Use-dependent effect is characteristic of certain antiarrhythmic drugs, mainly those included in Group I of Vaughan-Williams classification. There is an increasing interest in the study of this phenomenon in order to correlate it with the potential arrhythmogenic effect of currently used antiarrhythmic drugs. Use-dependent effect produces widening of the QRS ECG complex as the heart rate is increased. Thus, to produce the necessary changes in heart rate to clinically disclose this phenomenon, endocardial stimulation of the right ventricle is usually done both in control condition and under the effect of the tested drug. As this is an invasive method, the amount of information collected on this important aspect of the antiarrhythmic drug effects has been limited. Hence, we decided to confirm whether transesophageal cardiac pacing is a suitable method to produce controlled changes of the heart rate in order to analyse the use-dependent phenomenon. In this study we included 14 patients, 9 women and 5 men aged 47.85 +/- 13.91 years and ejection fraction of 54.64 +/- 7.19%. Transesophageal stimulation was performed up to the Wenckebach point and the previous rate producing 1:1 A-V response was considered. ECG was recorded in an ink-jet three-channel electrocardiograph at 100 mm/sec chart speed and QRS duration was measured. All patients were studied in the basal unsedated state, free of any medication and after the administration of 3.13 +/- 0.74 mg/kg of flecainide during 4.07 +/- 1.4 days. Atrial capture was obtained with pulses of 15 mA and 18 msec. Heart rate attained before treatment was 150 +/- 21.83 bpm and 144.28 +/- 19.88 bpm under the effect of flecainide (p = NS) (Table 1).(ABSTRACT TRUNCATED AT 250 WORDS)