Erythrocyte damage caused by the Haemotherm microwave blood warmer

Blood units (59) were warmed with the Haemotherm microwave blood warmer and seven units were warmed in a water bath for comparison. The influence of the final blood temperature, the size and the hematocrit of the units on the erythrocyte lesion was studied. Extracellular hemoglobin and potassium, hematocrit, osmotic fragility and mean cellular volume were used as indicators of red cell damage. Warming in the water bath caused no erythrocyte damage at temperatures below +46.8 degrees C. Higher temperatures caused progressive morphological changes and hemolysis. Units of red cells in saline warmed to temperatures above +46.3 degrees C with the Haemotherm showed intense hemolysis and changes in all parameters used. Below this temperature no erythrocyte damage occurred. When the amount of blood warmed was less than 300 g or when the hematocrit exceeded 0.70, the blood mixing mechanism became insufficient, leading to local overheating and hemolysis. Therefore, erythrocyte concentrates in a Fenwal blood bag should never be warmed by the Haemotherm. The mean plasma hemoglobin increase of the ten whole blood units warmed by the Haemotherm to +36.0-36.8 degrees C was 123 mg/l. The results of the present study indicate that microwaves per se are not harmful to erythrocytes but that poor penetrance of microwaves, together with insufficient blood mixing during warming, are the critical factors leading to hemolysis.