1. A method comprising: capturing a plurality of images corresponding to an area beneath a surface of skin using a fingerprint sensor configured to capture a live scan of a fingerprint pattern from a finger on a touch surface on a portable electronic device; comparing image characteristics corresponding to at least a first image of the plurality of images with image characteristics corresponding to at least a second image of the plurality of images; and determining a physiological parameter based on comparing the image characteristics, wherein the method further comprises: operating the fingerprint sensor in a fingerprint reader mode including capturing a live scan of a fingerprint pattern from a finger on the touch surface to authenticate a user of the portable electronic device; operating the fingerprint sensor at a first number of frames per second to authenticate a user of the portable electronic device; changing fingerprint sensor parameters to switch the fingerprint sensor from the fingerprint reader mode to a physiological parameter determination mode; and increasing the first number of frames per second to a second number of frames per second to operate the fingerprint sensor as a physiological parameter monitor, and wherein the physiological parameter comprises a heart rate or oxygen consumption.