Platelets in shock and trauma

Effects of low temperature on shear-induced platelet aggregation and activation

BACKGROUND

Hemorrhage is a major complication of trauma and often becomes more severe in hypothermic patients. Although it has been known that platelets are activated in the cold, studies have been focused on platelet behavior at 4 degrees C, which is far below temperatures encountered in hypothermic trauma patients. In contrast, how platelets function at temperatures that are commonly found in hypothermic trauma patients (32-37 degrees C) remains largely unknown, especially when they are exposed to significant changes in fluid shear stress that could occur in trauma patients due to hemorrhage, vascular dilation/constriction, and fluid resuscitation.

METHODS

Using a cone-plate viscometer, we have examined platelet activation and aggregation in response to a wide range of fluid shear stresses at 24, 32, 35, and 37 degrees C.

RESULTS

We found that shear-induced platelet aggregation was significantly increased at 24, 32, and 35 degrees C as compared with 37 degrees C and the enhancement was observed in whole blood and platelet-rich plasma. In contrast to observation made at 4 degrees C, the increased shear-induced platelet aggregation at these temperatures was associated with minimal platelet activation as determined by the P-selectin expression on platelet surface. Blood viscosity was also increased at low temperature and the changes in viscosity correlated with levels of plasma total protein and fibrinogen.

CONCLUSION

We found that platelets are hyper-reactive to fluid shear stress at temperatures of 24, 32, and 35 degrees C as compared with at 37 degrees C. The hyperreactivity results in heightened aggregation through a platelet-activation independent mechanism. The enhanced platelet aggregation parallels with increased whole blood viscosity at these temperatures, suggesting that enhanced mechanical cross-linking may be responsible for the enhanced platelet aggregation.

Preoperative plateletpheresis does not reduce blood loss during cardiac surgery

Acute preoperative plateletpheresis has been reported to be effective in reducing blood loss and blood component transfusion while improving haematological profiles in patients undergoing open-heart surgery. However, in these studies, the concomitant use of cell saver techniques may have been responsible for the beneficial effects because they remove free haemoglobin and activated procoagulants and, therefore, could mask the deleterious effects of combined plateletpheresis and cardiopulmonary bypass (CPB). In the present study, 40 patients undergoing primary myocardial revascularization were randomly divided into two groups: a control group without plateletpheresis performed, and a second group in which preoperative platelet-rich plasma 10 ml.kg-1 (PRP group) was collected and later reinfused after reversal of heparin. Standardized surgery, anaesthesia and CPB without concomitant cell saver techniques were employed. In the PRP group, blood transfusion was reduced (1.5 +/- 1.3 vs 2.4 +/- 1.3 units, P < 0.05) but this was accompanied by lower postoperative haemoglobin concentrations. There were no differences in blood loss (992.6 +/- 327.4 vs 889.6 +/- 343.7 ml), fresh frozen plasma (2/19 vs 3/20 patients) or platelet requirements (1/19 vs 1/20 patients). Reinfusion of autologous PRP did not improve platelet count and function, nor tests of coagulation. Fibrinogen concentrations were lower in the PRP group on the operative day (P < 0.05), suggesting increased fibrinogen consumption; and more patients in the PRP group had low haptoglobin levels during CPB (8/19 vs 0/20 patients, P < 0.005), which indicated greater haemolysis in this group. We conclude that acute preoperative plateletpheresis offers no advantage in haemostasis during elective primary myocardial revascularization surgery.

Autologous blood transfusion

Autologous blood transfusion is a procedure in which blood is removed from a donor and returned to his circulation at some later time. Autologous transfusion can be performed in three ways: (1) preoperative blood collection, storage, and retransfusion during surgery; (2) immediate preoperative phlebotomy with subsequent artificial hemodilution and later return of the phlebotomized blood; and (3) intraoperative blood salvage and retransfusion. All three methods of autologous transfusion offer a potentially superior method of blood transfusion which eliminates many of the problems and complications associated with the banking and administration of homologous donor blood.

Prognostic value of blood platelet counts, coagulation factors and serum fibrin/fibrinogen degradation products (FDP) in acute infections

Platelet counts, P&P, factor V and fibrin/fibrinogen degradation products (FDP) in serum were determined in 96 patients with 100 episodes of acute infections. The aim of the study was to evaluate the possible correlation between the tests, separately and in combination, and the outcome of the patients. The prognostic value was found to be dubious (0.05 greater than P greater than 0.01) as regards both a low platelet count and a decreased level of P&P and very low (P greater than 0.2) as regards factor V. On the other hand, the demonstration of a raised S-FDP as well as the presence of more than one pathological test was of distinct prognostic value (P less than 0.001). It is concluded that coagulation tests are useful as supportive aids in the prognostic evaluation of patients with acute infectious diseases.

Vagal reflexes in the bronchoconstriction occurring after induced intravascular platelet aggregation

Intravascular platelet aggregation induced in cats by i.v. infusions of collagen caused a transient increase in pulmonary vascular resistance (PVR) and in non-elastic pulmonary resistance (RL), and a transient decrease in dynamic lung compliance (dyn CL). PVR,dyn CL and platelet aggregation after collagen infusions were unaffected by bilateral cervical vagotomy and atropinization, wheras these procedures reduced the post-infusion rise in RL by about 50 per cent. The administration of indomethacin inhibited platelet aggregation as well as bronchoconstriction after collagen infusion. The present investigation indicates that intravascular platelet aggregation will cause reflex bronchoconstriction mediated by vagal efferent fibres.

The release of prostaglandin-like substances during platelet aggregation and pulmonary microembolism

During pulmonary microembolism subsequent to induced platelet aggregation in vivo, a pressor response is elicited in the lungs due to the release of vasoactive substances. During the prefusion of isolated cat lungs with whole blood, platelet aggregation induced by collagen released prostaglandin-like substances (PG-LS), and a rabbit aorta contracting substance (RCS) as measured by continuous biosassay of the venous effluent. This release occurred in parallel to the pulmonary pressor response. The same amount of PG-LS and RCS was released when the lungs were replaced by a blood filter to trap platelet aggregates. Apparently PG-LS and RCS are released from platelets during their aggregation. As E- and F-type PGs are rapidaly inactivated in the pulmonary circulation, there must either be an equal generation of PGs in the lungs themselves, or rather, the release of PGs from platelets must occur distal to the inactivation mechanism without any additional PG-release from the lungs. The exact role of PGs in the lung responses to intravascular platelet aggregation is not clear.