Single breath cardiac output--enhanced sampling and analysis techniques

The single breath method for measuring cardiac output (Q) was enhanced using new computer sampling and analysis techniques. The carbon dioxide (CO2) elimination rate was measured breath by breath using computer integration of expired flow rate and CO2 concentration. The concentration of CO2 in the arterial blood was determined from the CO2 partial pressure at the end of a normal breath. The concentration of CO2 in mixed venous blood was determined with a modification of the single breath method. The variability and relative accuracy of measurement of cardiac output were evaluated using four healthy men who exercised at three work rates from 0.5 L X min-1 (rest) to 2 L X min-1 oxygen consumption (VO2). Repeatability of measurements was assessed by making repeated measurements on one subject at a steady state exercise work rate. The coefficients of variation for repeated measurements made at 0.5 L X min-1 (rest), 1.0 L X min-1 VO2, and 2.0 L X min VO2 were 18.2, 10.5% and 8.8%, respectively. Regression lines relating cardiac output and oxygen consumption from three separate studies, employing the direct Fick method, and the described method were all similar. The probability of a successful breathing maneuver was increased to nearly 100%. Success was defined as a linear regression correlation coefficient (r2), relating instantaneous exchange ratio and CO2 during the breathing maneuver, of greater than 0.9.