Airway-based cardiac output monitor and methods for using same

Abstract

A respiratory gas analyzer for measuring the cardiac output of a subject includes a flow meter and an oxygen sensor interconnected with one another between a mouthpiece and a source of respiratory gases which may be a controlled source or the atmosphere. An oximeter provides measurements of the oxygen saturation of the subject. A computer connected to receive the signals from the flow meter, oxygen sensor, and oximeter can then calculate the subject's cardiac output.

1 . An apparatus for determining a cardiac output of a subject, the apparatus comprising: 
 a respiratory analyzer, having a flow path through which respiratory gases pass, a flow rate sensor, and an oxygen sensor, the respiratory analyzer providing a flow signal correlating with a flow rate of respiratory gases through the flow path, and a respiratory oxygen concentration signal correlating with an oxygen concentration of respiratory gases; and    a computation unit, wherein said computation unit receives the respiratory oxygen concentration signal and the flow signal, the computation unit being operable to determine an oxygen consumption of the subject, to determine an end-tidal partial pressure of oxygen of at least one breath of the subject, and to determine the cardiac output of the subject using the oxygen consumption, the end-tidal partial pressure of oxygen, and an arterial oxygen saturation.    
     
     
         2 . The apparatus of  claim 1 , further comprising a pulse oximeter operable to provide the arterial oxygen saturation.  
     
     
         3 . The apparatus of  claim 1 , wherein the arterial oxygen saturation is a predetermined value for the subject.  
     
     
         4 . The apparatus of  claim 1 , wherein the computation unit determines the cardiac output (C.O.) of the subject using a formula  
       
         
           
             
               
                 
                   C 
                   . 
                   O 
                   . 
                 
                 = 
                 
                   
                     VO 
                     2 
                   
                   
                     
                       CaO 
                       2 
                     
                     - 
                     
                       CvO 
                       2 
                     
                   
                 
               
               , 
             
           
           
           
               
           
         
       
       wherein VO 2  represents the oxygen consumption of the subject, CaO 2  represents an oxygen content of arterial blood, and CvO 2  represents an oxygen content of venous blood.  
     
     
         5 . The apparatus of  claim 4 , wherein the computation unit is operable to determine the oxygen content of arterial blood (CaO 2 ) using an equation of the form 
       CaO 2   =A (SaO 2 )(Hgb)+ B (PaO 2 ), 
       wherein A and B represent numerical values, SaO 2  represents the arterial oxygen saturation, Hgb represents a hemoglobin concentration, and PaO 2  represents a dissolved arterial oxygen concentration.  
     
     
         6 . The apparatus of  claim 5 , wherein the computation unit uses a predetermined value for SaO 2  when calculating the oxygen content of arterial blood.  
     
     
         7 . The apparatus of  claim 5 , wherein the computation unit uses the oxygen saturation signal and a predetermined value of Hgb when calculating the oxygen content of arterial blood.  
     
     
         8 . The apparatus of  claim 4 , wherein the computation unit is operable to determine the oxygen content of venous blood (CvO 2 ) using an equation of the form 
       CvO 2   =C (SvO 2 )(Hgb)+ D (PvO 2 ), 
       wherein C and D represent numerical constants, SvO 2  represents a venous oxygen saturation, Hgb represents a hemoglobin concentration, and PvO 2  represents a dissolved venous oxygen concentration.  
     
     
         9 . The apparatus of  claim 8 , wherein the computation unit determines PvO 2  using the end-tidal partial pressure of oxygen.  
     
     
         10 . The apparatus of  claim 8 , wherein the computation unit determines SvO 2  from PvO 2  using a predetermined relationship between SvO 2  and PvO 2 .