The effect of hematocrit during hypothermic cardiopulmonary bypass in infant heart surgery: results from the combined Boston hematocrit trials.

OBJECTIVE: Two randomized trials of hematocrit strategy during hypothermic cardiopulmonary bypass in infant heart surgery have been performed. The first suggested worse outcomes were concentrated in patients with lower hematocrit levels (approximately 20%), whereas the second suggested there was little benefit to increasing the hematocrit level above 25%. The form of the relationship between continuous hematocrit levels and outcomes requires further study.
METHODS: In the two trials, 271 infants who underwent biventricular repair not involving the aortic arch were enrolled. Analysis was undertaken of the effects of hematocrit level, as a continuous variable, at the onset of low-flow cardiopulmonary bypass.
RESULTS: Psychomotor Development Index scores at age 1 year varied nonlinearly with hematocrit levels, with increasing scores up to 23.5% hematocrit (P < .001) and a plateau effect beyond 23.5% (P = .42), based on a piecewise linear model. Lower hematocrit levels were associated with more positive intraoperative fluid balance (P < .001 for linear trend) and marginally associated with higher serum lactate levels at 60 minutes after bypass (P = .08 for linear trend), but not with blood products given, nadir of cardiac index in the first 24 hours, or Mental Development Index scores.
CONCLUSIONS: A hematocrit level at the onset of low-flow cardiopulmonary bypass of approximately 24% or higher is associated with higher Psychomotor Development Index scores and reduced lactate levels. Because the effects of hemodilution may vary according to diagnosis, age at operation, bypass variables such as pH strategy and flow rate, and other perioperative factors, this study cannot ascertain a universally "safe" hemodilution level.