THE EFFECT OF CARDIOPULMONARY BYPASS ON CIRCULATING MEGAKARYOCYTES

Megakaryocytes (Mks) are found in the lungs and the blood stream as we ll 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 venou s and arterial circulations before, during and after cardiopulmonary b ypass (CPB) in haematologically normal patients undergoing routine car diac surgery, Blood samples were taken immediately after the insertion of central venous (V) and arterial (A) catheters and after thoracotom y, immediately before bypass, Further samples were taken after 60-90 m in on-CPB and 180-240 min post-bypass, In comparison with the steady s tate before bypass, circulating Mk levels in blood on bypass increased dramatically, from (V) 10.93 +/- 3.94/ml (mean +/- SD) to 36.48 +/- 1 1.52/ml and from (A) 8.37 +/- 4.39/ml to 38.65 +/- 20.68/ml. This effe ct was still present, to a lesser extent, 180-240 min post-bypass, Cir culating levels of Mks were consistently lower in the arterial circula tion than in the venous circulation off bypass, but levels in the two circulations were comparable during CPB, confirming previous suggestio ns 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 th at the lungs selectively remove large Mlrs. The lungs appear to play a n active role in the regulation of Mk levels, This is lost during CPB and despite the extracorporeal 40 mu m arterial line filter, large Mks enter the systemic circulation, More effective extracorporeal filtrat ion of large Mks might reduce the neurological impairment seen in some patients who have undergone CPB.