A system including 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 plurality of R-waves in response to crossings of a first amplitude 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 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 plurality of R-waves sensed by the first sensing channel; determine a morphology match score for each of a plurality of the time segments of the second cardiac electrical signal, wherein the morphology match score is determined by performing a morphology matching analysis on a portion of the time segment compared to a R-wave template, and wherein the morphology match score indicates how well the morphology of the time segment matches the R-wave template; and withhold detection of a tachyarrhythmia episode based at least on the morphology match scores. 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 plurality of R-waves in response to crossings of a first amplitude 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 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 plurality of R-waves in response to crossings of a first amplitude 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 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 plurality of R-waves sensed by the first sensing channel; determine a morphology match score for each of a plurality of the time segments of the second cardiac electrical signal, wherein the morphology match score is determined by performing a morphology matching analysis on a portion of the time segment compared to a R-wave template, and wherein the morphology match score indicates how well the morphology of the time segment matches the R-wave template; and withhold detection of a tachyarrhythmia episode based at least on the morphology match scores. store a time segment of the second cardiac electrical signal in the memory in response to each one of the plurality of R-waves sensed by the first sensing channel; determine a morphology match score for each of a plurality of the time segments of the second cardiac electrical signal, wherein the morphology match score is determined by performing a morphology matching analysis on a portion of the time segment compared to a R-wave template, and wherein the morphology match score indicates how well the morphology of the time segment matches the R-wave template; and withhold detection of a tachyarrhythmia episode based at least on the morphology match scores.