An extra-cardiovascular implantable cardioverter defibrillator senses R-waves from a first cardiac electrical signal by a first sensing channel and stores a time segment of a second cardiac electrical signal in response to each sensed R-wave. The ICD determines intervals between successively sensed R-waves and, in response to at least a predetermined number of the intervals being less than a tachyarrhythmia detection interval, analyzes at least a portion of the time segment of the second cardiac electrical signal corresponding to a most recent one of the sensed R-waves to confirm the most recent one of the R-waves. The ICD updates an unconfirmed beat count in response to the most recent one of the R-waves not being confirmed and withholds detection of a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than a rejection threshold.