The fate of circulating megakaryocytes during cardiopulmonary bypass.

Megakaryocytes with intact cytoplasm normally leave the bone marrow, enter central venous blood, and are filtered in the lungs. During cardiopulmonary bypass, large megakaryocytes are not filtered by the lungs and may not be removed in the extracorporeal circuit by arterial line filters. In such circumstances, they could enter the systemic circulation and block smaller cerebral vessels, resulting in neurologic impairment. To investigate the fate of circulating megakaryocytes during cardiopulmonary bypass, central venous blood and oxygenated blood samples before and after arterial line filtration (40 microns polyester screen filter) were obtained from 10 patients undergoing cardiopulmonary bypass. Megakaryocytes were isolated by whole blood filtration and identified by their characteristic structure after May-Grunwald-Giemsa staining. In preliminary studies, megakaryocyte identification was verified by immunolabeling. All samples contained megakaryocytes with copious cytoplasm. Their frequencies in central venous blood and oxygenated blood before and after the arterial line filtration (corrected for hemodilution) were 23.4 +/- 9.3 per milliliter (mean +/- standard error of the mean, range 3.1 to 89.7 per milliliter), 21.0 +/- 8.2 per milliliter (2.0 to 84.2 per milliliter) and 17.1 +/- 7.4 per milliliter (3.1 to 80.4 per milliliter), respectively. Megakaryocytes with scant or no visible cytoplasm were also observed. The results confirm that circulating megakaryocytes are a normal physiologic component. During cardiopulmonary bypass, megakaryocytes with copious cytoplasm (mean diameter 42.7 microns, range 22 to 78 microns) can pass through the extracorporeal circuit. In the absence of filtration by the lungs, these large cells have access to the systemic circulation. This study supports a possible role for circulating megakaryocytes in the development of cerebral dysfunction after cardiopulmonary bypass.