The QTc interval and its dispersion in patients receiving two atypical antipsychotics

Treatment with atypical antipsychotics may prolong the rate-corrected Q-T interval (QTc) on electrocardiogram and increase the risk of dangerous ventricular arrhythmias. Polytherapy with atypical antipsychotics is becoming common, but the effect of this practice on the QTc has not been explored in detail.

Among 364 adults treated with atypical antipsychotics randomly selected from consecutive admissions to a single hospital, electrocardiograms with measurable Q-T intervals in at least six leads were available for 38 of 49 patients receiving polytherapy with two atypical antipsychotics. Daily chlorpromazine equivalent, QTc duration and QTc dispersion were assessed in this group and in 73 closely matched patients receiving atypical antipsychotic monotherapy.

The daily chlorpromazine equivalent of atypical antipsychotics was significantly greater in the polytherapy group (525.2 vs. 244.7 mg, P = 0.0003). Polytherapy and monotherapy patients were similar with regard to QTc duration, QTc dispersion and proportion of patients with gender-adjusted QTc prolongation (7.9% vs. 9.6%). The QTc duration had only a modest correlation with the total antipsychotic dose (P = 0.064). The presence of hypokalemia (3.0-3.5 mEq/l) was not associated with longer QTc intervals.

The common practice of polytherapy with two atypical antipsychotics does not seem to lead to significant QTc prolongation compared to monotherapy.