Corrected qt time among patients treated with clozapine or quetiapine under clinical routine conditions

QTc prolongation has been shown in vitro for clozapine (CLOZ) and quetiapine (QUET). The association of QT prolongation with antipsychotic dose and plasma levels, and the clinical relevance of these findings remain to be open. We assessed heart-rate corrected QT intervals (ECG) and correlations with CLOZ or QUET dose and plasma levels during steady state conditions. In a cross-sectional retrospective study, clinical data, ECGs, and plasma levels of patients with schizophrenia (ICD-10: F20), treated with CLOZ (N = 49) or QUET (N = 55), were analyzed in steady state. Comedication was considered using a QT prolongation risk score. Mean QT intervals corrected for heart rate according to Bazett or Fridericia were compared between CLOZ and QUET. Correlations between QTc intervals and antipsychotic dose, plasma levels, age, serum potassium levels, and gender were calculated. Groups treated with CLOZ or QUET were comparable regarding age (44 ± 12y) and gender (46% women). Mean daily doses were CLOZ 468 ± 210 mg and QUET 530 ± 227 mg, respectively. QTc intervals were slightly higher under treatment with CLOZ than with QUET (380 vs. 371 ms; P < 0.05), two QTc intervals (1.9%, both CLOZ) were between 460 and 480 ms. Mean heart rates were substantially higher in patients receiving CLOZ (95/min vs. QUET 79/min; P < 0.001). Partial correlations controlling for potassium levels and comedication revealed no significant associations of antipsychotic dose, plasma levels, age, and gender with QTc intervals.