Steady-state serum quinidine concentration: role in prophylactic therapy following acute myocardial infarction.

Steady-state serum quinidine concentrations were monitored in 24 patients with acute myocardial infarction who were on a 1,300-mg daily dosing regimen. Mean serum concentrations spanned the therapeutic range, from 2 to 6 microgram/ml in 21 patients. In no patient was the level of 7 microgram/ml exceeded. Mean levels were similar in patients with congestive heart failure [3.6 +/- 1.5 (SD) microgram/ml] as in those free of failure (3.2 +/- 1.3 microgram/ml), and did not vary with impairment of renal function. There was a significant correlation between mean individual serum quinidine concentrations and the rate-normalized QT interval prolongation (r = 0.54, P less than 0.01); however, variability of the response was high. Variability of the mean serum quinidine levels among individuals was 41%. Variability within individual patients was only 18%. In the individual patient receiving prophylactic oral quinidine therapy, monitoring serum quinidine levels appears to be an accurate, reproducible and pharmacologically significant guideline to therapy.