OBJECTIVE: Assessment of electrocardiographic (ECG) effects of dexmedetomidine. DESIGN: Prospective observational study including children 0-17 years of age with congenital heart disease (CHD) and children following cardiothoracic surgery. Patients who did not receive dexmedetomidine were used as a control group. All patients had two ECGs: one baseline, pre-dexmedetomidine (T1) and one during dexmedetomidine infusion (T2). MEASUREMENTS AND RESULTS: Fifty-one patients, median age of 0.5 years (IQR = 3.4), and 25 patients, age 0.25 (IQR = 2.9), were included in the dexmedetomidine and control groups, respectively. Forty received a dexmedetomidine-loading dose of 1 microg/kg (IQR = 0.5). At T2, the dexmedetomidine infusion was 1 microg/kg/h (IQR = 0.5). In the dexmedetomidine group, heart rate (HR) decreased from 140 +/- 22 to 115 +/- 23 (P < 0.001); PR, PRc and PR index changed from 115 +/- 28 to 122 +/- 29 ms (P = 0.01), 174 +/- 38 to 167 +/- 35 ms (P = 0.07) and 15,882 +/- 3,565 to 13,792 +/- 3,311 (P < 0.001), respectively. QRS decreased from 84 +/- 21 to 80 +/- 21 ms (P = 0.02), and QTc had no change (433 +/- 47 to 435 +/- 36 ms). When compared to the control group, none of the ECG intervals had any difference other than a trend towards lower HR (P = 0.08). Neonates and infants had a bigger drop in the HR compared to older children (P < 0.001), while other parameters were similar. At T2 none of the dexmedetomidine group patients had atrioventricular block or other arrhythmia. Four patients in the control group had accelerated junctional rhythm. CONCLUSIONS: Use of dexmedetomidine in patients with CHD and patients following cardiothoracic surgery is not associated with any significant ECG interval abnormalities other than a trend towards lower HR.