BACKGROUND There is debate on the optimal QT correction method to determine the degree of the drug-induced QT interval prolongation in relation to heart rate (DeltaQTc). METHODS Forty-one patients (71 +/- 10 years) without significant heart disease who had baseline normal QT interval with narrow QRS complexes and had been implanted with dual-chamber pacemakers were subsequently started on antiarrhythmic drug therapy. The QTc formulas of Bazett, Fridericia, Framingham, Hodges, and Nomogram were applied to assess the effect of heart rate (baseline, atrial pacing at 60 beats/min, 80 beats/min, and 100 beats/min) on the derived DeltaQTc (QTc before and during antiarrhythmic therapy). RESULTS Drug treatment reduced the heart rate (P < 0.001) and increased the QT interval (P < 0.001). The heart rate increase shortened the QT interval (P < 0.001) and prolonged the QTc interval (P < 0.001) by the use of all correction formulas before and during antiarrhythmic therapy. All formulas gave at 60 beats/min similar DeltaQTc of 43 +/- 28 ms. At heart rates slower than 60 beats/min, the Bazett and Framingham methods provided the most underestimated DeltaQTc values (14 +/- 32 ms and 18 +/- 34 ms, respectively). At heart rates faster than 60 beats/min, the Bazett and Fridericia methods yielded the most overestimated DeltaQTc values, whereas the other 3 formulas gave similar DeltaQTc increases of 32 +/- 28 ms. CONCLUSIONS Bazett's formula should be avoided to assess DeltaQTc at heart rates distant from 60 beats/min. The Hodges formula followed by the Nomogram method seem most appropriate in assessing DeltaQTc.