New predictors of fluid responsiveness have been obtained from plethysmographic waveforms displayed on pulse oxymeters. However, they require recordings on a PC and offline operator-dependent analysis. A new parameter called the plethysmography variability index (PVI) has been proposed by a pulse oxymetry manufacturer to be used for the purpose of fluid responsiveness. Its advantage is that it can be automatically calculated and displayed on the screen of the pulse oxymetry monitor. The aim of the study is to test the accuracy of this parameter to predict fluid responsiveness in critically ill patients.