ObjectiveTo examine the utility of single breath CO2analysis as a noninvasive measure of cardiac output.SettingAn animal laboratory in a university-affiliated medical center.DesignA prospective, animal cohort study comparing 21 parameters derived from single breath CO2analysis with cardiac output determined by an ultrasonic flow probe.SubjectsSix healthy adult sheep.MethodsThe single breath CO2analysis station consists of a mainstream capnometer, a variable orifice pneumotachometer, a signal processor, and computer software with capability for both on- and off-line data analysis. Twenty-one derived components of the CO2expirogram were evaluated as predictors of cardiac output. Cardiac output was manipulated by successive injections of a hydraulic constrictor placed around the inferior vena cava.Measurements and Main ResultsThirty-four measurements of cardiac output were available for comparison with derived variables from the CO2expirogam. Stepwise linear regression identified two variables that were most predictive of cardiac output: a) the angle between the slope lines for phases II and III of the CO2expirogram divided by the volume of CO2per breath (angle/mL CO2); and b) the slope of phase II. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2= .94, p < .0001). The bias and precision of the calculated cardiac output were .00 and .23, respectively. The mean percent difference for the cardiac output estimate derived from the single breath CO2analysis station was 0.36%.ConclusionsOur data indicate that analysis of the CO2expirogram can yield accurate information about the cardiovascular system. Specifically, two variables derived from a plot of expired CO2concentration vs. expired volume predict changes in cardiac output in healthy adult sheep with an adjusted coefficient of determination of .94. Prospective application of this technology in the setting of lung injury and rapidly changing physiology will be essential in determining the clinical usefulness of the technique.(Crit Care Med 1996; 24:1701-1705)