A method of detecting a cardiac event in a medical device that includes sensing cardiac signals from a plurality of electrodes forming a first sensing vector and a second sensing vector, determining inflections of the sensed cardiac signals, generating a pulse amplitude threshold in response to the determined inflections, determining whether the sensed cardiac signals are corrupted by noise in response to the determined inflections and the generated pulse amplitude threshold, and determining, in response to the sensed cardiac signals being corrupted by noise, whether the sensed cardiac signals are both corrupted by noise and shockable.