Shock lead impedance alert: replace or reconsider?

A 61-year-old woman presented to our outpatient department because of audible alerts by her implantable cardioverter-defibrillator (ICD) starting about 2 weeks ago. The ICD (Medtronic Maximo VR 7232) was implanted in 2004 after cardiopulmonary resuscitation caused by spontaneous sustained ventricular tachycardia. Her medical history was significant for restrictive cardiomyopathy, with predominant right ventricular dysfunction, high-grade tricuspid valve insufficiency treated with tricuspid prosthetic valve replacement in 2000, mildly impaired left ventricular function, and permanent atrial fibrillation. Because of the prosthetic valve replacement, stimulation electrodes were implanted epimyocardially and 2 single-finger defibrillation leads were positioned subcutaneously (Medtronic 6996/SQ-Finger). All electrodes were connected to the ICD, which was placed beneath the diaphragm. High-voltage B (HVB) lead impedance is measured against the ICD can and superior vena cava (SVC) lead in parallel. The SVC lead impedance is measured between both finger leads. Interrogation of the ICD revealed a decrease in HVB lead impedance below the limit of 20 Ω about 2 weeks before presentation, causing the acoustic alert (Figure 1). At the time of presentation, HVB lead impedance was 21 Ω, SVC lead impedance was 36 Ω, pacing impedance was 544 Ω, pacing threshold was stable at 0.1 ms at 3 V, and ventricular amplitude was >5.6 mV. Since the last interrogation 3 months earlier, 15 episodes of nonsustained ventricular tachycardia were detected without the need for antitachycardia therapy. It is noteworthy that …