Plasma concentrations of antipsychotics and QTc prolongation: a pilot study

Background: Certain antipsychotics are known to cause QTc interval prolongation, which has been associated with increased risk of arrhythmia and sudden death. Previous studies have investigated whether there is an association between oral antipsychotic dose and QTc interval prolongation, however only few have examined the association between antipsychotic plasma concentrations and QTc interval. Material and methods: We performed a cross-sectional study with 22 forensic psychiatric in-patients. We measured the plasma concentration of the prescribed antipsychotics and performed an ECG simultaneously. We used Bazett's formula to calculate QTc and defined QTc as prolonged when: >460 ms for women and >450 ms for men. Results: Seventy-seven percent (n = 17) of the subjects were men (mean age = 40 years) and 91% (n = 20) were diagnosed with schizophrenia. QTc’s ranged from 369 to 437 ms. Patients receiving QTc prolonging drugs had significantly greater QTc interval compared to patients receiving non-prolonging drugs. Weak to moderate negative correlations were found between QTc interval and both defined daily dose (DDD) and antipsychotic plasma concentration. There was no statistical difference between the correlations for DDD and plasma concentration versus QTc interval. Conclusion: We did not find a stronger association between antipsychotic plasma concentration and QTc than between antipsychotic dose and QTc. We suggest close monitoring with regular electroencephalogram’s until the development of a better marker for predicting the risk of cardiac arrhythmia.