A medical device for sensing cardiac events that includes a plurality of electrodes sensing cardiac signals utilized to identify a cardiac event, a plurality of light sources capable of emitting light at a plurality of wavelengths, and a detector to detect the emitted light. A processor determines a plurality of light measurements in response to the emitted light detected by the detector, an isobestic blood volume index in response to determined light measurements of the plurality of light measurements from a first light source of the plurality of light sources emitting light at an isobestic wavelength, determines an oxygen index associated with light measurements of the plurality of light measurements from a light source of the plurality of light sources other than the first light source, and verifies the identifying of the cardiac event in response to the determined isobestic blood volume index and the determined oxygen index.