Patients with an implantable cardioverter defibrillator (ICD) can undergo inappropriate therapies (shock or antitachycardia pacing) if electrical activity not originating in the ventricle is wrongly recognized as ventricular by the device. Inappropriate therapy leading to shocks is not innocuous; consequences include pain and its psychological sequelae, particularly anxiety, as well as induction of ventricular arrhythmias and decreased battery longevity. Inappropriate therapies might result from (1) detection of supraventricular tachyarrhythmias or (2) oversensing
artifacts by the device. To avoid inappropriate therapies due to detection of supraventricular tachyarrhythmias, enhanced detection criteria, such as sudden onset and stability, have been developed. Sudden onset refers to the degree of prematurity of the initial beat of tachycardia with respect to the previous ones. Stability refers to the degree of regularity of arrhythmia. If programmed on, both criteria must be met to initiate the therapy. Previous history of atrial fibrillation and NYHA class I should be considered as risks for the occurrence of inappropriate therapies in single-chamber ICD recipients.