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Antiplatelet drugs are used in the prevention or treatment of arterial thrombosis; their sites of action are illustrated in Figure 6.3. As described above, aspirin inhibits cyclo-oxygenase and hence reduces the formation of TXA2. Because it also has the effect of reducing the formation of PGI2, which itself has antiplatelet activity, agents acting more specifically as thromboxane synthase inhibitors or thromboxane receptor antagonists have also been investigated as potential antiplatelet agents, but do not appear to be more effective than aspirin. Dipyridamole acts by reducing the availability of ADP, and ticlopidine and clopidogrel inhibit the ADP receptor (Fig. 6.3). These drugs have antithrombotic effects similar to those of aspirin, but cause less gastric bleeding because they do not interfere with synthesis of prostaglandins in the
stomach. Recently, GPIIb-IIIa antagonists have been used in acute coronary thrombosis. Each of these newer antiplatelet drugs adds to the antithrombotic efficacy of aspirin; but also increases the risk of bleeding when used in combination.
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| PATIENT SELF-MONITORING OF ORAL ANTICOAGULANT THERAPY |
| Oral anticoagulant therapy (e.g. with warfarin) is given long-term to patients at risk of thrombosis within the chambers of the heart (e.g. patients with atrial fibrillation, or heart valve prostheses), which may embolize to the brain causing a stroke. Monitoring of the prothrombin time every few weeks is essential to minimize not only risk of thromboembolism, but also of excessive bleeding. Up to 1 per cent of the adult population in developed countries now receive long-term oral anticoagulants, hence traditional monitoring by doctors and nurses (taking blood samples, sending them to the laboratory, getting results, and giving dosage instructions to patients) has become a huge workload. In recent years, portable prothrombin time measuring devices have been developed which selected patients can use for self-monitoring. A 'finger-stick' capillary sample is drawn into the machine, and the result displayed to the patient (similar to blood glucose self-monitoring by persons with diabetes). Computer algorithms have also been developed which track the patient's prothrombin time results and oral anticoagulant doses, and can recommend appropriate changes in dose not only to their supervising doctor or nurse, but also directly to the patient. |
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