The changes in the arterial to end-tidal carbon dioxide gradient. P(a-ET)CO2, were studied in postoperative cardiac surgery patients from the time of admission to the intensive care unit, during changing cardiorespiratory support, up to the time of tracheal extubation. Individual factors evaluated for their effects on P(a-ET)CO2 included rate of mechanical ventilation, infusion of vasoactive agents (nitroglycerin, nitroprusside, dopamine, dobutamine, and metariminol), and associated changes in haemodynamic pathophysiology (cardiac index, pulmonary artery pressure, pulmonary vascular resistance index, systemic vascular resistance index, and pulmonary capillary wedge pressure). After approval by the Clinical Investigation Committee, 59 patients, age 63 +/- 9 (41 to 75) yr, were studied and 382 individual gradient determinations made. Mean P(a-ET)CO2 was 5.47 +/- 5.21 mmHg, with the mean PaCO2, 36.51 +/- 5.89 mmHg and mean ETCO2, 31.04 +/- 6.44 mmHg. For the population as a whole, the correlation between PaCO2 and ETCO2 determined by regression analysis was maintained (R = 0.644, P less than 0.001). Comparison of the individual and population correlation coefficients by Student's t test showed no significant difference, but a normal population distribution of the gradients (P less than 0.05). None of the variables assessed could be determined to influence P(a-ET)CO2 significantly. For many of the individual patients, however, the relationship between PaCO2 and ETCO2 did not maintain a significant correlation throughout the study period. In the postoperative cardiac surgery patient population P(a-ET)CO2 follows a normal distribution and PaCO2 and ETCO2 maintain a statistically significant correlation. However, when evaluating individual patients, this relationship has wide variability.