The limits of bloodwarming: Maximally heating blood with an inline microwave bloodwarmer

Background: Bloodwarmers, mandatory for use in trauma resuscitation, are currently limited to a maximum temperature of 42°C by the American Association of Blood Banks. Using newly available inline microwave bloodwarming technology, we sought to identify the maximal temperature to which blood may be safely heated. Methods: Using an inline microwave bloodwarmer, we warmed refrigerated packed red blood cells to settings ranging from 'Off' to 60°C. We evaluated heated blood for changes in red cell structure and function by measuring hemoglobin/hematocrit, potassium, lactate dehydrogenase, plasma hemoglobin, blood smear, osmotic fragility, PO₂ giving 50% O₂ saturation, and hemoglobin electrophoresis. Results: Measures of hemolysis showed no increase above control until temperatures of 51 to 53°C were achieved (p < 0.05). Red cell size remained unaffected until temperatures of 53°C were achieved (p < 0.05). Osmotic fragility was not elevated until 60°C (p < 0.05). PO₂ giving 50% O₂ saturation was low for all samples. Hemoglobin electrophoresis remained unchanged at all temperature settings. Conclusion: An inline microwave bloodwarmer may be used to heat blood safely to 49°C. Blood warmed to this temperature may significantly increase the amount of heat returned to the hypothermic trauma patient.