Electrocardiographic ST-segment changes during acute, severe isovolemic hemodilution in humans

Background: Controversy exists regarding the lowest blood hemoglobin concen tration that can be safely tolerated. The authors studied healthy resting h umans to test the hypothesis that acute isovolemic reduction of blood hemog lobin concentration to 5 g/dl would produce an imbalance in myocardial oxyg en supply and demand, resulting in myocardial ischemia. Methods: Fifty-five conscious healthy human volunteers were studied. Isovol emic removal of aliquots of blood reduced blood hemoglobin concentration fr om 12.8 +/- 1.2 to 5.2 +/- 0.5 g/dl (mean +/- SD). Removed blood was replac ed simultaneously with intravenous fluids to maintain constant isovolemia. Hemodynamics and arterial oxygen content (Cao(2)) were measured before and after removal of each aliquot of blood. Electrocardiographic (ECG) changes were monitored continuously using a Holter ECG recorder for detection of my ocardial ischemia. Results: During hemodilution, transient, reversible ST-segment depression d eveloped in three subjects as seen on the electrocardiogram during hemodilu tion. These changes occurred at hemoglobin concentrations of 5-7 g/dl while the subjects were asymptomatic. Two of three subjects with ECG changes had significantly higher heart rates than those without ECG changes at the sam e hemoglobin concentrations. When evaluating the entire study period, the s ubjects who had ECG ST-segment changes had significantly higher maximum hea rt rates than those without ECG changes, despite having similar baseline va lues. Conclusion: With acute reduction of hemoglobin concentration to 5 g/dl, ECG ST-segment changes developed in 3 of 55 healthy conscious adults and were suggestive of, but not conclusive for, myocardial ischemia. The higher hear t rates that developed during hemodilution may have contributed to the deve lopment of an imbalance between myocardial supply and demand resulting in E CG evidence of myocardial ischemia. However, these ECG changes appear to be benign because they were reversible and not accompanied by symptoms.