Pleth Variability Index (PVI) is a non-invasive and automated measure of the respiratory variations of plethysmography during mechanical ventilation. PVI is extracted via an algorithm implemented on Masimo Radical 7 device (Masimo, Irvine, CA). PVI is commonly used in anesthesiology and has been validated to predict fluid responsiveness and optimize fluid administration in the surgical setting. However, the signal/noise ratio of PVI makes acute changes in PVI somewhat difficult to interpret at the bedside. Subsequently, a new algorithm entitled RPVI (Rainbow Pleth Variability Index), has been developed in order to improve the signal/noise ratio of PVI and facilitate its clinical use by practitioners. The details of this new proprietary built-in algorithm are unknown and no data are available to date. Therefore, the main objective of the study was to compare RPVI and PVI during dynamic changes in venous return induced by a tidal volume (Vt) challenge during mechanical ventilation in anesthetized patients. The hypothesis was that the agreement between both dynamic indices would be good.

Inclusion Criteria:

• Age > 18
• Low or intermediate risk surgery or noninvasive procedures in the supine position
• General anaesthesia and volume-controlled mechanical ventilation.

Exclusion Criteria:

• Non sinus heart rhythm
• Left Ventricular Ejection Fraction < 50%
• Documented right ventricle failure
• Lack of patient consent
• Pregnancy
• Minor patient or under tutorship
• Presence of spontaneous ventilatory movements (attested by a real respiratory rate higher than the set respiratory rate)
• Report HR / RR <3.6 (HR: heart rate, RR: respiratory rate)
• Open chest surgery
• Laparoscopic surgery
• Vital or lateral decubitus surgery

All adult patients undergoing closed-chest surgery in the supine position under general anaesthesia.