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, and generates a blood volume index in response to a light source of the plurality of light sources emitting light at an isobestic wavelength. The blood volume index is then utilized to verify the identifying of the cardiac event.