Point-of-care assessment of hypothermia and protamine-induced platelet dysfunction with multiple electrode aggregometry (Multiplate®) in patients undergoing cardiopulmonary bypass.

Background: Coagulopathy is common after cardiopulmonary bypass (CPB), and platelet dysfunction is frequently considered to be a major contributor to excessive bleeding. Exposure to hypothermia may exacerbate the platelet function defect. We assessed platelet function during and after deep hypothermia with multiple electrode aggregometry (Multiplate(®); Verum Diagnostica GmbH, Munich, Germany).
Methods: Twenty adult patients undergoing pulmonary endarterectomy for chronic pulmonary hypertension were cooled on CPB to 20°C and deep hypothermic arrest was used to facilitate surgery. We analyzed platelet aggregation in whole blood samples at 12 measuring points during and after the procedure. Platelet aggregation was stimulated via the thrombin receptor (TRAPtest) at the patient's actual body temperature (AUC-CT) and after rewarming the samples to 37°C (AUC-37). In addition, we tested samples at 2 time points after 2 minutes of in vitro incubation with 20 μg protamine (0.067 μg/μL). Results are expressed as area under the aggregation curve (AUC).
Results: Cooling resulted in a marked decrease of platelet aggregation to a minimum AUC-CT of 20.5 (95% confidence interval [CI] 8.9-32.1) at 20°C body temperature. AUC-CT was significantly different from baseline (92.8, 95% CI 82.5-103.1) for temperatures of ≤28°C (P < 0.001), whereas the change in AUC-37 only became significant at the lowest body temperature (59.4, 95% CI 41.3-77.4). After rewarming to 36°C, AUC-CT and AUC-37 had recovered to 67.6 (95% CI 53.9-81.3) and 71.7 (95% CI 52.5-90.8), respectively. The mean AUC-CT was significantly lower than the mean AUC-37 from cooling at 28°C to warming at 24°C inclusive, and the relationship with temperature during cooling was significantly different between AUC-CT and AUC-37 (regression coefficients 4.7 [95% CI 4.2-5.2] vs 1.3 [95% CI 0.7-1.9]; P < 0.0001). After administration of protamine, mean aggregation decreased significantly for both measurements by 38.2 (95% CI -27.9 to -48.5; P < 0.001) and 44.5 (95% CI -58.5 to -30.5; P < 0.001), respectively. Similarly, adding protamine in vitro resulted in a decrease of mean aggregation by 35.1 (95% CI -71.0 to 0.8; P = 0.055) when measured after administration of heparin, and 56.5 (95% CI -94.5 to -18.5; P = 0.005) at the end of CPB.
Conclusion: Platelet aggregation, assessed by multiple electrode aggregometry (Multiplate), was severely affected during deep, whole-body hypothermia. This effect was partially reversible after rewarming, and was distinct from a general decline of platelet aggregation during CPB. Protamine also caused a significant decrease in platelet aggregation in vivo and in vitro.