Plethysmographic Perfusion Index and Fluid Responsiveness

In patients with acute circulatory failure, it is recommended to assess fluid responsiveness. Fluid responsiveness is defined by an increase in cardiac output of 10 to 15% after fluid loading. The assessment of fluid responsiveness usually needs cardiac output monitoring (or stroke volume). However, in limited resource settings or during the initial management of patients with acute circulatory failure, cardiac output measurement is usually unavailable. The perfusion index (PI) is derived from the plethysmographic signal of the pulse oximeter and represents the ratio between the ratio of pulsatile on non-pulsatile light absorbance of the plethysmography signal. The PI is influenced by vascular and stroke volume. The investigators hypothesized that the peripheral index could track the changes in cardiac output induced by fluid loading and therefore detect fluid responsiveness. Adult patients with acute circulatory failure in whom physicians want to test the fluid responsiveness will be included. At baseline, the PI will be recorded. An initial echocardiography will be performed to measure the left ventricular outflow tract velocity time integral (a surrogate of stroke volume). A fluid loading with 500 ml of 0.9% Saline or Ringer Lactate will be performed. After fluid administration, the velocity time integral and the PI will be collected. Fluid-responsive patients are defined by a 15% increase in velocity time integral. The investigators will analyze the ability of the PI to detect fluid responsiveness.