A medical device comprising: a sensing circuit comprising: a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device and to sense a first plurality of R-waves in response to crossings of a first R-wave sensing threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device and different than the first extra-cardiovascular sensing electrode vector; a memory; and a control circuit coupled to the sensing circuit and the memory and configured to: store a time segment of the second cardiac electrical signal in the memory in response to each one of the first plurality of R-waves sensed by the first sensing channel; determine a first plurality of intervals between successive ones of the first plurality of R-waves sensed by the first sensing channel; in response to at least a first predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, analyze at least a portion of the time segment of the second cardiac electrical signal corresponding to a most recent one of the R-waves sensed by the first sensing channel to confirm the most recent one of the R-waves; update an unconfirmed beat count in response to the most recent one of the R-waves not being confirmed based on the analyzing of at least the portion of the corresponding time segment; in response to a second predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, compare the unconfirmed beat count to a rejection threshold; and withhold detection of a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than the rejection threshold. a sensing circuit comprising: a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device and to sense a first plurality of R-waves in response to crossings of a first R-wave sensing threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device and different than the first extra-cardiovascular sensing electrode vector; a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device and to sense a first plurality of R-waves in response to crossings of a first R-wave sensing threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device and different than the first extra-cardiovascular sensing electrode vector; a memory; and a control circuit coupled to the sensing circuit and the memory and configured to: store a time segment of the second cardiac electrical signal in the memory in response to each one of the first plurality of R-waves sensed by the first sensing channel; determine a first plurality of intervals between successive ones of the first plurality of R-waves sensed by the first sensing channel; in response to at least a first predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, analyze at least a portion of the time segment of the second cardiac electrical signal corresponding to a most recent one of the R-waves sensed by the first sensing channel to confirm the most recent one of the R-waves; update an unconfirmed beat count in response to the most recent one of the R-waves not being confirmed based on the analyzing of at least the portion of the corresponding time segment; in response to a second predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, compare the unconfirmed beat count to a rejection threshold; and withhold detection of a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than the rejection threshold. store a time segment of the second cardiac electrical signal in the memory in response to each one of the first plurality of R-waves sensed by the first sensing channel; determine a first plurality of intervals between successive ones of the first plurality of R-waves sensed by the first sensing channel; in response to at least a first predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, analyze at least a portion of the time segment of the second cardiac electrical signal corresponding to a most recent one of the R-waves sensed by the first sensing channel to confirm the most recent one of the R-waves; update an unconfirmed beat count in response to the most recent one of the R-waves not being confirmed based on the analyzing of at least the portion of the corresponding time segment; in response to a second predetermined number of the first plurality of intervals being less than a tachyarrhythmia detection interval, compare the unconfirmed beat count to a rejection threshold; and withhold detection of a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than the rejection threshold.