in outpatient settings, pulse oximeters are commonly worn on the finger. however, in cases of shock, hypothermia, etc. blood flow to the periphery can be reduced, resulting in a ppg without a discernible cardiac pulse. in this case, a ppg can be obtained from a pulse oximeter on the head, with the most common sites being the ear, nasal septum, and forehead. ppg can also be configured as multi-site photoplethysmography (mppg), e.g. making simultaneous measurements from the right and left ear lobes, index fingers and great toes, and offering further opportunities for the assessment of patients with suspected peripheral arterial disease, autonomic dysfunction, endothelial dysfunction, and arterial stiffness. mppg also offers significant potential for data mining, e.g. using deep learning, as well as a range of other innovative pulse wave analysis techniques.