Correlation Between Verapamil Plasma Concentration and P-R Prolongation in Essential Hypertension

Plasma verapamil concentration was correlated with serial electrocardiographic P‐R intervals in patients with essential hypertension receiving immediate‐release (80 to 120 mg three times a day) or sustained‐release (240 mg daily) verapamil. The mean P‐R interval in 22 patients taking placebo and immediate‐release verapamil was 0.18 second. The borderline first‐degree atrioventricular block of three patients did not change during treatment. Plasma verapamil concentrations of patients with a P‐R interval longer than 0.20 second and of those with a P‐R interval of 0.20 second or less were 169 ± 73 ng/mL and 63 ± 8 ng/mL, respectively. Six patients taking sustained‐release verapamil had a maximal mean P‐R interval of 0.19 ± 0.01 second during 24‐hour ambulatory electrocardiographic monitoring. P‐R intervals were 0.22 second or more in two patients, but they returned to normal by hour 7 for one and by hour 20 for the other patient. In summary, transient P‐R prolongation occurred with oral verapamil therapy, but no patient, regardless of baseline P‐R interval, developed high‐grade atrioventricular block.