Objective: To review the pathophysiology of lactic acidosis in patients undergoing open-heart surgery, with special reference to the splanchnic circulation.
Data source: MEDLINE search of pertinent experimental and clinical research studies.
Results: Lactate is an end-product of anaerobic metabolism and is in dynamic equilibrium with its precursor, pyruvate. The ratio of serum lactate to pyruvate concentrations in arterial blood is normally < or = 10:1. In patients with lactic acidemia, measurement of serum pyruvate concentrations may yield valuable clinical information. Lactate/pyruvate ratios > 10:1 suggest that oxygen delivery (DO2) is inadequate to meet metabolic demand, whereas increases in both lactate and pyruvate values with preservation of normal lactate/pyruvate ratios suggest a defect in oxidative utilization (e.g., a fractional increase in the inactive form of the pyruvate dehydrogenase enzyme complex) despite adequate DO2. Meaningful changes in regional oxygen kinetics occur during extracorporeal circulation. Increased production of endogenous vasoconstrictors in response to nonpulsatile flow, together with a decrease in arterial oxygen content resulting from the addition of 2 L of pump prime to the patient's circulation at the beginning of cardiopulmonary bypass, decrease DO2 to the gastrointestinal tract. The effect of this reduction is mitigated, in part, by lowering core temperature and reducing tissue oxygen demand.
Conclusions: The abdominal organs tolerate reductions in DO2 when they are cold (25 degrees C), since gastric intramucosal pH (a marker of inadequate DO2), and hepatic venous lactate/pyruvate ratios and oxygen saturation during the first half of cardiopulmonary bypass are normal. As surgery nears completion and core temperature is increased, tissue oxygen demands escalate. The presence of gastric mucosal acidosis, coupled with lactic acidemia and oxygen desaturation of hepatic venous blood, suggest that delivery of oxygen to the abdominal organs at the conclusion of cardiopulmonary bypass is insufficient to meet demand. A growing proportion of cardiac surgery patients are older and many have concomitant medical problems that can impair their recovery. Useful strategies are needed to reduce the occurrence of splanchnic ischemia in these and other high-risk populations if surgical outcome is to improve in the future.