We examined the relationship between cardiac output (CO) and mixed venous-arterial Pco 2 gradient (v-aPco 2 ) along with the other variables derived from arterial and/or mixed venous blood gases in sodium bicarbonate-treated dogs. Six dogs with low cardiac output following cardiopulmonary resuscitation were used. CO, blood gases, and hemoglobin measurements were repeated every 20-30 min after administration of sodium bicarbonate or normal saline. All measurements were performed after the confirmation of a steady state of CO 2 elimination by end-tidal CO 2 monitoring. Arteriovenous oxygen content difference (Cao 2 -Cv-O2), mixed venous oxygen saturation (Sv-O2), and v-aPco 2 were highly correlated with CO. The correlation coefficients between CaO-2Cv-O2, Sv-O2, and v-aPco 2 were r = -0.81 (P < 0.001), r = 0.70 (P < 0.001), and r = -0.77 (P < 0.001), respectively. The results suggest that, if v-aPco 2 is measured during the steady state, except for the period during the transient increase in CO 2 elimination just after the administration of sodium bicarbonate, v-aPco 2 can be used as an index of systemic perfusion even after the administration of sodium bicarbonate.