The effects of heparin and extracorporeal circulation on platelet counts and platelet microaggregation during cardiopulmonary bypass

AbstractBackground: Cardiopulmonary bypass is associated with platelet activation and reduced platelet counts. Platelet activation may artifactually lower platelet counts by causing aggregation. In vivo platelet activation may increase existent platelet microaggregation ex vivo. We studied platelet counts and existent platelet microaggregation at different stages of cardiopulmonary bypass. Methods: Twenty-one patients were studied before and after heparinization (300 U · kg–1) and at the end of cardiopulmonary bypass. Unaggregated (or single) platelets were counted in hirudin-anticoagulated blood, and total platelets were counted in ethylenediaminetetraacetic acid–anticoagulated blood. Results: The total platelet count, 198 ± 61 × 109 · L–1, was unaffected by heparin and stayed at 197 ± 60 × 109 · L–1 (P =.7) but fell during extracorporeal circulation; the hemodilution-corrected count was 163 ± 52 × 109 · L–1 (P =.0004). Heparinization reduced the unaggregated platelet count from (mean ± 1 SD) 178 ± 62 × 109 · L–1 to 155 ± 60 × 109 · L–1 (P =.0001). Extracorporeal circulation had little additional effect. The hemodilution-corrected count was 142 ± 48 × 109 · L–1 (P =.6). Conclusions: Heparinization caused platelet activation and increased existent platelet microaggregation ex vivo. During extracorporeal circulation, there was a reduction in total platelets that was greater than could be explained by hemodilution alone, but the unaggregated platelet count did not change significantly when corrected for hemodilution. Furthermore, the increased platelet microaggregation observed after heparinization was no longer evident after this loss. These findings suggest that during extracorporeal circulation, the platelets that formed into microaggregates after heparinization were lost from the circulation in preference to single platelets. (J Thorac Cardiovasc Surg 2000;120:538-43