Blood lactate and mixed venous-arterial PCO2 gradient as indices of poor peripheral perfusion following cardiopulmonary bypass surgery.

Conventional indices of tissue perfusion after surgery involving cardiopulmonary bypass (CPB) may not accurately reflect disordered cell metabolism. Venous hypercarbia leading to an increased veno-arterial difference in CO2 tensions (V-aCO2 gradient) has been shown to reflect critical reductions in systemic and pulmonary blood flow that occur during cardiorespiratory arrest and septic shock. We therefore measured plasma lactate levels and V-aCO2 gradients in 10 patients (mean age 57.2 years) following CPB and compared them with conventional indices of tissue perfusion. Plasma lactate levels, cardiac index (CI) and oxygen uptake (VO2) all increased significantly (p less than 0.05 vs baseline levels) up to 3 h following surgery. Oxygen delivery (DO2) did not change. Plasma lactate levels correlated significantly with CI (r = 0.47, p less than 0.01). V-aCO2 fell significantly with time (p less than 0.01 vs baseline). There was an inverse relationship between V-aCO2 and cardiac index and V-aCO2 and lactate (r = -0.37, p less than 0.05; r = -0.3, p less than 0.05 respectively). We conclude that blood lactate, CI and VO2 increase progressively following CPB. An increase in lactate was associated with a decrease in V-aCO2. An increase in V-aCO2 was not therefore associated with evidence of inadequate tissue perfusion as indicated by an increased blood lactate concentration.