Platelet behaviour in dogs during and after moderate and severe haemorrhage

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.

Use of aspirin in haemorrhagic shock

Haemorrhagic shock was induced in two similar groups of dogs for two hours. One group received aspirin before shock and the other group served as a control. When blood was retransfused the PVR which was markedly elevated during shock returned to pre-shock value in the aspirin group but was elevated 100 per cent in the control group, despite correction of a cidosis. Aspirin reduces collagen-induced platelet aggregation and thereby inhibits the formation of platelet micro-emboli without affecting the coagulationfactors. This effect of aspirin is thought to be responsible for the lowering of the elevated PVR to pre-shock values in the aspirin group following retransfusion. Because of the metabolic acidosis associated with the shock state, concurrent administration of sodium bicarbonate is recommended when transfusing shocked patients with blood. A clinical trial of aspirin in early treatment of shocked patients as well as for prophylaxis in high risk situations is justified.