The effect of posture and anaesthetic technique on the surgical pleth index

Abstract

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

Assessing pain objectively: the use of physiological markers

“…Although pre‐clinical and clinical (intra‐operative and postoperative) research indicated its ability to discriminate strong noxious stimuli from no stimulation , the method could not consistently differentiate stimulus intensities . The surgical plethysmographic index is not specific to nociception, and is influenced by both peripheral and central sympathetic tone , sex , intravascular volume status , heart rate , drugs , location of the probe , posture , levels of consciousness and in awake patients, pain anticipation and emotional stress. Although the non‐invasive nature of the clinical application is appealing, this is offset by large inter‐patient variability.…”

Surgical pleth index: prediction of postoperative pain and influence of arousal

“…We also avoided patients in a steep Trendelenburg or anti-Trendelenburg position because this has been found to alter SPI. 13 The SPI values obtained in everyday clinical practice may therefore be different. However, the limitation of SPI recordings to the last 10 min of surgery may make the cut-off suggested by us a bit more useful.…”

Pulse Photoplethysmographic Analysis Estimates the Sympathetic Activity Directed to Heart and Vessels

“…11,[24][25][26] These effects are the principal determinants of PPGA changes because of sympathetic stimulation. [31][32][33][34] All these factors alter the ANS without affecting the nociception. The blood volume pulsations (ΔV) are related to both the systemic intravascular pulse pressure (ΔP) and the distensibility of the vascular wall (D) according to the relationship: 27 The distensibility is influenced by intravascular volume status and sympathetic activity directed to the vessels, whereas vagal fibers are absent in the peripheral vascular bed.…”