Background Hypertrophic obstructive cardiomyopathy(HOCM)carries an increased risk for sudden cardiac death.Nodata regarding the percutaneous transseptal myocardial ablation(PTSMA)and epicardial left ventricular pacing(LVP)were reported.Methods Seven patients with recurrent symptoms and increased resting left ventricular outflow tract pressure gradient(LVOTG)after PTSMA and another 14 patients with HOCM without history of PTSMA were studied.Both resting anddobutamine stress echocardiography,PTSMA and LVP were routinely performed.Results In patients without previous PTSMA procedure,mild reduction of resting LVOTG was detected at 5 minutesafter left ventricular pacing,and this reduction became significant at 10 minutes.All patients were divided into successfuland unsuccessful groups according to their response to LVP.In contrary to patients in unsuccessful group,resting andR-S2 stimuli-induced LVOTG during PTSMA procedure were decreased dramatically((9±5)mmHg vs(58±12)mmHg,(12±2)mmHg vs(113±27)mmHg,P<0.001).Analysis of Logistic regression demonstrated that only LVOTG levelduring left ventricular pacing was an independent factor predicting the reduction of LVOTG immediately after PTSMA(odds ratio(OR),0.59;95% CI 2.67 to 5.82;P=0.0002).Conclusion Left ventricular endocardial temporary pacing plays a critical role in predicting acute effect on the reductionof LVOTG immediately after PTSMA procedure. Background Hypertrophic obstructive cardiomyopathy (HOCM) carries an increased risk for cardiac arrest. Nodata regarding the percutaneous transseptal myocardial ablation (PTSMA) and epicardial left ventricular pacing (LVP) were reported. Methods Seven patients with recurrent symptoms and increased resting left ventricular outflow tract pressure gradient (LVOTG) after PTSMA and another 14 patients with HOCM without history of PTSMA were studied. Both resting and dobutamine stress echocardiography, PTSMA and LVP were routinely performed. Results In patients without previous PTSMA procedure, mild reduction of resting LVOTG was detected at 5 minutesafter left ventricular pacing, and this reduction became significant at 10 minutes. All patients were divided into successfuland unsuccessful groups according to their response to LVP. contrary to patients in unsuccessful group, resting and R-S2 stimuli-induced LVOTG during PTSMA procedure were decreased dramatically ((9 ± 5) mmHg vs (58 ± 12) mmHg, (12 ± 2) mmHg v (113 ± 27) mmHg, P <0.001) .Analysis of Logistic regression that only LVOTG levelduring left ventricular pacing was an independent factor predicting the reduction of LVOTG immediately after PTSMA (odds ratio (OR), 0.59; 95% CI 2.67 to 5.82; P = 0.0002) .Conclusion Left ventricular endocardial temporary pacing plays a critical role in predicting acute effect on the reduction of LVOTG immediately after PTSMA procedure.