Application of Surgical Pleth Index (SPI) in Pain Monitoring

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【Definition】

The Surgical Pleth Index (surgical pleth index, SPI) is a monitoring tool that measures the photoplethysmographic signals at the fingertip to monitor the nociception-anti-nociception balance during general anesthesia.

【Principle】

Stress stimuli activate the peripheral sympathetic nervous system, inducing distal vasoconstriction. By measuring the absorption of red light at the fingertip, we can calculate the photoplethysmographic pulse wave amplitude (PPGA) and the heart beat interval (HBI) to construct a bivariate model.

SPI = 100 – (0.33 × HBI + 0.67 × PPGA)

SPI is measured and calculated using a reusable SpO2 monitoring probe on the monitor (GE), characterized by non-invasiveness, simplicity, and speed.

Application of Surgical Pleth Index (SPI) in Pain Monitoring

Multiple studies have confirmed the effectiveness of SPI in quantifying the intensity of nociceptive stimulation during general anesthesia:

Implementing intraoperative analgesia guided by SPI significantly reduces the consumption of opioids during surgery compared to traditional hemodynamic parameters.

It shortens extubation time and reduces the incidence of postoperative nausea and vomiting.

Setting the SPI target range of 20-50, avoiding a rapid increase in SPI >10

In children, SPI < 40 is the target for adequate intraoperative analgesia.

3.2 Anesthesia Protocol

1. Total Intravenous Anesthesia

Most randomized controlled studies have adopted total intravenous anesthesia, finding that implementing analgesia guided by SPI can reduce the consumption of opioids during surgery.

2. Inhalation Anesthesia

In a study by Gruenewald et al., using sevoflurane, analgesia guided by SPI did not reduce the consumption of opioids (sufentanil) during surgery; in another study by Jain et al., using isoflurane, the consumption of fentanyl during analgesia guided by SPI was higher.

3.3 Cardiovascular Drugs and Cardiovascular Diseases

1. Vasoactive Drugs

Ephedrine, norepinephrine, and nicardipine may interfere with the accuracy of SPI monitoring by altering PPGA and HBI.

2. Atropine, Pacemaker

Hocker et al. found that in patients under general anesthesia, using a pacemaker or atropine to increase HR would cause SPI to rise; after resetting the pulse oximeter, the SPI increase caused by the pacemaker was calibrated, but this did not occur in the atropine group.

3.4 Blood Volume and Position

1. Blood Volume

SPI may decrease with the worsening of hypovolemia;

2. Position

In urological surgeries, an increase in SPI was observed at a 30° head-up position, while a decrease occurred at a head-down position, lasting for at least 45 minutes; another study also reported the impact of prone position on SPI during spinal surgery under general anesthesia, where prone position caused a significant increase in SPI, possibly due to increased sympathetic tone, gradually decreasing over the following 20 minutes, and moving patients from supine to prone position increased SPI without nociceptive stimulation.

3.5 Conscious State

The balance of sympathetic and vagal nerves is influenced by awakening and emotions;

In the awake state, SPI has no correlation with stress hormones (including ACTH, cortisol, epinephrine, and norepinephrine), but shows moderate correlation in patients under general anesthesia.

Summary

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1. SPI monitors the nociception-anti-nociception balance during general anesthesia by measuring the photoplethysmographic signals at the fingertip;

2. A large body of evidence shows that in adults without serious underlying diseases, the target range for adequate analgesia during general anesthesia is usually 20-50;

3. In children, SPI < 40 is the target for adequate intraoperative analgesia;

4. The effectiveness of SPI in monitoring analgesia during surgeries using total intravenous anesthesia has been confirmed, while its effectiveness in inhalation anesthesia surgeries remains controversial;

5. Drugs such as ephedrine, norepinephrine, nicardipine, atropine, and pacemakers may interfere with the accuracy of SPI monitoring;

6. Blood volume and position may also interfere with the accuracy of SPI monitoring.

●Other applications of BIS monitoring in clinical practice

●Electrical injuries caused by muscle relaxant monitoring

●Invasive arterial blood pressure monitoring: The unsung hero of precision medicine

●Monitoring urine output intraoperatively is important and closely related to postoperative renal dysfunction

●Literature study | Ward Monitoring 4.0: Understanding perioperative organ dysfunction through continuous blood glucose monitoring

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The original or reprinted content published on this WeChat public platform does not represent the views or positions of Voice of Miller. The content regarding drug use, disease diagnosis and treatment is for reference by medical professionals only.

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Application of Surgical Pleth Index (SPI) in Pain Monitoring

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