Many pulse oximetry technologies display a processed and filtered representation of the photoplethysmosgraphic
waveform. Each manufacturer uses unique proprietary algorithms to calculate the waveform displayed on the
pulse oximeter. Masimo has extracted and processed information from the waveform to create two physiologic
indices, perfusion index (PI) and pleth variability index (PVI). PI is calculated by indexing the infrared (IR)
pulsatile signal against the nonpulsitile signal and expressing this number as a percentage. Masimo SET®
PI has been shown to be useful to gauge the severity of illness in newborns1, 2 to assess the effectiveness of
epidural blocks3, 4 to indicate successful interscalene nerve block placement in awake patients5
and to quantify
peripheral perfusion for diagnosis of congenital heart disease in newborns.6
PVI is the first and only commercially
available measurement that automatically and continuously calculates the respiratory variations in the
photoplethysmosgraphic waveform. This paper will review why the photoplethysmosgraphic waveform reflects
changes that occur during the respiratory cycle and how monitoring those changes can be used to help clinicians
assess the physiological status of patients, including fluid responsiveness. We also will review the limitations
of PVI.