he implantable cardioverter-defibrillator (ICD) effectively terminates ventricular tachycardia (VT) and ventricular fibrillation (VF) and can prevent sudden cardiac death. 1,2 Tachycardia is identified primarily from the measurement of heart rate and/or the morphology of an intraventricular electrogram, so ICD therapy can be prescribed for supraventricular tachycardia or other causes of increased heart rate. [3][4][5] However, for supraventricular tachycardia, a dual-chamber ICD would decrease the frequency of inappropriate shocks from the ICD. Other mechanisms of inappropriate discharge of the ICD include non-sustained VT, pacemaker interference or T-wave oversensing. The autogain control system of the ICD detects the small amplitude electrograms during VF, but increased sensitivity leads to increased sensing of diastolic signals, particularly T-waves. On the other hand, decreased sensitivity for T-wave might result in a failure of VF sensing. 6 Moreover, ICD discharges due to T-wave oversensing during sinus rhythm can induce ventricular tachyarrhythmias. Thus, T-wave oversensing is an important problem for patients with an ICD. We report 2 cases of inappropriate shocks caused by T-wave oversensing in ICDs.
Case Reports
Case 1A 65-year-old female was referred for further treatment of cardiac arrest caused by VF. She had been diagnosed as Japanese Circulation Journal Vol.65, July 2001 having cardiac sarcoidosis and a daily 50-mg oral administration of prednisolone (PSL) had been started in another hospital. However, VF developed during tapering of the PSL dosage.When she was subsequently admitted to hospital, her chest X-ray showed mild cardiomegaly and bilateral hilar lymphadenopathy, but no pulmonary congestion. The 12-lead ECG during sinus rhythm showed complete right bundle branch block. Two-dimensional echocardiography revealed no abnormality of left ventricular wall motion or chamber size, and 99m Tc-pyrophosphate revealed no abnormal uptake. After obtaining written informed consent, an electrophysiological study (EP) was performed using the standard technique. 7 Although only non-sustained VT were induced by programmed stimulation, ICD therapy was indicated because VF had been documented clinically.In April 1998, a transvenous ICD system (Medtronic Micro Jewel II, 7223Cx, model 6943 right ventricular screw-in lead, Minneapolis, MN, USA) was implanted. At operation, shocks of 20 J were twice confirmed to be capable of terminating VF induced by T-wave shock, and 30 J was set as the initial treatment of VF. The amplitude of the ventricular electrogram from the tips of the defibrillation lead was 7.5 mV during sinus rhythm and the pacing threshold at the site was 2.0 V at a pulse width of 0.10 ms. One month later, the patient experienced 4 discrete inappropriate shocks during sinus rhythm of 94 beats/min. The stored electrograms showed decreased R-wave amplitude (2.1 mV) and increased T-wave amplitude (2.3mV). Both R-and T-waves were detected at the ventricular sensing threshold of 0.3 mV and this led to...