Adverse effects of low hematocrit during cardiopulmonary bypass in the adult: should current practice be changed?

Hemodilutional anemia during cardiopulmonary bypass can lead to inadequate oxygen delivery and, consequently, to ischemic organ injury. In adult bypass, the nadir hematocrit can vary widely with body size and prebypass hematocrit variations, yet its effects on perioperative organ dysfunction and patient outcomes remain largely unknown.

To elucidate these effects, we retrospectively analyzed operative results and resource utilization data from 5000 consecutive cardiac operations with cardiopulmonary bypass performed on adults (1994 to 2000). Rolling decile groups (500 patients each; 75% overlapping) of increasing lowest hematocrit values were used to characterize hemodilution-outcome relationships. Intermediate-term (0 to 6 years) survival was assessed for coronary artery bypass patients (n = 3800) via Kaplan-Meier analysis in quintile subgroups based on lowest hematocrit. Multivariate logistic regression (operative mortality and morbidity) and Cox proportional hazard model (0- to 6-year mortality) analyses were used to determine independent predictors of poor outcomes.

Stroke, myocardial infarction, low cardiac output, cardiac arrest, renal failure, prolonged ventilation, pulmonary edema, reoperation due to bleeding, sepsis, and multiorgan failure were all significantly and systematically increased as lowest hematocrit value decreased below 22%. Consequently, intensive care requirements, hospital stays, operative costs, and operative deaths were also significantly greater as a function of hemodilution severity. Longer-term survival was improved systematically for increasing lowest hematocrit coronary artery bypass grafting quintiles; for example, 6-year survival was 80.5% and 92.3% for quintiles I (lowest hematocrit = 16.1%) and V (lowest hematocrit = 27.5%). The continuous variable lowest hematocrit was an independent predictor of (1) operative mortality, (2) prolonged cardiovascular intensive case (>2 days) and postoperative hospital (>8 days) stays, and (3) worse 0- to 6-year survival.

Increased hemodilution severity during cardiopulmonary bypass was associated with worse perioperative vital organ dysfunction/morbidity and increased resource use, as well as greater short- and intermediate-term mortality. We speculate that these results derive from inadequate oxygen delivery causing ischemic and/or inflammatory vital organ injury, as recently demonstrated intravitally in cerebral tissues. Although this analysis of a large observational study offers evidence linking low on-pump hematocrit values to these adverse outcomes, prospective randomized trials are needed (1) to establish whether a causal effect of hemodilution on poor outcomes actually exists and (2) to test the potential efficacy of maintaining on-pump hematocrit above 22% for improving outcomes of cardiopulmonary bypass.