1. A plethysmograph variability method comprising: receiving a plethysmograph waveform having pulses corresponding to pulsatile blood flow within a tissue site; determining a plurality of perfusion values corresponding to the pulses, wherein determining the plurality of perfusion values comprises: identifying peaks and valleys of the pulses, and calculating differential values for the pulses from the peaks and the valleys of the pulses; determining a plurality of variability values each indicative of the variability of a series of the perfusion values, wherein determining a variability value of the plurality of variability values includes calculating a difference between a first perfusion value of a series of perfusion values and a second perfusion value of the series of perfusion values; determining a plethysmograph variability parameter representative of the plurality of variability values; and causing a display device to display the plethysmograph variability parameter.