In the context of opioid sparing goal during general anaesthesia, the Analgesia nociception index (ANI) is an analgesia/nociception balance monitoring tool computed from high frequency heart rate variability analysis, and developed by MetroDoloris (Lille, France). Its sensitivity to detect noxious stimulus was studied in several studies. However, its specificity is not established in particular in the presence of factors influencing autonomic nervous system other than analgesia/nociception balance. In the context of anaesthesia, one of the most relevant factor is hypovolemia. This study aims to assess the effect of blood volume variations on ANI under general anaesthesia, independently of analgesia/nociception balance status. Variations of systemic venous return (SVR) (cardiac preload), which reflects blood volume status, will be induced by changes of operating table slope, from head-down tilt (Trendelenburg), with increased SVR, to head-up tilt position, with decreased SVR. Variations of SVR will be attested by changes in the arterial pulse pressure variation, a well-established indice of preload responsiveness in the mechanically ventilated anaesthetized patient.
The purpose of this study is to determine whether the value of analgesia nociception index (ANI) is influenced by variations of systemic venous return (cardiac preload) under general anaesthesia.