A 56-year-old woman with ischemic cardiomyopathy, left ventricular ejection fraction of 20%, complete left bundle branch block and NYHA functional class III underwent implantation of an InSync Sentry CRT-D for primary prevention, with VF induced at the end of the procedure.
During this induction, the operator noted considerable ventricular undersensing due to a wide variability in the amplitude of the ventricular signal interfering with the device’s automatic sensitivity control. The failure to detect the low amplitude VF signals occurred after a higher amplitude signal. Recall that the detection level fits the amplitude of the preceding QRS. Because of the risk of delayed treatment, the operator proceeded with the delivery of a manual shock. In this patient, the 3.2 mV ventricular sensing in sinus rhythm was insufficient. The lead was moved from the apex of the right ventricle to a high septal position, a site that allowed reliable sensing to 5.6 mV. On subsequent induction VF was accurately sensed and effectively terminated by a 20-J shock.
Following VF induction, the 25 J first shock required a >4 sec charge time; the shock impedance was 46 ohms. The second shock of 35 J required a >6 sec charge time; the shock impedance was 47 ohms.
