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

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 all increased significantly (p<0.05 vs baseline levels) up to 3h following surgery. Oxygen delivery did not change. Plasma lactate levels correlated significantly with CI (r=0.47,p<0.01). V-aCO₂ fell significantly with time (p<0.01 vs baseline). There was an inverse relationship between V-aCO₂ and cardiac index and V-aCO₂ and lactate (r=–0.37,p<0.05;r=–0.3,p<0.05 respectively). We conclude that blood lactate, CI and 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.