 3A; 
     FIG. 4A is a cross of an alternative embodiment of the dual-chamber reservoir; 
     FIG. 4B is an end view of the embodiment of the invention shown in FIG. 4A; 
     FIG. 5 illustrates a further alternative embodiment of the dual-chamber reservoir; 
     FIG. 6 illustrates a further alternative embodiment of the dual-chamber reservoir; 
     FIG. 7 illustrates a further alternative embodiment of the dual-chamber reservoir; 
     FIG. 8A illustrates the preferred embodiment of the diverting adapter; 
     FIG. 8B is a cut-away view of the embodiment of the diverting adapter shown in FIG. 8A; 
     FIG. 8C is a longitudinal cross-sectional view of the embodiment of the diverting adapter shown in FIG. 8A; 
     FIG. 8D is a further view of the embodiment of the diverting adapter shown in FIG. 8A; 
     FIG. 9A illustrates an alternative embodiment of the diverting adapter; 
     FIG. 9B is a cut-away view of the embodiment of the diverting adapter shown in FIG. 9A; 
     FIG. 9C is an end view of the embodiment of the diverting adapter shown in FIG. 9A; 
     FIG. 10A illustrates a further alternative embodiment of the diverting adapter; 
     FIG. 10B is a cut-away view of the embodiment of the diverting adapter shown in FIG. 10A; 
     FIG. 10C is an end view of the embodiment of the diverting adapter shown in FIG. 10A; 
     FIG. 11 illustrates a further alternative embodiment of the diverting adapter; 
     FIG. 11B is a cut-away view of the embodiment of the diverting adapter shown in FIG. 11A; and 
     FIG. 11C is an end view of the embodiment of the diverting adapter shown in FIG.  11 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts the inventive re-breathing circuit  80  in relation to the respiration circuit as a whole, indicated generally at  5 . Tubular airway  20  communicates air flow to and from the lungs of a patient. Tubular airway  20  may be placed in flow communication with the trachea of the patient (not shown) by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. During normal breathing (i.e., without re-breathing), fresh gas is provided to the patient from a ventilator or from the atmosphere via inspiratory hose  30 , while expired gas is returned to the ventilator or vented to the atmosphere via expiratory hose  40 . Y-piece  50  connects inspiratory hose  30  and expiratory hose  40  with diverting adapter  60 , which is interposed between and in flow communication with tubular airway  20  and Y-piece  50 . The primary respiratory path is indicated generally by broken lines at  10 . The inventive re-breathing circuit, indicated generally at  80 , is connected to the primary respiratory path  10  via diverting adapter  60 . The inventive re-breathing circuit includes dual-chamber reservoir  100  (also referred to as reservoir  100 ), which includes a first chamber  101  and a second chamber  102  connected to diverting adapter  60  by hoses  91  and  92 , respectively. In order to accomplish re-breathing, expired gases are drawn into chamber  102  of reservoir  100 , and subsequently injected into the circuit during re-breathing. Chambers  101  and  102 , or portions thereof, are compressed and expanded in a reciprocal fashion so that as one expands by a given volume, the other is compressed by the same volume, keeping the total volume of gas within the reservoir  100 , and in the system  5  as a whole, constant. 
     The principle of operation of dual chamber reservoir  100  is described with reference to FIGS. 2A through 2D. FIGS. 2A through 2D depict a system which includes the presently preferred embodiment of the dual-chamber reservoir  100 ; however, the basic principal of operation is the same for all embodiments of the reservoir. During normal breathing when no re-breathing is desired, chamber  101  of reservoir  100  is held at its maximum volume, while chamber  102  of reservoir  100  is at its minimum volume, as shown in FIG.  2 A. Fresh air, which is present in chamber  101  when it is expanded to its maximum volume state is stored in chamber  101  during normal ventilation. No air flows into or out of either chamber of the reservoir  100  during normal ventilation. 
     As shown in FIG. 2B, during the last expiratory breath prior to the start of re-breathing, chamber  102  is caused to expand, drawing expired gases into the chamber  102  via tube  92 , while chamber  101  contracts to inject the gases which were stored therein into the circuit via tube  91 . Chamber  102  thus provides a reservoir for storing expired gas for re-breathing. Diverting adapter  60  provides for the injection of fresh gases from chamber  101  into circuit  5  at a point downstream (i.e. further from the patient) than the point at which expired gases are withdrawn into chamber  102 . Diverting adapter  60  is designed to minimize mixing of fresh and expired gases in its interior during filling and emptying of the reservoir chambers. 
     The expansion of chamber  102  (and simultaneous contraction of chamber  101 ) is completed by the end of the expiratory breath prior to the start of re-breathing, with the result that chamber  102  is “primed” to a desired volume with expired gases and prepared for the start of re-breathing to begin with the next breath, as shown in FIG. 2C, and chamber  101  is collapsed to a minimum volume commensurate with the expansion of chamber  102 . 
     Timed with the ventilator inspiratory phase, chamber  102  is compressed to inject the expired gases stored therein into the circuit for re-breathing by the patient, as shown in FIG.  2 D. Concurrently, chamber  101  is expanded to draw in fresh gases from inspiratory hose  30 . In the steps shown in FIGS. 2A-2D, the volume of air withdrawn from the circuit  5  is always the same as the volume of air injected into the circuit  5 , so no net accumulation or deficit of gases occurs in the circuit between inspiration and expiration, and operation of the re-breathing circuit is transparent to the ventilator and patient. 
     The chambers may be directly mechanically coupled as in the bellows design of FIGS. 2A through 2D,  3 , and  4 , indirectly coupled through a mechanical linkage as in the designs of FIGS. 5,  6 , and  7 , or actuated separated but coupled via control logic. Specifically, if desired, the chambers used in the designs of FIGS. 5,  6 , and  7  could be coupled via control logic rather than via the mechanical linkage shown in the figures. In the directly and indirectly mechanically coupled systems the amount of gas withdrawn from the system matches the amount of gas injected into the system at each moment. In a system with no mechanical coupling, it would be possible to modify the control logic to adjust the withdrawal and injection of gases such that the total quantities of gases withdrawn and injected from the system during a single breathing cycle were substantially equal, but that on a moment-by-moment basis, an imbalance could occur. This would lead to fluctuations in the pressure of the system, so it would be necessary to ensure that such pressure fluctuations remained within a range which would not interfere with ventilator function or patient respiration. 
     A presently preferred embodiment of dual-chamber reservoir  100  is shown in FIGS. 3A and 3B. It is a cylindrical, bellows-like structure with two flat, preferably circular end walls  305  and  306 , and two chambers  101  and  102  separated by internal wall  307 . The lengths of side wall  311  of chamber  101  and side wall  312  of chamber  102  can be varied to allow shortening or lengthening of each chamber  101  and  102  without changing the diameter of the chamber. Side wall  311  and side wall  312  are preferably accordion-pleated, but could also be helically pleated or otherwise configured to permit them to be collapsed and expanded (shortened or lengthened). A linear relationship between chamber length and volume will be obtained if shortening and lengthening occur without change in chamber cross-sectional area. However, there is no need for the volume change to be linear, in this and other embodiments of the invention. In any embodiment of the invention, the control algorithm must take into account the relationship between chamber actuation and change in chamber volume. Side walls  311  and  312  are preferably circular in cross section, but could also have another cross-sectional shape. Chamber  101  has an outlet  315  which is connected to tube  91  leading to diverting adapter  60 . Chamber  102  has an outlet  316  which is connected to tube  92  leading to diverting adapter  60 . Outlets  315  and  316  are depicted here at the ends of the chambers; however, any outlet location may be selected which does not interfere with expansion and contraction of the chambers. For example, if only a portion of the chamber wall was subject to expansion or contraction, the outlet could be placed on the chamber wall on the non-expanding and non-contracting portion. During use, end walls  305  and  306  are held at a fixed distance from each other, which distance defines the total volume of reservoir  100 . Movement of internal wall  307  toward end wall  305  results in a decrease in the volume of chamber  101  and an increase in the volume of chamber  102 , while movement of internal wall  307  toward end wall  306  results in an increase in the volume of chamber  101  and an decrease in the volume of chamber  102 . The longitudinal distance between end walls  305  and  306  can be adjusted for each patient to provide the optimal total volume for reservoir  100 . The distance moved by internal wall  307 , which may also be adjusted for each patient, determines the actual volume of gas stored in and delivered by chamber  102  during re-breathing. The bellows structure shown in FIGS. 3A and 3B can be manufactured as a one piece blow-molded component, inexpensive enough to be disposable. Various actuation mechanisms might be used for moving internal wall  307 , for example a linear stepper motor or a pneumatic, hydraulic, or solenoid-based mechanism. The practice of the invention is not limited to the particular actuation mechanism used. Actuation of internal wall  307  may be controlled by a programmed microprocessor, switches, or other means. The use of an actuator controlled by a microprocessor, in this and other embodiments of the invention, is particularly advantageous because it makes it possible for the volume of gases exchanged to be varied to tune the percent re-breathing for each patient, to take into account differences in tidal volume, physiological response, etc. Moreover, re-breathing could be under closed-loop control, if desired. 
     FIGS. 4A and 4B illustrates an alternative embodiment of the dual-chamber reservoir, which is a rigid-walled cylinder  400  which is formed by rigid side wall  413  and end walls  405  and  406 . The interior of cylinder  400  is divided into chambers  101  and  102  by movable piston wall  407 . Movable piston wall  407  forms a sliding, substantially gas-tight seal with side wall  413 , to prevent the leakage of gases between chambers  101  and  102 . This is achieved, as known in the art, by use of an O-ring or other annular seal structure around the periphery of movable piston wall  407 . Movable piston wall  407  is driven by piston rod  408 , which passes through opening  409  in end wall  406 , the bore of opening  409  also being provided with a sliding seal. Gases enter and exit chamber  101  through outlet  415 , to which is connected tube  91 . Similarly, gases enter and exit chamber  102  through outlet  416 , to which is connected tube  92 . Shaft  408  may be driven by various mechanism, for example a linear stepper motor or a pneumatic, hydraulic, or solenoid-based mechanism. The practice of the invention is not limited to the particular actuation mechanism used. The drive mechanism may be controlled by various methods known in the art, including a programmed microprocessor, switches, and so forth. 
     FIG. 5 illustrates a further alternative embodiment of the dual-chamber reservoir, in which two separate bellows structures serve as chambers  101  and  102 . The two bellows are preferably identical; in this illustration the components of each are numbered identically, distinguished by the letters “a” and “b”. The first bellows  501 , which contains inner chamber  101 , includes a fixed end wall  505   a , movable end wall  507   a , and variable length side wall  511   a . Gases enter and exit chamber  101  through outlet  515   a  in end wall  505   a , which is connected to tube  91 . The second bellows  502 , which contains inner chamber  102 , includes a fixed end wall  505   b , movable end wall  507   b , and variable length side wall  511   b . Variable length side walls  511   a  and  511   b  are preferably accordion pleated, but may also be helically pleated or otherwise configured to permit chambers  101  and  102  to be collapsed and expanded (shortened and lengthened). Gases enter and exit chamber  102  through outlet  515   b  in end wall  505   b , which is connected to tube  92 . Linkage  521   a  is attached at its first end to movable end wall  507  a and attached at its second end to a first end of arm  525 . Linkage  521   b  is attached at its first end to movable end wall  507   b  and attached at its second end to the second end of arm  525 . Arm  525  is mounted on rotatable shaft  526 ; linkages  521   a  and  521   b  are driven in reciprocating fashion by arm  525  responsive to the direction of rotation of shaft  526 . Shaft  526  is driven by, for example, a rotary two-way electric motor, or a motor or cylinder with a spring return, under switch or microprocessor control. Various actuation and control mechanisms can be used in the practice of the invention. 
     FIG. 6 illustrates a further alternative embodiment of the dual-chamber reservoir, in which two separate cylinders  601  and  602  contain as chambers  101  and  102 . The two cylinders are preferably identical; in this illustration the components of each are numbered identically, distinguished by the letters “a” and “b”. The first cylinder  601 , which contains inner chamber  101 , includes rigid side wall  611   a , end wall  605   a , and movable piston wall  607   a  driven by piston rod  621   a . Gases enter and exit chamber  101  through outlet  615   a  in end wall  605   a , which is connected to tube  91 . The second cylinder  602 , which contains inner chamber  102 , includes rigid side wall  611   b , end wall  605   b , and moving piston wall  607   b  driven by piston rod  621   b . Movable piston walls  607   a  and  607   b  form sliding, substantially gas-tight seals with rigid side walls  611   a  and  611   b , respectively, by use of an O-ring or other annular seal structure on the periphery of each piston wall. Gases enter and exit chamber  102  through outlet  615   b  in end wall  605   b , which is connected to tube  92 . Piston rod  621   a  is attached at its first end to movable end wall  607   a  and attached at its second end to a first end of arm  625 . Piston rod  621   b  is attached at its first end to movable end wall  607   b  and attached at its second end to the second end of arm  625 . Arm  625  is mounted rotatable shaft  626 ; piston rods  621   a  and  621   b  are driven in reciprocating fashion by arm  625  responsive to the direction of rotation of shaft  626 . Actuation and control of shaft  626  is as described above in connection with the embodiment of the invention shown in FIG.  5 . 
     FIG. 7 illustrates a further alternative embodiment of the dual-chamber reservoir. In this embodiment, chamber  101  is provided within variable volume chamber  731   a  which in this example of the invention is a flexible, thin-walled bag  731   a . The variable volume chamber  731   a  is contained within fixed volume chamber  711   a . The volume of variable volume chamber  731   a  (and hence chamber  101 ) is controlled by adjusting the pressure in space  735   a  between variable volume chamber  731   a  and fixed volume chamber  711   a . Similarly, chamber  102  is provided within variable volume chamber (e.g., a flexible, thin-walled bag)  731   b  which is contained within fixed volume chamber  711   b . The volume of variable volume chamber  731   b  (and hence chamber  102 ) is controlled by adjusting the pressure in space  735   b  between variable volume chamber  731   b  and fixed volume chamber  711   b . Vacuum pump or vacuum/positive pressure pump  740  is connected to space  735   a  by line  741   a , and to space  735   b  by line  741   b . For example, an increase in the volume of variable volume chamber  731   a  and decrease in the volume of variable volume chamber  731   b  is caused by generating a negative pressure (vacuum) in space  735   a  relative to the pressure in the variable volume chamber of bag  731   a , while simultaneously generating a positive pressure in space  735   b  relative to the pressure in the interior of variable volume chamber  731   b . Although flexible, thin-walled bags are used in the embodiment presented here, variable volume chambers  731   a  and  731   b  can be any structure which will expand or compress in response to a difference in external and internal pressure. As a further alternative, chambers  101  and  102  could be provided in spaces  735   a  and  735   b , respectively, while the increases and decreases in pressure which drive gases in and out of chambers  101  and  102  could be produced by varying the volumes of bags  731   a  and  731   b . Fixed volume chambers  711   a  and  711   b  may include pressure relief valves  712   a  and  712   b , respectively. The pressure relief valves are used to permit contraction of variable volume chambers  731   a  and  731   b  by permitting release of a vacuum in space  735   a  and  735   b  in the case that only a vacuum pump  740  is used. Gauge/absolute pressure sensors  742   a  and  742   b  are placed in lines  741   a  and  741   b , respectively to monitor pressures in chambers  711   a  and  711   b  to ensure proper reciprocal expansion and contraction of chambers  731   a  and  731   b . In the present exemplary embodiment of the invention, outlet  43   a  of chamber  101  (variable volume chamber  731   a ) is connected to tube  732   a . Tube  732   a  passes through opening  733   a  in the wall of fixed volume chamber  711   a . Opening  733   a  is sealed to the exterior of tube  732   a  to prevent the flow of air or gases into or out of space  735   a  at opening  733   a . Tube  732   a  is joined to tube  91  at connector  734   a . Flow sensor  744   a  is included on tube  732   a . Similarly, outlet  43   b  of variable volume chamber  731   b  may be connected to tube  732   b , which passes through opening  733   b  in the wall of fixed volume chamber  711   b . Opening  733   b  is sealed to the exterior of tube  732   b . Tube  732   b  is connected to tube  92  at connector  734   b , and a flow sensor  744   b  may be included on tube  732   b . In general, each variable volume chamber communicates with the main breathing circuit through a tubular element connected to the outlet of the variable volume chamber and passing through the wall of the fixed volume chamber with an air-tight seal between the tube and the wall of the fixed volume chamber. In the example presented here, the tubular elements are made up of the tube  732   a  and tube  91 , and tube  732   b  and tube  92 . It will be appreciated by one of ordinary skill in the art that it would be possible to use various combinations of tubes and connectors, and that the invention is not limited to the particular arrangement of tubes and connectors presented here. 
     It will be appreciated that various mechanisms can be devised for controlling the volumes of the two chambers of the different embodiments, and that actuation of the mechanism can be microprocessor-controlled if desired. In general, the volumes of the chambers are controlled by controlling the pressures in spaces  735   a  and  735   b , through generation of a vacuum or positive pressure with vacuum pump or vacuum/positive pressure pump  740  and/or release of pressure through pressure relief valves  712   a  and  712   b . Accordingly, pump  740  and pressure relief valves  712   a  and  712   b  could be microprocessor controlled, while flow sensor and Gauge/absolute pressure sensors could be used to provide feedback signals. The invention is not limited to the use of a particular actuation mechanism or control scheme. 
     A presently preferred embodiment of the diverting adapter  60  is shown in FIGS. 8A through 8D. In general, as shown in FIG. 1, diverting adapter  60  includes a main flow passage which is an integral part of primary respiratory path  10 , and two diverting passages  61  and  62 , which provide for the flow of gases between main the flow passage of the adaptor and chambers  101  and  102  of dual chamber reservoir  100 . The main flow passage of the adaptor is positioned in primary respiratory path  10 , between tubular airway  20  and Y-piece  50 , as shown in FIG.  1 . Referring now to FIGS. 8A through 8D, cylindrical element  801 , which defines main flow passage  800 , has a diameter d. First end  802  and second end  803  of cylindrical element  801  are connected to Y-piece  50  and tubular airway  20 , respectively, by, for example, an adhesive or thermal welding. Tube  91  from chamber  101  of the dual-chamber reservoir  100  is connected to first tube connector  815 , while tube  92  from chamber  102  is connected to second tube connector  816 . In this preferred embodiment of the invention, the first diverting passage runs through first tube connector  815  and is continuous with tube  91 , while the second diverting passage runs through second tube connector  816  and is continuous with tube  92 . Although it is preferred to have tubes connecting between diverting adapter  60  and the chambers of dual chamber reservoir  100 , to allow the reservoir to be placed at a variable distance from the breathing circuit, in an alternative embodiment it would be possible for diverting adapter  60  to be connected directly to the chambers of dual chamber reservoir  100 . Tube connectors  815  and  816  are preferably substantially parallel to each other and substantially perpendicular to the longitudinal axis of cylindrical element  801 , with tube connector  816  on the upstream side of tube connector  815  (i.e., on the side closer to the patient). First tube connector  815  defining first bore  815  a joins to first angled wall region  805 . First angled wall region  805  is planar and intersects cylindrical element  801  at an angle relative to the longitudinal axis of cylindrical element  801 . First angled wall region  805  also intersects a planar section  807  which is parallel to the longitudinal axis of cylindrical element  801  and substantially perpendicular to tube connectors  815  and  816 . Planar section  807  is rectangular and intersects cylindrical element  801  along its edges parallel to the longitudinal axis of cylindrical element  801 . Planar section  807  is located at a distance h from the longitudinal axis of cylindrical element  801 ; distance h is always less than d. Second tube connector  816  defining second bore  816  a joins to second angled wall region  806 , which is planar and intersects cylindrical element  801  at an angle θ relative to the longitudinal axis of cylindrical element  801 . Second angled wall region  806  also intersects planar section  807 . The lines of intersection between planar section  807  and angled wall regions  805  and  806  are perpendicular to the long axis of cylindrical element  801 . The end of tube  815  is flush with first angled wall region  805 , with the effect that the upstream edge of opening  821  of tube connector  815  extends further into primary passage  800 . Conversely, because the end of tube connector  816  is flush with angled wall region  806 , the downstream edge of opening  822  of tube connector  816  extends further into primary passage  800 . The orientations of angled wall regions  805  and  806 , and openings  821  and  822  therein, serve to minimize the mixing of fresh and expired gases as they flow into and out of chambers  101  and  102  of reservoir  100 . An advantage of the combination of the inventive reservoir system and diverting adapter is that in the event of a malfunction which causes the reservoir system to stop functioning, gas will still be able to flow from the ventilator to the patient, through the diverting adapter. In prior art systems which use a valve to divert gases to be stored for re-breathing, a malfunction of the valve could block the flow of gases between the ventilator and the patient. 
     Alternative embodiments of diverting adapter  60  are shown in FIGS. 9A-11C. In the embodiment of diverting adapter  60  shown in FIGS. 9A-9C, tube connectors  915  and  916  connect to and have bores  915   a  and  916   a  which have openings flush with the outer wall of cylindrical element  901  having first end  902  and second end  903 . The primary respiratory path  10  is through primary flow tube  940 , which is preferably co-axial with cylindrical element  901 . Primary flow tube  940  is supported by annular support element  930 , which is connected at its periphery to the interior of cylindrical element  901 , substantially perpendicular to the longitudinal axis of cylindrical element  901  and midway between tube connectors  915  and  916 . During normal breathing or ventilation, gas flows through primary flow tube  940 . During re-breathing, gas flows through the space between cylindrical element  901  and primary flow tube  940 , on either side of annular support element  930 , and then through either tube connector  915  or tube connector  916 . Annular support element  930  prevents mixing of the gases flowing through tube connectors  915  and  916 . 
     FIGS. 10A-10C illustrate a further alternative embodiment of the diverting adapter. In this embodiment, the primary passage  800  is defined by cylindrical element  1001 . Air flows to chambers  101  and  102  of reservoir  100  through diversion channels  1010  and  1011 , respectively. Inlets  1040  and  1041 , which communicate with diversion channels  1010  and  1011 , respectively, are tubular and run substantially parallel to the longitudinal axis of cylindrical element  1001 . A single connector  1008  extends from cylindrical element  1001 , substantially perpendicular to its longitudinal axis. Connector  1008  is essentially cylindrical and is split by divider  1009  to form diversion channels  1010  and  1011 . Inlet  1040 , which communicates with diversion channel  1010 , extends in the downstream direction from its junction with channel  1010 , while inlet  1041  extends upstream from its junction with channel  1011 . Tubes  91  and  92 , which connect chambers  101  and  102  of reservoir  100  to diverting adapter  60 , are connected to channels  1010  and  1011  by means of an adapter which extends from the connector, which defines half-circular channels  1010  and  1011 , to circular outlets compatible with tubes  91  and  92 . 
     FIGS. 11A-11C illustrate a further alternative embodiment of diverting adapter  60 . This embodiment is similar to that shown in FIGS. 10A-10C in that the primary passage  800  is defined by a cylindrical element  1101 . Air flow is diverted to chambers  101  and  102  of reservoir  100  by two diversion channels  1110  and  1111 , respectively. However, in place of a single connector ( 1008  in the previously described embodiment), two tubular connectors  1115  and  1116  are used to connect to tubes  91  and  92 . Tubular connectors  1115  and  1116  are joined along junction line  1119 . Tubes  91  and  92  which connect chambers  101  and  102  to diverting adapter  60  are connected to channels  1110  and  1111 , respectively, by sizing them to fit over the tips of connectors  1115  and  1116  and adhesive bonding or welding them thereto. Inlets  1140  and  1141  to diversion channels  1110  and  1111 , respectively, are tubular and run parallel to the longitudinal axis of cylindrical element  1101 , and substantially perpendicular to the diversion channels. Inlets  1140  and  1141  are separated by divider  1109 , which is contiguous with junction line  1119 . Inlet  1140 , which is in flow communication with diversion channel  1101 , extends in the downstream direction from its junction with channel  1101 , while inlet  1140  extends upstream from its junction with channel  1111 . 
     Diverting adapter  60 , as depicted in FIGS. 8A-11C, may be constructed by injection molding from rigid, sterilizable, polymeric materials, including but not limited to polycarbonate, ABS, and acrylic. The bellows, tubing, and diverting adapter can be preassembled as a unit and sterilized, and be fully disposable after use to prevent cross-patient contamination and associated hazards. Of course, the drive and control mechanisms would be non-disposable, with the drive mechanisms configured to quickly receive and release the bellows or elements thereof. 
     While the present invention has been described and illustrated in terms of certain specific embodiments, those of ordinary skill in the art will understand and appreciate that it is not so limited. Additions to, deletions from and modifications to these specific embodiments may be effected without departing from the scope of the invention as defined by the claims. Furthermore, features and elements from one specific embodiment may be likewise applied to another embodiment without departing from the scope of the invention as defined herein.