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1 The plasma elimination half-life of mexiletine was measured in four subjects when the urine was rendered (a) acidic and (b) alkaline. 2 Urinary acidification (pH 5.0) was associated with a plasma elimination half-life of 2.8 h and 57.5% of the intravenous dose was excreted in the urine within 48 hours. When the urine was alkaline (pH 8.0) the half-life increased to 8.6 h with negligible amounts of drug appearing in the urine. 3 Urinary pH varies widely in cardiac patients and should be controlled or monitored to provide better therapeutic precision with mexiletine.

Plasma procainamide levels achieved by oral procainamide treatment were studied in patients with recent myocardial infarction or ischaemia. Procainamide therapy was started after intravenous lignocaine had been used to control ventricular arrhythmias in the acute phase. A i g loading dose did not provoke toxicity and achieved 4-hour levels in or near the therapeutic range in 79 per cent ofpatients. Some patients with cardiac failure absorbed oral procainamide very slowly. A maintenance regimen of 375 mg 4 hourlyfailed to maintain effective procainamide levels in 73 per cent of patients. On a dosage of 500 mg 4 hourly 36 per cent had ineffective levels before each dose. Measurement of procainamide levels is desirable during treatment, since the variation between patients given a standard dosage is considerable and unpredictable. To maintain adequate procainamide levels frequent doses are necessary but inconvenient.