Methods for non-invasivelyestimating pulmonary capillary blood flow or cardiac output

Abstract

Methods for non-invasively determining the cardiac output or pulmonary capillary blood flow of a subject include monitoring the subject's respiration during two ventilatory states. Such a method may include determining parameters of at least one of the ventilatory states based on one or more characteristics of the subject. Data obtained from monitoring the subject's respiration, such as an amount of gas present in exhaled gases and respiratiory flow, may be used to estimate respiratory or blood gas parameters, such as an amount of gas exchanged between blood and gases in the subject's lungs or an indicator of a content of the gas in the subject's blood.

1 . A method for estimating the cardiac output of an individual, comprising: 
 determining a carbon dioxide elimination (V CO     2   ) of the individual in a before phase before a change in effective ventilation and in a during phase during a change in effective ventilation;    calculating a carbon dioxide elimination difference between the carbon dioxide elimination of the before phase and the carbon dioxide elimination of the during phase;    estimating a partial pressure of carbon dioxide in alveoli (P A CO 2 ) of the individual for the before phase and for the during phase based on partial pressure of end tidal carbon dioxide (PetCO 2 ) measurements of the individual from the before phase and the during phase, respectively;    converting each estimation of the partial pressure of carbon dioxide in alveoli to a carbon dioxide content (C CO     2   );    calculating a carbon dioxide content difference between the carbon dioxide content of the before phase and the carbon dioxide content of the during phase; and    dividing the carbon dioxide elimination difference by the carbon dioxide content difference to estimate cardiac output.    
   
   
       2 . The method of  claim 1 , wherein determining the carbon dioxide elimination comprises accounting for at least one of serial dead space and parallel dead space.  
   
   
       3 . The method of  claim 1 , wherein determining the carbon dioxide elimination comprises accounting for functional residual capacity.  
   
   
       4 . The method of  claim 3 , wherein accounting for functional residual capacity comprises determining a corrected carbon dioxide elimination (V CO     2 corrected   ) during at least one of the before rebreathing period and the during rebreathing period as follows:  
         V   CO     2 corrected     =V   CO     2     +FRC ×ΔPet CO     2     /P   bar ,  
     where V CO     2    is the carbon dioxide elimination that has been determined, ΔPet CO2  is the breath-to-breath change in Pet CO2 , and P bar  is barometric pressure.  
   
   
       5 . The method of  claim 1 , wherein determining the carbon dioxide elimination for at least one of the before phase and the during phase comprises determining an average carbon dioxide elimination over a period of time.  
   
   
       6 . The method of  claim 5 , wherein determining the average carbon dioxide elimination over a period of time comprises determining the average carbon dioxide elimination over a plurality of breaths.  
   
   
       7 . The method of  claim 5 , wherein determining the average carbon dioxide elimination is effected at at least about twenty-five seconds into the during phase.  
   
   
       8 . The method of  claim 1 , wherein estimating the partial pressure of carbon dioxide in alveoli includes accounting for at least one of functional residual capacity and carbon dioxide in parallel dead space.  
   
   
       9 . The method of  claim 8 , wherein accounting comprises employing the following formula:  
         P   CO2 PDS ( n )=[ FRC /( FRC+V   t )]· P   CO2 PDS ( n− 1)+( P   bar ·(([ Vi   CO2 +deadspace·(Pet CO2 ( n− 1)/ P   bar )]/ V   t ) [ V   t /( V   t   +FRC )])),  
     where (n) represents one breath, (n−1) represents a prior breath, FRC is the functional residual capacity, V t  is the tidal volume, Vi CO2  is the volume of inspired carbon dioxide, P CO     2 PDS    is the partial pressure of carbon dioxide in the parallel dead space, and P bar  is barometric pressure.  
   
   
       10 . The method of  claim 1 , wherein converting at least one estimation of the partial pressure of carbon dioxide in alveoli to a carbon dioxide content comprises predicting a stable carbon dioxide content.  
   
   
       11 . The method of  claim 10 , wherein predicting the stable carbon dioxide content comprises plotting the partial pressure of carbon dioxide in alveoli from at least one of the before rebreathing phase and the during rebreathing phase against a breath-to-breath change in a content of carbon dioxide in blood in the alveolar capillaries (ΔC A CO 2 ).  
   
   
       12 . A rebreathing method, comprising: 
 monitoring a subject's respiration during a non-rebreathing phase;    determining a selected amount of carbon dioxide to be collected based on at least one characteristic of the subject;    collecting the selected amount of carbon dioxide from gases exhaled by the subject;    introducing the selected amount of carbon dioxide into gases inhaled by the subject during a rebreathing phase; and    monitoring the subject's respiration during the rebreathing phase.    
   
   
       13 . The method of  claim 12 , wherein monitoring the subject's respiration during the non-rebreathing phase and during the rebreathing phase include monitoring an amount of at least one gas and respiratory flow.  
   
   
       14 . The method of  claim 13 , further comprising; 
 estimating an amount of the gas exchanged between the subject's blood and gases in at least one lung of the subject; and    estimating an indicator of a content of the gas in the subject's blood.    
   
   
       15 . The method of  claim 13 , wherein monitoring an amount of at least one gas comprises monitoring an amount of carbon dioxide as the subject exhales.  
   
   
       16 . The method of  claim 15 , wherein: 
 estimating the amount of the gas exchanged includes estimating carbon dioxide elimination; and    estimating the indicator of the content of the gas includes estimating carbon dioxide content of the subject's blood.    
   
   
       17 . The method of  claim 16 , further comprising: 
 dividing a difference between carbon dioxide elimination from the non-rebreathing and rebreathing phases by a difference between carbon dioxide content from the non-rebreathing and rebreathing phases to estimate pulmonary capillary blood flow or cardiac output.    
   
   
       18 . The method of  claim 12 , further comprising: 
 defining a volume within which the act of collecting is to be effected.    
   
   
       19 . A method for estimating a pulmonary capillary blood flow or cardiac output of a subject, comprising: 
 monitoring a subject's respiration during a first ventilatory phase;    determining a selected amount of at least one gas to be administered to the subject based on at least one characteristic of the subject;    administering the selected amount of the at least one gas to the subject during a second ventilatory phase; and    monitoring the subject's respiration during the rebreathing phase.    
   
   
       20 . The method of  claim 19 , wherein administering the selected amount of the at least one gas includes inducing a change in effective ventilation.