Abstract:
Apparatus and method for providing breathing gases to a subject employs an exchanger taking up a quantity of a given component, such as CO 2 , from expiratory breathing gases passing through the exchanger and thereafter releasing the given component in inspiratory breathing gases subsequently passing through the exchanger. The exchanger may be selectively inserted in a flow path for the breathing gases for this purpose. Or, the breathing gases may be selectively passed through and bypassed around the exchanger. The apparatus and method may be used for non-invasive determination of the functional cardiac output of a patient using the differential form of the Fick equation.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to apparatus and method for use in non-invasively determining a condition of the circulatory system of a subject. More particularly, the present invention is directed to an apparatus and method for non-invasively determining the functional cardiac output of the heart. 
   The physiological function of the heart is to circulate blood through the circulatory system to the body and lungs. For this purpose, the heart receives blood in arterial chambers during its relaxed or diastolic phase and discharges blood from its ventricle chambers during the contractile or systolic phase. The amount of blood discharged from a ventricle chamber of the heart per unit time is the cardiac output (CO). A typical cardiac output for the heart of a normal adult (at rest) is 5-6 liters per minute. 
   During circulation through the body, the blood is depleted of oxygen (O 2 ) and is enriched with carbon dioxide (CO 2 ) as a result of the metabolic activity of the body. A major purpose for blood circulation is to take venous blood that has been depleted in O 2  and enriched in CO 2  as a result of its passage through the tissues of the body and supply it to the lungs. In the alveoli of the lungs, O 2  is supplied to the blood from the breathing gases, typically air, and CO 2  is discharged into the breathing gases. The oxygenated arterial blood is then supplied to the body tissues. The gas exchange takes place in the capillaries of the lung because of the differences in concentration, or partial pressure, of O 2  and CO 2  in breathing gases, such as air, and in the venous blood. That is, the blood is low in O 2  and high in CO 2  whereas air is high in O 2  and low in CO 2 . 
   A common condition reducing the gas exchange efficiency of the lungs is the presence of shunt perfusion or blood flow in the lungs. A shunt comprises pulmonary blood flow that does not engage in gas exchange with breathing gases, due to blockage or constriction in alveolar gas passages, or for other reasons. This shunt blood flow thus bypasses normal alveoli in which gas exchange is carried out. Upon leaving the lungs, the shunt blood flow mixes with the non-shunt blood flow. The former reduces the oxygen content and increases the CO 2  content in the mixed arterial blood supplied to the body tissues. 
   It will be appreciated that only the non-shunt pulmonary blood flow through the lungs participates in the gas exchange function of the lungs and in oxygenation and CO 2  removal in the blood of the subject. The quantity of blood that participates in such pulmonary gas exchange in the lungs is termed functional cardiac output (FCO). For diagnostic or other purposes, it is frequently desirable or essential to know this quantity. 
   While shunt conditions can occur in the lungs due to blockage brought about by disease, mechanical ventilation, particularly when the respiratory muscles of a subject are relaxed as during anesthesia, can result in an increase in the pulmonary shunt. The breathing gases supplied to the lungs can be enriched with oxygen under such conditions to assist in oxygenation of the blood. However, a sufficient amount of CO 2  may not be removed from the blood when the pulmonary shunt is increased, giving rise to potentially adverse consequences to the subject. 
   The classic technique for determining the functional cardiac output of the heart is through use of the Fick equation 
             FCO   =       VCO   2         CvCO   2     -     CcCO   2                 (   1   )             
 
where,
         VCO 2  in ml/min. is the amount of CO2 released from the blood in the circulatory system of the subject,   CvCO 2  is the mixed venous blood CO 2  content, for example in ml CO 2 /ml of blood, and   CcCO 2  is the end capillary blood CO 2  content, i.e. the CO 2  content in the blood leaving the ventilated lungs.       

   The Fick equation states that, knowing the amount of CO 2  gas released from the blood in a unit of time (e.g. the rate of gas transfer as a volume/minute) and the concurrent gas transfer occurring per unit of blood (i.e. volume of gas/volume of blood), the blood flow through the lungs (i.e. FCO expressed in volume/minute) can be determined. 
   If a portion of the pulmonary blood flow of the subject is in shunt, this will decrease the amount of CO 2  released from the blood and the computation of Equation (1) provides an indication of the resulting decrease in functional cardiac output. In computing functional cardiac output using the Fick equation, the quantity VCO 2  can be determined non-invasively by subtracting the amount of CO 2  of the inhaled breathing gases, for example air, from the amount of CO 2  of the exhaled breathing gases, taking into account changes in the amount of CO 2  stored in the lungs and the deadspace in the breathing organs of the subject, such as the trachea and bronchi. The amount of CO 2  stored in the lungs can be computed from the alveolar CO 2  gas concentration, as determined from an end tidal breathing gas measurement, and the end expiratory volume V EE  of the lungs. The end capillary blood CO 2  content (CcCO 2 ) can be determined non-invasively, with a fair degree of accuracy, from a measurement of the concentration of CO 2  in the breathing gases exhaled at the end of the expiration of a tidal breathing gas volume, i.e. the end tidal (ET) CO 2  level. See also  Respiratory Physiology,  by J. F. Nunn, published 1993 by Butterworths. 
   The venous blood CO 2  content (CvCO 2 ), is often determined invasively. An alternate non-invasive approach for the determination of the CvCO 2  can be seen in U.S. Pat. No. 6,042,550 and WO 01/62148. In these approaches, exhaled CO 2  enriched breathing gases are rebreathed by the subject in subsequent inhalations. As rebreathing of the exhaled breathings gases continues, breath-by-breath, the end tidal CO 2  partial pressure (P ET CO 2 ) increases until the end capillary blood CO 2  partial pressure (P c CO 2 ) is reached. At this point, it is postulated that the end tidal CO 2  partial pressure (P ET CO 2 ), the alveolar CO 2  partial pressure (P A CO 2 ), the end capillary blood CO 2  partial pressure (P c CO 2 ), and the venous blood CO 2  partial pressure (P v CO 2 ) are all equal and that this partial pressure can be converted to the venous CO 2  content (C v CO 2 ) for use in the Fick equation. 
   The need for the determination of the venous blood CO 2  content (C v CO 2 ) is eliminated by the use of a differential form of the Fick equation which arises from the following circumstances. As a subject rebreathes exhaled breathing gases, the end tidal CO 2  partial pressure (P ET CO 2 ) and thus the alveolar CO 2  partial pressure (P A CO 2 ) and end capillary CO 2  content increases. This reduces the venous blood-alveolar CO 2  partial pressure differences and because this is the driving force for CO 2  elimination in the lungs, CO 2  elimination is also reduced. It has been shown that the ratio of the change in CO 2  elimination to the change in the end capillary blood CO 2  content is equal to the functional cardiac output. See Gedeon A., et al. Med. Biol. Eng. Comp. 18:411-418 (1980). It is set forth in equation form, as follows: 
             FCO   =           VCO   2   N     -     VCO   2   R           CcCO   2   R     -     CcCO   2   N         =       Δ   ⁢           ⁢     VCO   2         Δ   ⁢           ⁢     CcCO   2                   (   2   )             
 
   In the differential form of the Fick equation, the superscript N indicates values obtained in “normal” breathing conditions. The superscript R indicates values obtained during a short term “reduction” in the CO 2  partial pressure difference between that in the alveoli and that in the blood. This results in reduced CO 2  transfer in the lungs. 
   In using the differential form of the Fick equation, a first set of values for VCO 2  and CcCO 2  are obtained, as in the manner described above, under normal breathing conditions. These are identified by the superscript N. Thereafter, the amount of CO 2  in the breathing gases for the subject is increased. This maybe accomplished by a partial re-breathing of exhaled breathing gases. See U.S. Pat. Nos. 5,836,300 or 6,106,480 and published International Patent Appln. WO 98/26710 that employ valve mechanisms, to vary the re-breathed gas volume, for this purpose. Or, this may be accomplished by injecting CO 2  into the inhaled breathing gases as described in U.S. Pat. No. 4,608,995. Further possibilities for altering the alveolar CO 2  content include varying lung ventilation. This may be accomplished by altering the tidal volume or the respiration rate. Single breath maneuvers such as a deep breath as presented by Mitchell R R in Int J Clin Mon Comp 5:53-64 (1988), inspiratory hold as presented in WO 99/25244, or expiratory hold, may also be used for the purpose. 
   The CO 2  enrichment increases the concentration of CO 2  in the alveoli in the lungs and reduces the CO 2  partial pressure difference between that of the breathing gases in the lungs and that in the venous blood. As noted above, it is that CO 2  partial pressure difference that drives the CO 2  gas transfer from venous blood to the breathing gases in the alveoli of the lungs. The reduced CO 2  partial pressure difference reduces CO 2  gas transfer in the lung and causes an elevation of the CO 2  content in the blood downstream of the lung, i.e. in the arterial blood of the subject. In the time interval before the blood with elevated CO 2  content circulates through the body and returns to the lungs, the CO 2  content of venous blood (CvCO 2 ) entering the lungs can be taken to be the same for both the initial, normal breathing conditions (N) and the subsequent, reduced CO 2  partial pressure difference conditions labeled by the superscript R. This similitude permits the factor CvCO 2  to be dropped out of the Fick equation when expressed in the differential form as Equation 2 so that the cardiac output is determined by the ratio of the change in released CO 2  amounts (VCO 2 ) between the normal (N) and reduced (R) gas exchange conditions to the corresponding change in the end capillary blood CO 2  content (CcCO 2 ) in the normal and reduced (R) gas exchange conditions. The need to determine the venous blood CO 2  content (CvCO 2 ) from the subject is thus eliminated. 
   The foregoing approach is also advantageous with ventilated or anesthetized subjects since the alteration of the CO 2  content of the breathing gases can be effected by altering the ventilation provided to the subject. In the case of a subject anesthetized with a breathing circuit of the recirculating type, the alteration in CO 2  content may be carried out by bypassing the CO 2  absorber in the breathing circuit. The CO 2  absorber removes CO 2  from exhaled breathing gases of the subject thereby allowing the breathing gases to be recirculated to form inspiratory breathing gases for the subject. Bypassing the absorber increases the amount of CO 2  in the breathing gases that are recirculated to the subject for inspiration. 
   While the above described techniques avoid the need to invasively determine venous blood CO 2  content, other problems are created. In cases in which a subject is being provided with a fixed volume of breathing gases, an increased re-breathing volume is accompanied by a decreased volume of inspired oxygen. This may produce an undesired reduction in the oxygen content in the blood or require increased oxygen concentrations in the inspired breathing gases, following a cardiac output measurement, to restore oxygen levels in the blood to desired values. Also the tubing required for the large re-breathing volume adds to the size of associated valve systems making them big and bulky when assembled at the very crowded area near the mouth and nose of the subject. Such apparatus also adds to the overall ventilation dead-space volume between the breathing circuit for the subject and the subjects lungs. This increases the amount of ventilation required, adding to the risk of lung distension. 
   The injection of carbon dioxide into inspired breathing gas overcomes the problems of reduced oxygenation and bulky valve systems, but raises analogous problems. The CO 2  is obtained from a gas source and is typically injected using a gas tube. Such a tube is not normally present at the point of care for the subject and adding such a tube, with the accompanying high-pressure regulators and supply conduits, into the already crowded care environment is also undesirable. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an improved apparatus and method for carrying out an alteration in the CO 2  content of breathing gases inspired by a subject for purposes of non-invasively determining a circulatory system condition, e.g. the functional cardiac output, of a subject. 
   Another object of the present invention is to provide an apparatus and method that can carry out such alteration without affecting the exchange of other respiratory gases, such as oxygen, in the lung. 
   Yet another object of the present invention is to provide such apparatus that minimizes disturbance to a patient care environment and minimizes the overall increase in the breathing circuit-lung dead-space volume. 
   Briefly, in accordance with the improved apparatus and method of the present invention for altering the CO 2  content of the breathing gases, and the lung CO 2  partial pressure, the breathing gas flow is selectively guided through a CO 2  exchanger in a flow path for the breathing gases. The CO 2  exchanger selectively takes up CO 2  from the expired breathing gases of the subject and releases it to the breathing gases inhaled in a subsequent inspiration. Such an exchanger can be made of a gas porous element, for example, activated charcoal or zeolite, with pore sizes suitable for the adsorption CO 2 . 
   The CO 2  exchanger can be in a form of a moveable element, that can, with the aid of a transfer mechanism, be moved into and out a flow path of the breathing gases. Alternatively, especially during prolonged artificial ventilation of a subject in intensive care, when the dry inspiration breathing gas is often humidified and warmed with a heat and moisture exchanger (HME), the CO 2  exchanger can be connected in parallel with such an HME. Using a control valve, the breathing gas flow can be directed either through the HME, thereby forming a CO 2  exchanger bypass channel, or through the CO 2  exchanger. With such an arrangement, an increase of the dead space in the breathing gases pathway is avoided. The temporary interruption of the humidification when the breathing gas is directed through the CO 2  exchanger is easily tolerated by the subject. To keep the gas exchange conditions unchanged gases other than CO 2 , the volume of the CO 2  exchanger and associated components is advantageously equal to the volume of the by-pass channel containing the HME. 
   Breathing gas measurements obtained when the breathing gases are not passing through the exchanger and when they are passing through the exchanger may be used to determine the functional cardiac output of the subject using the differential Fick equation, in the manner described above. 
   Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     In the drawing: 
       FIG. 1  is a graph showing data obtained from the breathing gases of a subject under normal breathing conditions and under conditions of reduced gas exchange in the lungs of the subject; 
       FIG. 2  shows a breathing device using the apparatus of the present invention in order to determine functional cardiac output; 
       FIG. 3   a  is a detailed cross sectional view of the apparatus according to the present invention showing a moveable CO 2  exchanger element in a position in which the breathing gases of the subject bypass the CO 2  exchanger element; 
       FIG. 3   b  is a similar view showing the CO 2  exchanger element transferred to a position in which it is in the breathing gas flow path; 
       FIG. 4  is a graph of the breathing gas CO 2  concentration when the breathing gas is passed through the CO 2  exchanger element and when it by-passes the exchanger element; and 
       FIG. 5  is an alternative embodiment of the CO 2  exchanger apparatus of the present invention connected in parallel to a heat and moisture exchanger. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The basic principles of the analytical technique in which the apparatus and method of the present invention find use are as follows. For one or more normal (N) breaths of the subject, values are obtained for the amount of CO 2  released from the blood (VCO 2   N ) and for a quantity indicative of the end capillary blood CO 2  content, for example CcCO 2   N . One or more values for the same quantities are obtained under conditions of reduced (R) gas exchange in the lungs of the subject, to comprise VCO 2   R  and CcCO 2   R  values. This is accomplished by enriching the inspired breathing gases with CO 2 . The breathing gases are then, again, returned to the normal condition. 
   The normal (N) breathing values (N) and reduced (R) gas transfer values (R) are used as data points for a regression analysis, such as a linear regression analysis. Graphically, the data points may be plotted on a graph in which the end capillary CO 2  blood quantity values, such as CcCO 2 , are scaled along the abscissa and values for the released amount of CO 2  (VCO 2 ) are scaled along the ordinate. Such a graph is shown in FIG.  1 . For simplicity only, a single set of N and R data points are shown in  FIG. 1  as points  10  and  12 , respectively. The regression analysis produces a straight line  14  providing the best fit for the data points. In the simplified example shown in  FIG. 1 , this is a straight line intersecting the two data points. The downward slope of line  14  makes it clear that the greater the amount of CO 2  that is released in the exhalations of the subject, the less will be the end capillary blood CO 2  content of the subject. 
   It will also be appreciated that the slope of line  14  represents the functional cardiac output of the subject as expressed in the differential form of the Fick equation, Equation 2. That is, the difference between the amount of CO 2  (VCO 2 ) released under normal (N) conditions and that released under reduced (R) gas transfer conditions shown along the ordinate of  FIG. 1  represents the numerator of Equation 2. The corresponding situation exists with respect to the difference in end capillary blood CO 2  content (CcCO 2 ) shown on the abscissa of FIG.  2  and forming the denominator of Equation 2. When Equation 2 is presented graphically in the manner shown in  FIG. 1 , the functional cardiac output thus determined will have a negative sign due to the transposition of the quantities forming the denominator of the equation. 
     FIG. 2  shows a device suitable for incorporating the apparatus of the present invention and carrying out the method of the present invention. The breathing organs of the subject, including lungs  20  are supplied with breathing gases through breathing circuit  22  of conventional construction. Breathing circuit  22  includes inspiration limb  24  that supplies breathing gases to the subject and expiration limb  26  that receives exhaled gases from the subject. Inspiration limb  24  and expiration limb  26  are connected to two arms of Y-connector  28 . A third arm of Y-connector  28  is connected to patient limb  30 . Patient limb  30  supplies and receives breathing gases to/from the subject through an endotracheal tube, face mask, or other appliance (not shown). 
   The other ends of inspiration limb  24  and expiration limb  26  are connected to ventilator  32 . Ventilator  32  provides breathing gases in inspiration limb  24  and receives breathing gases from expiration limb  26 . 
   The patient limb accommodates also a flow sensor  34  connected through a signal line  36  to the monitor  38 . A flow measuring apparatus suitable for use in breathing circuit  22  is shown in U.S. Pat. No. 5,088,332 to Instrumentarium Corp. of Helsinki, Finland. A hot wire anemometer may also be used for this purpose. The flow sensor may also be placed elsewhere in the circuit than at the location shown in  FIG. 2. A  CO 2  sensor  40  is also located at the patient limb. This sensor can be of mainstream type when the signal line  42  is an electrical one and the active sensor element, typically based on infrared light absorption, is measuring the gas flow in the patient limb. Alternatively, the CO 2  sensor  40  may be of sidestream type, when the element in the patient limb is a sampling port and the line  42  is a sampling line conveying a sample gas flow to the infrared analysis within the monitor  38 . The CO 2  sensor is used to determine the end-tidal CO 2  concentration and, together with the flow signal from flow sensor  34 , is used to determine the CO 2  elimination from the lungs by integrating the product of instantaneous flow and the corresponding CO 2  concentration. 
   The output of sensors  34  and  40  are provided in signal lines  36  and  42  to monitor  38  in which the integration of flow rates to obtain volumes, filtering, or other signal processing is carried out to produce values for the sensed quantities. 
   Sensors  34  and  40  and monitor  38  measure gas flows, expired CO 2  concentrations, and end tidal CO 2  gas concentrations. Measured expired CO 2  concentrations and gas flows can be used to determine the amount of CO 2  (VCO 2 ) released from the blood. The end tidal CO 2  concentration is used to determine quantities indicative of the CO 2  content of the blood, such as CcCO 2 , as described above. 
   As shown in  FIG. 2 , the CO 2  exchanger apparatus  50  of the present invention is located in the patient limb  30 . One embodiment of the exchanger apparatus is shown in  FIGS. 3   a  and  3   b.  CO 2  exchanger apparatus  50  has housing  52  with ports  54  and  56  for connecting the CO 2  exchanger apparatus in patient limb  30 , as shown in FIG.  2 . As shown in  FIG. 2 , CO 2  exchanger apparatus  50  is connected in patient limb  30  upstream of CO 2  sensor  40 . That is, CO 2  sensor  40  is positioned between CO 2  exchanger apparatus  50  and the subject, i.e. subject&#39;s lungs  20 . Housing  52  of CO 2  exchanger apparatus  50  includes a moveable element  58  containing a substance capable of taking up a quantity of CO 2  from expiration breathing gases passing through the element and thereafter releasing the taken up quantity of CO 2  to inspired breathing gases subsequently passing through the element. For this purpose and by way of example, element  58  may comprise a porous housing  60  containing activated charcoal rods. Such a material adsorbs the CO 2  from the high CO 2  partial pressure expiration breathing gases, and due to the weakness of the bonding of the CO 2  to the absorption material, thereafter releases or relinquishes the CO 2  to the low CO 2  partial pressure inspiration breathing gases. The two-way taking up and releasing action of the CO 2  exchanger of the present invention distinguishes it from a CO 2  absorber conventionally found in recirculating breathing circuits. The function of a CO 2  absorber is to permanently remove CO 2  from the breathing gases of a patient. The activated charcoal rods may, for example, be 1 mm in diameter and 1-5 mm in length. A typical volume of material for taking up CO 2  and releasing a sufficient quantity to adequately increase the alveolar CO 2  partial pressure is 10-30 ml, depending the exact geometry of apparatus  50  and element  58 . For an apparatus suitable for pediatric patients the volume of CO 2  absorption/release material may be smaller. Other materials, such as zeolite with pore sizes suitable for the adsorption of CO 2  may also be used. 
   Element  58  may be moved from a position which is shown as an upper position in  FIG. 3   a,  to a lower position shown in  FIG. 3   b.  In the simplest embodiment of the invention, a manual actuator  64  may be employed as a transfer mechanism for this purpose. In a typical, practical embodiment of the present invention shown in  FIG. 2 , manual actuator  64  is replaced with an electrical solenoid or linear motor  66  operable by a signal in line  68  from monitor  38 . It would also be possible to provide a pneumatic actuator in apparatus  50 . 
   With element  58  in the raised, upper position shown in  FIG. 3   a,  breathing gases to/from the patient proceed directly between ports  54  and  56  of housing  52  of apparatus  50 . With element  58  in the lowered position, shown in  FIG. 3   b,  breathing gases passing between ports  54  and  56  pass through element  58  and the gas take up/release substance  62 . A seal  67  may be provided in the lower portions of housing  52  to accommodate element  58  when it is in the lowered position. 
   The method for carrying out the method of the present invention is as follows. The method is described as in an instance using air for the breathing gases. Respiration may be either spontaneous on the part of the subject or assisted by the ventilation apparatus shown in FIG.  2 . 
   Element  58  of apparatus  50  is placed in the upper position shown in  FIG. 3   a.  The subject breathes, or is ventilated, with breathing gases such as air. The normal (N) breathing action of the subject is allowed to stabilize. This may, for example, require a minimum of five breaths or a half a minute to a minute of time. The amount of CO 2  released from the blood in the lungs of the subject and the CO 2  concentration in the breathing gases are then measured, for at least one breath, or preferably for each of a plurality of breaths, of the subject using sensors  34  and  40 . Typically, the CO 2  concentration is measured as the end tidal CO 2  concentration (P ET CO 2   N ). One or more values of VCO 2  (N) are determined. In this exemplary description, the quantity used to describe the end capillary blood CO 2  condition is the CO 2  content (CcCO 2 ). The measured end tidal CO 2  concentrations are thus used to determine CcCO 2  and one or more CcCO 2  N values are obtained from the end tidal CO 2  levels for the breaths. 
   Thereafter, the CO 2  content of the breathing gases inhaled by the subject is increased to increase the CO 2  concentration in the lungs of the subject and to reduce CO 2  gas transfer, i.e. (R) breathing conditions. Using the apparatus shown in  FIG. 3   a,  this may be accomplished by lowering element  58  to place the element in the breathing gas flow path between ports  54  and  56 , as shown in  FIG. 3   b.    
   The end tidal CO 2  levels are examined as the subject breathe under these conditions.  FIG. 4  shows a read out of the CO 2  levels of the breathing gas passing CO 2  sensor  40  downstream of apparatus  50 . Prior to time  70 , element  58  in apparatus  50  is in the raised position so that the breathing action of the subject is in the normal (N) one described above. For each breath, the CO 2  level starts at essentially zero during inhalation and rises to about 5% in the exhaled breathing gases. 
   At time  70 , element  58  is lowered into the breathing gas passage between parts  54  and  56 . Element  58  commences its CO 2  taking up and releasing action. This causes the CO 2  content of the inhaled breathing gases to rise to over 1% and the CO 2  content of the exhaled breathing gases to increase to about, or over, 6%, as shown in FIG.  4 . The result is an increase in the inspired CO 2  content of about 1.0% which is considered optimal in carrying out the determination of functional cardiac output. 
   When the end tidal CO 2  levels no longer change, this indicates that the alveolar CO 2  concentration in the lungs is constant, which means that CO 2  storage in the lungs has been accommodated. The measurement of the amount of gas released from the lungs of the subject and CO 2  concentrations of the breathing gases, i.e. end tidal CO 2  concentration, is then commenced. After measurements are taken, the enrichment of CO 2  in the inhaled breathing gases may thereafter be terminated by raising the CO 2  take up/release element  58  to the upper position shown in  FIG. 3   a  at time  72 . 
   The exact amount and duration of the CO 2  enrichment will depend on numerous physical and physiological factors of the patient and on the data needed to accurately determine functional cardiac output. For a typical adult, CO 2  enrichment would last about 6 or 10 breaths. 
   The amount of end-tidal CO 2  increase is governed by somewhat conflicting considerations. The larger the increment, the larger will be the alveolar CO 2  concentration in the lungs and the end capillary blood CO 2  content (CcCO 2 ). This will place the R data point  12  farther from the abscissa of FIG.  1  and improve the accuracy of the FCO determination. On the other hand, the larger the CO 2  increase is, the less CO 2  gas exchange occurs in the lungs of the subject resulting in higher CO 2  blood levels that require a longer time to return to normal levels. The optimum of CO 2  increase a combination of these factors and need be no greater than that required to achieve the desired results. 
   The amount of CO 2  released from the blood of the subject (VCO 2   R ) is determined by subtracting the amount of CO 2  in the enriched, inhaled breathing gases from the CO 2  amount measured in the exhaled breathing gases. The measured end tidal CO 2  levels are used to determine the end capillary blood CO 2  content CcCO 2   R . These determinations are carried out from measurements obtained within the circulation period of the blood in the body of the subject following the switching of actuator  64 ,  66  to transfer the CO 2  take up/release element  58  into the breathing gas flow path. This is a period of approximately 20 seconds to one minute. In this period, the venous blood CO 2  content (CvCO 2 ) remains constant since it has not yet returned to the lungs to undergo gas exchange. 
   If desired, an administration of increased CO 2  in the inhaled breathing gases to the subject can be repeated after an appropriate interval during which CO 2  levels in the blood return to normal. 
   A regression analysis, such as a linear regression analysis, is then performed using the normal (N) values obtained from the initial breaths of the patient prior to time  70  in FIG.  40  and the reduced (R) gas transfer values obtained following the increase in the CO 2  content of the inhaled breathing gases, i.e. after time  70 . It will be appreciated that the data used to perform the regression analysis can include many normal (N) values obtained from the plurality of normal breaths taken by the patient. There will be a smaller number of R values due to the time limitation set by the blood recirculation. 
   As noted above, the slope of line  14  produced by the regression analysis is the negate of the functional cardiac output (FCO) of the patient. 
     FIG. 5  presents an alternate embodiment in which the CO 2  take the CO 2  up/release element is positioned in parallel with a heat and moisture exchanger (HME). Specifically, apparatus  501  contains CO 2  take up/release element  581 . Element  581  may be similar in construction to element  58  except that it is not moveable in the housing  502  of apparatus  501 . Housing  502  contains ports  504  and  506 . Part  504  may be connected in patient limb  30 . Part  506  is connected to valve  80 . 
   Heat and moisture exchanger  82  is connected in parallel with apparatus  501  between patient limb  30  and valve  80 . Valve  80  is also connected to patient limb  30 . Heat and moisture exchanger  82  may be of conventional construction and includes a component  84 , schematically shown in  FIG. 5 , for carrying out its intended purpose. 
   By the appropriate operation of valve  80 , the breathing gases of the subject can bypass apparatus  501  and pass through heat and moisture exchanger  82 , as prior to time  70  and subsequent to time  72 , or pass through apparatus  501 , as between timer  70  and  72 . 
   It is preferable that the volumes of the apparatus  501  and its associated flow paths and the volume of heat and moisture exchanger  82  and its associated flow paths be made essentially equal to avoid changes in the gas exchange of gases other than CO 2 . An adult heat and moisture exchanger is typically 40 ml by volume, and for pediatric patients the volume may be 15 ml. 
   It is recognized that other equivalents, alternatives, and modifications aside from those expressly stated, are possible and within the scope of the appended claims.