MEASUREMENT OF THE CARDIAC OUTPUT 



573 



FIG. 15. Records of cyclic changes in the rate of perfusion of blood through the lungs of normal 

 man at rest and during exercise. Data were derised from the pressure changes resulting from the 

 absorption of nitrous oxide within a body plethysmograph. [From Lee & DuBois (88).] 



this possibility is the ingenious maneuver of Lee & 

 DuBois (88), working in Comroe's laboratory, en- 

 closed the subject in a body plethysmograph with the 

 idea of measuring the pressure changes resulting 

 from the .solution of nitrous oxide in the pulmonary 

 capillary stream. The subject at first was asked to hold 

 a breath of air inspired from the plethysmograph in 

 his lungs with open glottis. The pressure changed 

 very slowly due to final temperature drift and the 

 respiratory quotient. The subject was then asked to 

 inspire 80 per cent nitrous oxide from a bag inside of 

 and in temperature equilibrium with the plethysmo- 

 graph. The pressure in the plethysmograph began sud- 

 denly to decrease due to the reduction of gas volume 

 from solution of N2O2 in the pulmonary capillary 

 stream (see fig. 15). This reduction in pressure oc- 

 curred in pulses synchronous with the surges in pulmo- 

 nary capillary flow as measured by Cournand (16) 

 from the pressure differences between the pulmonary 

 "capillary" (wedge) pressure and the pulmonary ar- 

 terial pressure. These pulsations and the minute vol- 

 ume flow calculated from their mean height would in- 

 increase with exercise and would give resting values 

 within normal limits by accepted methods. 



A modification of this method has been suggested 

 by Wasserman & Comroe (138). They require the 



subject to maintain the thorax at a constant size 

 controlled by a stethogram with the glottis and mouth 

 open to a sensitive volume recorder. Control and 

 nitrous o.xide recordings were made as described above 

 and reproducible records of the cardiac output made 

 which were within physiologically normal limits. 

 Prompt improvement in the ability of most subjects 

 to hold the chest steady was reported. 



INJECTION .^ND INFUSION METHODS 



This is another variety of the dilution method 

 sharing that classification with the direct and indirect 

 Fick method and with the various respiratory foreign 

 gas methods. There are many sorts of information 

 that may be had from the results of the injection 

 method besides the measurement of the cardiac out- 

 put. Whereas this chapter is confined in the main to 

 discussion of the cardiac output, attention will be 

 called here to various "extra dividends" that may be 

 had from the use of the method. Most of these will be 

 discussed in other chapters of this section of the Hand- 

 book. A detailed and clear review of the literature has 

 recently been published (24). 



The injection method or indicator dilution method 



