We studied 37 patients with severe sepsis and systemic hypoperfusion to assess changes in PvCO2. Before fluid administration, the cardiac index (CI) was 2.64 +/- 0.14 L/min.m2. The PvCO2 was 38 +/- 1 torr and mixed venous pH was 7.32 +/- 0.02. The venous-arterial CO2 tension gradient (P[v-a]CO2) was 6 +/- 1 torr. After fluid administration, the CI increased to 3.45 +/- 0.14 L/min.m2 (p less than .001) and the P(v-a)CO2 decreased to 5 +/- 1 torr. The correlation between the change in CI and the change in P(v-a)CO2 was r = .42, p less than .01. P(v-a)CO2 was elevated in 19 (51%) patients before fluid administration (P[v-a]CO2 greater than 6 torr) (hypercarbic group). The P(v-a)CO2 gradient in this group was 9 +/- 1 compared with 4 +/- 1 torr in 18 patients with a normal P(v-a)CO2 gradient (p less than .001) (normocarbic group). PvCO2 was 41 +/- 2 torr in the hypercarbic group compared with 35 +/- 2 torr in the normocarbic group (p less than .05). No difference was noted in PaCO2. Venous arterial pH and HCO3- gradients were of greater magnitude in the hypercarbic group, -0.05 +/- 0.003 and 2.4 +/- 0.3 mEq/L compared to -0.02 +/- 0.004 (p less than .001) and 1.1 +/- 0.2 mEq/L (p less than .001), respectively. CI in the hypercarbic group was 2.3 +/- 0.2 compared to 3.0 +/- 0.2 L/min.m2 in the normocarbic group (p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)