Surgical pleth index monitoring in perioperative pain management: usefulness and limitations

Abstract

Surgical pleth index (SPI) monitoring is a representative, objective nociception-monitoring device that measures nociception using photoplethysmographic signals. It is easy to apply to patients and the numerical calculation formula is intuitively easy to understand; therefore, its clinical interpretation is simple. Several studies have demonstrated its efficacy and utility. Compared with hemodynamic parameters, the SPI can detect the degree of nociception during surgery under general anesthesia with greater accuracy, and therefore can provide better guidance for the administration of various opioids, including remifentanil, fentanyl, and sufentanil. Indeed, SPI-guided analgesia is associated with lower intraoperative opioid consumption, faster patient recovery, and comparable or lower levels of postoperative pain and rates of adverse events compared with conventional analgesia. In addition, SPI monitoring allows for the degree of postoperative pain and analgesic requirements to be predicted through the SPI values immediately before patient arousal. However, because patient age, effective circulating volume, position, concomitant medication and anesthetic regimen and level of consciousness may be confounding factors in SPI monitoring, clinicians must be careful when interpreting SPI values. In addition, as SPI values can differ depending on anesthetic and analgesic regimens and the underlying disease, an awareness of the effects of these variables with an understanding of the advantages and disadvantages of SPI monitoring compared to other nociception monitoring devices is essential. Therefore, this review aimed to help clinicians perform optimal SPI-guided analgesia and to assist with the establishment of future research designs through clarifying current usefulness and limitations of SPI monitoring in perioperative pain management.

Keywords: Analgesia; Autonomic nervous system; General anesthesia; Intraoperative monitoring; Nociception test; Pain measurement; Photoplethysmography..

Fig. 1.

Determinants of the SPI and mechanism of increased SPI values by surgical stimulus. A surgical stimulus increases the heart rate and vascular tone by increasing sympathetic tone; consequently, both the HBI and PPGA decrease, which inversely increases the SPI value. SPI: surgical pleth index, HBI: heartbeat interval, PPGA: photoplethysmographic waveform amplitude, LED: light-emitting diode.

Fig. 2.

Schematic diagram of the change in the SPI in response to nociceptive stimuli according to population age groups. In adults without severe underlying diseases, the target range of the SPI for adequate intraoperative analgesia during general anesthesia is usually 20–50, based on evidence from numerous studies (A). In children, the increased baseline heart rate results in less of a change in the HBI, and children are less likely to show a prominent reduction in the PPGA from sympathetic stimulation owing to lower vascular wall stress with higher distensibility. These conditions result in an underestimation of the SPI value; thus, an SPI < 40 is the target range for adequate intraoperative analgesia (B). In the elderly, the magnitude of change in the PPGA resulting from sympathetic stimulation is higher due to increased stiffness in the small and large arteries . In contrast, the change in the heart rate in response to stimuli decreases with age because of autonomic functional degeneration. These two factors have the effect of offsetting each other; thus, the SPI is maintained at a reference range of 20–50, similar to that in normal healthy adults (C). ∆: change, SPI: surgical pleth index, HR: heart rate, HBI: heartbeat interval, PPGA: photoplethysmographic waveform amplitude.