New Indirect Fick Procedure for the Determination of Cardiac Output

Abstract

THE indirect Fick method for determining cardiac output, which requires neither catheterization nor arterial puncture, would be the method of choice but for one serious drawback, namely, the difficulty of obtaining a reliable value for the carbon dioxide tension in (oxygenated) venous blood. It is now generally recognized that, when the breath is held, no equilibrium between alveolar and mixed venous Pco₂ is attained in the 15–20 sec. before recirculation intervenes. It has been proposed to plot the increase in pressure of alveolar carbon dioxide, Pco₂ while the breath is held, and to compute the mean value of venous Pco₂ by means of extrapolation¹. Since alveolar Pco₂ cannot be measured while the breath is held, this method requires the determination of alveolar Pco₂ at the end of a number of successive periods of holding the breath of different lengths. This results in considerable scatter of the experimental points and, thus, in an appreciable degree of uncertainty regarding the value for alveolar Pco₂.