Influence of the sample anticoagulant on the measurements of impedance aggregometry in cardiac surgery

Abstract

Background: The standard method of assessment of platelet function is represented by light transmission aggregometry (LTA), performed in citrated platelet-rich plasma (PRP). With LTA, decrease and subsequent post-cardiopulmonary bypass (CPB) recovery of platelet function have been reported during cardiac surgery. Multiple electrode aggregometry (MEA) may be used as point-of-care method to monitor perioperative changes in platelet function. Since MEA assesses macroaggregation which is influenced by the plasmatic levels of unbound calcium, citrate may be inadequate as anticoagulant for MEA. We used citrate and heparin for MEA samples, to see with which anticoagulant the intraoperative decrease and postoperative recovery in platelet function previously described with other aggregometric methods in cardiac surgery may be observed with MEA.

Methods: Blood was obtained from 60 patients undergoing routine cardiac surgery and the samples were collected in standard tubes containing unfractionated heparin (50 U/mL) or trisodium citrate (3.2%). The samples were obtained before CPB, at 30 minutes on CPB, end of CPB and on the first postoperative day. MEA was performed using the Multiplate® analyzer. Collagen (COLtest, 100 μg/mL) and TRAP-6 (thrombin receptor activating peptide, TRAPtest, 1mM/mL) were used as aggregation agonists.

Results: Platelet aggregometric response decreased significantly during CPB. Platelet aggregation assessed using TRAP-6 as agonist on heparinized blood significantly correlated with the duration of CPB (r = -0.41, p = 0.001, 2-tailed Pearson test). The aggregometric analysis performed on the first postoperative day showed a significant recovery in platelet activity in the samples containing heparin (increase from 30 ± 22 U to 46 ± 27 U for the COLtest and from 70 ± 34 U to 95 ± 32 U for the TRAPtest, p < 0.001, Student's t-test), while no significant recovery of platelet function could be established in the MEA measurements performed with citrated blood.

Conclusions: The choice of blood sample anticoagulant used for impedance aggregometry influenced the platelet aggregation response. Postoperative platelet function recovery was only detected in the heparinized samples. Heparin seems to be better suited than citrate for the analysis of impedance aggregometry in heart surgery.

Keywords: cardiac surgery; impedance aggregometry; platelet recovery; sample anticoagulant.

Figure 1

Time course of aggregation results. Data are presented as Box-and-Whisker plots showing mean, 5%, 25%, 75%, and 95% percentiles. Grey box plots represent the results of the analysis of heparin blood, white box plots of citrated blood. n = 60 patients; time point (1) before the beginning of cardiopulmonary bypass (CPB); (2) at 30 minutes on CPB; (3) at the end of CPB; (4) on the first postoperative day. There was a significant difference of the results of citrated vs heparin blood for all time points and tests Notes: *p < 0.05 (closed testing procedure; Hotelling’s T² test). The most important difference between the analyses of citrated and heparin blood is that an increase in platelet function is detected between the end of operation and the first postoperative day only in heparin blood but not in citrated blood. Abbreviations: CPB, cardiopulmonary bypass; ns, not significant.

Figure 2

Bland-Altman plot for COLtest (A) and TRAPtest (B) performed in blood anticoagulated with heparin or citrate, respectively. n = 60 patients; time point (1) before the beginning of cardiopulmonary bypass (CPB); (2) at 30 minutes on CPB; (3) at the end of CPB; (4) on the first postoperative day. Particularly for TRAPtest, some samples showed an excellent aggregation in heparin blood while almost no aggregation in citrated blood was observed.