Anti-tachycardia pacing degenerated fast ventricular tachycardia into undetectable life-threatening tachyarrhythmia in a patient with non-ischemic dilated cardiomyopathy.

A 45-year-old man with dilated cardiomyopathy was admitted to our hospital due to congestive heart failure (CHF). Despite the optimal medical treatment, his condition had not improved because of severe left ventricular dysfunction. Because he experienced non-sustained ventricular tachycardia (VT), a biventricular implantable cardioverter-defibrillator (Bi-V ICD) was implanted for reduction of dyssynchrony and primary prevention of lethal tachyarrhythmia. After discharge, he developed CHF and was transported to our hospital by ambulance. In the ambulance, monomorphic sustained VT with 200 bpm suddenly occurred. The ICD detected it as fast VT and anti-tachycardia pacing (ATP) was delivered. After the ATP therapy, RR intervals of VT became irregular and prolonged. Ventricular fibrillation-like electrical activity was recorded by a far-field electrogram from the defibrillator, but the tachycardia cycle length exceeded 400 ms which is under the tachycardia detection rate. The device failed to deliver a shock and the patient had to be rescued with an external shock. This is a rare case of fast VT that degenerated into undetectable life-threatening tachyarrhythmia by ATP.