BACKGROUND: Paroxysmal supraventricular tachycardia(PSVT) is frequently associated with ST segment depression or T-wave inversion. However, the mechanism of ST-T changes in the context of various mechanisms of PSVT is not clear. The purpose of this study was to evaluate the prevalence of ST depression or T-wave inversion during PSVT and determine whether these changes are related to the mechanism of PSVT or the rate of the tachycardia. METHODS: Twelve-lead electrocardiograms were recorded during sinus rhythm and during PSVT in 163 patients who underwent an electrophysiologic study for ablation. Tachycardia cycle length, presence of ST depression or T-wave inversion during PSVT and the mechanism of tachycardia were evaluated. Significant ST depression was defined as at least 1mm horizontal or downsloping depression, measured 80ms after the J point and T-wave inversion as inversion of T-wave which was positive in the same lead during sinus rhythm. RESULTS: 1) The mechanism of PSVT analysed for ST segment depression was atrioventricular nodal reentry tachycardia in 60 cases and atrioventricular reentry tachycardia in 111 cases. The mean tachycardia cycle length was 373.8+/-68.0 msec. 2) ST depression and T-wave inversion was observed during PSVT in 56%(96/171) and 45%(77/171) of cases, respectively. 3) Tachycardia cycle length, degree of ST depression and number of leads with ST depression are not different according to the mechanism of PSVT. 4) ST depression and tachycardia cycle length had significant correlation, especially in atrioventricular reentry tachycardia. 5) Leads with T-wave inversion during tachycardia was observed more frequently in atrioventricular reentry tachycardia than atrioventricular nodal reentry tachycardia(p<0.05), but no difference between manifest and concealed bypass tract. CONCLUSIONS: ST segment depression is rate-related phenomenon and not different according to the mechanism of PSVT. Leads with T-wave inversion during tachycardia was observed more frequently in atrioventricular reentry tachycardia.