Respiratory variations in the pulse oximeter plethysmographic waveform amplitude (ΔPOP) have been shown to be able to predict fluid responsiveness in mechanically ventilated patients. The main advantage of this index is that it is noninvasive, widely available, and inexpensive. Perfusion index (PI), the percentage between the infrared pulsatile and nonpulsatile signal, reflects the amplitude of the pulse oximeter waveform. Recently, Pleth Variability Index (PVI), derived from perfusion index, affords a continuous monitoring of ΔPOP. PVI has been shown to be correlated to ΔPOP and PPV and has been demonstrated to be equivalent to SVV as a predictor of fluid responsiveness in ventilated patients during major surgery. However, whether the PVI can predict the fluid responsiveness in acute respiratory distress syndrome necessitating low tidal volume and high PEEP is not clear.