Photoplethesmography (PPG) transmits infrared light from an emitting diode into the tissue. The light is reflected back from the blood within the cutaneous microcirculation. The amount of reflected light varies with the blood content of the microcirculation. The phototransducer is taped to the end of the toe and a small digital blood pressure cuff is placed at the base of the digit (TBI). The recordings reflect flow through the subpapillary venous plexus and are most useful at documenting skin flow and differentiating between organic and vasospastic disease.
When PPG is inconclusive, you can use the laser Doppler to detect minute microcirculatory blood because it is based on reflected light from moving blood particles, not on pulsatile changes in blood filling the digit.10