A METHOD FOR DETERMINING CARDIAC OUTPUT BY THE DIRECT FICK PRINCIPLE WITHOUT GAS ANALYSIS1'2

With the reintroduction of cardiac catheteriza-tion (1) which facilitated the withdrawal of mixed venous blood for gas analysis, the Fick principle has been widely applied for the determination of cardiac output in man (2, 3). While this method provides the most accurate measurement of the cardiac output available at present and has con-tributed considerably to our knowledge of cardio-vascular dynamics in health and disease, it is open, under certain circumstances, to several potential sources of error. Chief among these is the requirement of gaseous equilibrium between the alveoli, blood and tissues. Any change in the alveolar gas content during the collection of exhaled air may produce not only significant error in the apparent oxygen consump-tion, but possibly changes in the arterial-mixed venous blood (A-MV) oxygen differences as well. Consequently, variations in respiratory function occasioned by anoxia, hyperventilation, exercise or inhalation of oxygen may lead to significant errors in the measured cardiac output. Since re-establishment of hematopulmonic equilibrium, or the steady state, following a change in respiratory function may require forty to sixty minutes (4), the immediate or early effects of altered respira-tory dynamics on cardiac output cannot be evaluated. Furthermore, the measurement of cardiac output by gas methods involves special equipment and time-consuming analyses of gases in both blood and respired air. In most laboratories, therefore, only a limited number of measurements are pos