Correlation of Different Time Measurements of the Surgical PLETH Index With Postoperative Pain; a Prospective Observational Study

Despite the major progress in anesthetic techniques, postoperative pain is still considered a major problem during practice. (1-3). Leading to many co-morbidities, one to two-thirds of patients will suffer postoperative pain. These co-morbidities can include; pulmonary complications, cardiac complications, and delirium(4). Numerous risk factors are associated with the emergence of postoperative pain, including; younger age, female sex, preoperative pain, and extensive surgical procedure. The severity of postoperative pain may vary among patients undergoing the same operation (5-10).

Many techniques have been evolved to monitor nociception and predict postoperative pain intensity; one of the most recent techniques is the surgical pleth index (SPI)(11).SPI is a noninvasive dimensionless score; its value is obtained from heartbeat interval and pulse wave amplitude monitored by pulse oximetry probe.SPI reflects the sympathetic response of the patient to the surgical stimuli(12-13). It was reported that SPI is better than other parameters like heart rate and blood pressure for detecting the balance between nociceptor activation and analgesia(14-15), and its value is correlated with the severity of postoperative pain. SPI values range from 0 to 100, and higher values indicate strong surgical stimulus (16). SPI can be used as a guide for intraoperative analgesia; hence, it can be a valuable tool to assess the analgesic requirement and limit opioid consumption, both preoperative and postoperative(17).

Several studies have been performed to predict the severity of postoperative pain using SPI in adults and children (16,18). It was also used successfully to expect a hemodynamic response to tracheal intubation and skin incision (19) and monitor nerve block success. (20).

However, since SPI has emerged, the most sensitive cut-off value that correlates well with postoperative pain severity remained debatable. Recent studies reported a value of 30 as a cut-off value of SPI. (21). On the other hand, the time of measurement to rely on was debatable. Most studies recommended that a measure of SPI before recovery can be used. However, a more recent study suggested that SPI response to surgical incision is highly correlated with postoperative pain and opioid consumption (18). Based on these data, we hypothesized that both measurements are correlated with postoperative pain and aimed to test which measure is more correlated.