Correspondence

Abstract

We were concerned about the findings of Tigchelaar et al. in which ADP-induced platelet dysfunction was reportedly seen following cardiopulmonary bypass [1]. We are glad that they found HES to be a cost-effective plasma expander which neither caused bleeding nor perceptibly worsened platelet function, thus confirming previous work [2]. We were also pleased that they found that administration of gelatin solution during cardiopulmonary bypass was associated with decreased agglutination in response to ristocetin. This finding also gratifyingly confirms previous work [3] in which similar numbers of patients were studied using hirudinised blood and platelet rich plasma (PRP). We detected a specific impairment of vWF-dependent aggregation/agglutination by gelatin, an effect which was not seen with collagen-induced aggregation.We are disturbed by the choice of citrate as an anticoagulant since findings with ADP might be obscured by artefacts associated with low Ca 2 + concentrations. It has been shown that a low Ca 2 + level in plasma favours thromboxane A 2 formation and secretion in response to ADP [4,5]. The exaggerated response to ADP-induced aggregation might be more evident before cardiopulmonary bypass and since there is some impairment of largely ADP-dependent 'spontaneous' platelet aggregation in stirred normocalcaemic blood during and after cardiopulmonary bypass [3], haemodilution during cardiopulmonary bypass in addition to the 2-fold dilution of blood samples for aggregometry might have greatly exaggerated the degree of impairment of ADP-induced aggregation reported by Tigchelaar et al. [1].Although it is conventional to dilute whole blood 2-fold when using impedance aggregometry, it could markedly influence other results (as with ADP) when blood samples are taken following haemodilution. In the case of the 2L priming volume used by Tigchelaar et al. [1], there is likely to be a more marked effect on aggregation in whole blood than in PRP.Considering the above points, in which pre-cardiopulmonary bypass ADP-induced aggregation may have been exaggerated and the later effects could be attributed to haemodilution, we suggest that the change in aggregation was more apparent than real.