In fully awake patients under spinal anaesthesia, the SSI does not reflect the nociception-antinociception balance. This may be due to the influence of mental stress on the sympathetic nervous system. Even light sedation attenuates these influences.

SummaryThe surgical pleth index has been shown to correlate with surrogate variables of nociception during general anaesthesia, and it has been suggested to be of use as a depth of anaesthesia monitor. However, little is known about confounding factors. As the main determining variables are based on both central and peripheral autonomic regulatory mechanisms, we hypothesised that changing a patient's posture may produce a marked effect. We studied the effects of posture change in 45 patients who were randomly assigned to receive general (n = 15) or spinal anaesthesia with (n = 15) or without sedation (n = 15), as well as 15 awake volunteers. Mean (SD) values of the surgical pleth index after adoption of the lithotomy position were reduced from 57 (22) to 21 (6) under general anaesthesia, 63 (15) to 31 (9) under spinal anaesthesia alone, and 52 (14) to 22 (8) under spinal anaesthesia with sedation (all p < 0.01). In healthy volunteers, the surgical pleth index increased from 37 (13) to 57 (11) (p < 0.01) after 30°head-up tilt and was reduced from 35 (11) to 25 (11) after head-down tilt (p < 0.05). Change in posture has a marked effect on the surgical pleth index which lasts for at least 45 min, and this must be considered when interpreting the displayed values. The surgical pleth index (SPI) (formerly known as the surgical stress index, GE Healthcare, Helsinki, Finland) was developed as a tool to quantify the physiological reactions caused by nociception during general anaesthesia, and consists of a combined measurement of central (normalised heart beat interval) and peripheral (plethysmographic pulse wave amplitude) sympathetic tone. A more detailed description of the underlying algorithm has been published previously [1]. Several studies have investigated its performance during general anaesthesia [2][3][4][5][6], general anaesthesia combined with regional anaesthesia [7], emergence from anaesthesia [8] and spinal anaesthesia with and without sedation [9]. A number of confounding factors have already been demonstrated, including atropine administration, cardiac pacing [10], level of sedation [9] and intravascular volume status [11]. The SPI ranges from 0 to 100, and a high value is claimed to be associated with a high stress level. The optimal range has not yet been determined, but a SPI of 50 is supposed to reflect a mean stress level, with nociceptive and anti-nociceptive factors balanced.It is well known that changes in patients' position can lead to changes in the balance of the autonomic nervous system, and the tilt-table test is an accepted method of assessing the interplay between the sympathetic and parasympathetic branches of the autonomic nervous