The effect of cardiopulmonary bypass on circulating megakaryocytes

Megakaryocytes (Mks) are found in the lungs and the blood stream as well as in the bone marrow. We modified a whole blood filtration method for Mks by immunostaining for CD61 using biotin streptavidin, and used this technique to study Mks and their morphology in the central venous and arterial circulations before, during and after cardiopulmonary bypass (CPB) in haematologically normal patients undergoing routine cardiac surgery. Blood samples were taken immediately after the insertion of central venous (V) and arterial (A) catheters and after thoracotomy, immediately before bypass. Further samples were taken after 60–90 min on‐CPB and 180–240 min post‐bypass. In comparison with the steady state before bypass, circulating Mk levels in blood on bypass increased dramatically, from (V) 10.93 ± 3.94/ml (mean ± SD) to 36.48 ± 11.52/ml and from (A) 8.37 ± 4.39/ml to 38.65 ± 20.68/ml. This effect was still present, to a lesser extent, 180–240 min post‐bypass. Circulating levels of Mks were consistently lower in the arterial circulation than in the venous circulation off bypass, but levels in the two circulations were comparable during CPB, confirming previous suggestions that the lungs are net removers of Mks from the circulation. Type 4 Mks, the largest and most normal morphologically, were rarely seen in arterial blood, but increased significantly during CPB, indicating that the lungs selectively remove large Mks. The lungs appear to play an active role in the regulation of Mk levels. This is lost during CPB and despite the extracorporeal 40 μm arterial line filter, large Mks enter the systemic circulation. More effective extracorporeal filtration of large Mks might reduce the neurological impairment seen in some patients who have undergone CPB.