Prolongation of the QT interval by ketanserin.

In hypertensive patients single doses of ketanserin 40 mg prolonged the corrected QT interval (QTc) for at least 8 hours, with a maximal increase of 35 ms (P less than 0.001, n = 6) after 2 hours. During chronic dosing (20 and 40 mg b.d.) the QTc was further prolonged, by 46 and 45 ms respectively. QTc prolongation after treatment with a mean dose of 73 mg/day for 7 weeks (n = 26) was significantly related to body weight (r = -0.58, P less than 0.01), and to the dose of ketanserin corrected for body weight (r = 0.63, P less than 0.01), but not to plasma concentrations of ketanserin, ketanserinol, potassium or calcium. High doses of ketanserin (mean dose 167 mg/day, n = 9) increased the QTc by 40 ms (P less than 0.001), with prolongation of up to 80 ms in individual patients. Treatment with ketanserin at doses proposed for clinical use (40-80 mg/day) may carry a risk of ventricular arrhythmias.