Cardiovascular side effects of lithium have been reported to occur mainly at higher-than-therapeutic serum levels. We aimed to investigate the impact of the long-term lithium use on electrocardiogram (ECG) parameters in association with the serum levels in patients with bipolar disorder (BD) and in healthy controls (HCs) serving as the reference group.

The study sample consisted of 53 euthymic BD type I patients on lithium monotherapy at therapeutic serum levels (M=0.76, S.D.=0.14, range=0.41–1.09 mmol/l) for at least 12 months and 45 HCs. A 12-lead surface ECG was obtained from all participants at resting state for at least half an hour for 5-min recording. Heart-rate, Pmax, Pmin, QRS interval, QT dispersion, QT dispersion ratio (QTdR) and Tpeak-to-end interval (TpTe) were measured.

Regression analyses revealed that QTdR (B=14.17, P=.001), TpTe (B=18.38, P<.001), Pmax (B=17.84, P<.001) and Pmin (B=25.10, P<.001) were increased in BD patients who were on chronic lithium treatment than in HCs after controlling for age, sex and strict Bonferroni correction for multiple testing. There were no associations between serum lithium levels and ECG parameters.

Our findings suggest that the use of lithium is associated with both atrial and ventricular electrical instability, even when lithium levels are in the therapeutic range.