A 76-year-old man with ischemic cardiomyopathy, left ventricular ejection fraction of 20%, QRS duration of 150 ms, in NYHA functional class III, underwent implantation of an InSync Sentry CRT-D, followed by induction of VF at the end of the procedure.
1 : Sinus rhythm (AS-BV); 2 : Induction with T-wave shock; 3 : Detection of a tachycardia in the VF zone with undersensed ventricular events (alternating FS-VS); 4 : VF detected (FD) and capacitor charge (while charging all sensed ventricular events are labeled VS); 5 : Delivery of 20-J DC shock; 6 : Successful shock and sinus rhythm.
During the induction test, ventricular sensing is usually set to the highest programmable value (1.2 mV), i.e. the system’s lowest sensitivity, enabling the observation of faulty sensing. A reliable detection of VF at this high sensing value (low sensitivity) suggests that the safety margin for detection is wide, and that the choice of a 0.3 mV nominal value is appropriate. In this patient, while sensing was not flawless, it delayed only slightly the detection of VF and delivery of the shock. It is, therefore, critical to closely scrutinize the EGM during and immediately after the test, in order to not miss the presence of undersensing. As in the previous case, undersensing was due to a wide variability in the amplitude of the ventricular signal, which interfered with the automatic control of sensitivity. Sensing in sinus rhythm was measured at 4.5 mV. Relocation of the lead to a septal position was associated with a modest increase in sensitivity (to 5.1 mV) during sinus rhythm, though enabled a reliable detection of the episode of induced VF.
Programmed for standard induction (ventricular sense : 1.2 mV; single treatment zone programmed at >171 bpm), then delivery of 20-J shock requiring a >3 sec charge time; the shock impedance was 40 ohms.
