Aspirin and bleeding-time: dependency of age

Interrelationships between prostacyclin and thromboxane A2

Prostacyclin is a product of arachidonic acid metabolism generated by the vessel wall of all mammalian species studied including man. Prostacyclin is a potent vasodilator and the most potent inhibitor of platelet aggregation so far described. It inhibits platelet aggregation through stimulation of adenylate cyclase leading to an increase in cyclic AMP in the platelets. The enzyme which synthesizes prostacyclin is mainly localized in the endothelial layer of the vascular wall. Prostacyclin can also be a circulating hormone constantly released by the pulmonary circulation. On the basis of these observations we proposed that platelet aggregability in vivo is controlled via a prostacyclin mechanism. In contrast to the vessel wall, in blood platelets arachidonic acid is converted by the enzyme thromboxane synthetase to a potent vasoconstrictor and proaggregating substance, thromboxane A2. Therefore arachidonic acid is metabolized in the vessel wall and the platelets to potent substances with opposing biological activities. The balance between the activities of these substances is important in the homeostatic interaction of the platelets and the vessel wall. The different ways of interfering with this balance and its impact in the development of thrombosis and atherosclerosis are discussed. The balance between thromboxane A2 and prostacyclin might be important in the control of the pulmonary circulation. This possibility is discussed in the light of the present evidence.

Aspirin, a silent risk factor in urology

Aspirin is a widely used drug for its analgetic, antiinflamatory and antipyretic effects as well as for prophylactic effect in cardiovascular diseases. However, an increased number of operative hemorrhagic complications in patients on daily aspirin have been reported, an adverse effect highly relevant in urology. In this review the normal hemostatic mechanism and the chief pharmacological effect of aspirin on hemostasis is described. The literature is reviewed for hemorrhagic complications to aspirin in urology. Few reports indicate that aspirin increases bleeding and need for transfusion following prostatectomy, but no placebo-controlled clinical trials with large patient groups have been carried out. Following prostate biopsy and extracorporeal shock wave lithotripsy aspirin-induced hemorrhagic complications have been reported. Cessation of aspirin ingestion one week prior to invasive urologic procedures and correction of bleeding complications with desmopressin, platelet concentration or fresh whole blood is described.

A study of bleeding time in 120 long-term aspirin trial patients

The bleeding time was measured in 120 patients participating in a longterm randomised double-blind trial of aspirin in thromboembolic prophylaxis (UK-TIA aspirin Study). In 70 patients taking aspirin 300 mg or 1,200 mg daily for a mean duration of 35 months the bleeding time averaged 228 seconds. In comparison with 30 patients randomised to placebo and not taking aspirin whose bleeding time averaged 217 seconds, there was no significant difference. Stratification of bleeding time estimation by duration of treatment suggested no significant trend in either placebo or aspirin groups over several years. These results suggest that the longterm trials of aspirin should be looked at again from the point of view of efficacy of treatment by time from randomisation.

The ability of vascular tissue to produce prostacyclin decreases with age

The ability of aortae from young and mature swine to produce prostacyclin (PGI2) has been determined. PGI2 was measured as its hydration product, 6-keto-PGF1 alpha and assayed by stable isotope dilution GCMS. There was no significant difference in 6-keto-PGF1 alpha production between intimal strips from young and mature aortae in the basal state. In the presence of saturating concentrations of arachidonic acid, however, intimal strips from young aortae synthesized twice as much 6-keto-PFG1 alpha as did older tissues. Fatty acid compositions of young and mature aortae were virtually identical, making dietary difference an unlikely explanation for the age-related decrease in PGI2 synthesis. Both young and mature vascular tissues produced essentially only PGI2; insignificant amounts of PGE2 and PGF2 alpha were found.