Utility of Surgical Plethysmographic Index (SPI) as a predictor of post operative pain and Emergence agitation in patients undergoing elective Otolaryngological surgeries: A prospective cohort study

Introduction

The Surgical Plethysmographic Index (SPI) is a proprietary technology of the GE Healthcare ventilators, which is used intra operatively as a measure of adequacy of analgesia. Pain from surgical stimulus causes tachycardia and vasoconstriction due to sympathetic activation.

SPI is calculated using the formula: 100 - ( 0.33* HBI + 0.67* PPGA) where HBI is the Heart Beat Interval and PPGA is the Photoplethysmographic waveform Amplitude. Tachycardia leads to decrease in HBI and vasoconstriction leads to decrease in PPGA which would thereby give an increased SPI value.SPI ranges from 1 to 100 and an ideal range should be kept between 20 to 50 for adequate analgesia.

Emergence agitation is a post anaesthetic phenomenon in which emergence from general anaesthesia is accompanied by psychomotor features such as agitation, confusion, disorientation and violent behavior.

Intra operative inadequacy of analgesia is one of the primary causes for post operative pain and emergence agitation

Need for the study

The incidence of post operative pain in patients undergoing general anesthesia can range from 20% up to 80% . Post operative pain often leads to prolonged hospital stay and delayed recovery.

The incidence of Emergence agitation in patients undergoing general anaesthesia is around 21.3%  but in ENT surgeries it can go up to 55.4% . This can lead to serious complications such as self extubation, removal of arterial/ IV lines, injury to medical staff and damage to hospital property.

There is a dearth of literature which studies the utility of SPI in predicting postoperative pain and Emergence agitation.

The conventional method of measuring intra operative adequacy of analgesia by using hemodynamics alone (NIBP and Heart Rate) is not always accurate and leads to either inadequate or excess of analgesic administration.

SPI is a simple, non invasive, real time monitoring of adequacy of intra operative analgesia and its utility will be tested through this study, to check if incidence of post operative pain and emergence agitation can be better predicted

Methodology

All relevant patient demographic details: Age, gender, surgery name, ASA grade, medication history and pre op ECG will be noted

Intraoperative

• Pre induction values of SPI, Heart Rate, Mean Arterial Pressure will be noted.

• Premedication with 0.03 mg/kg of injection midazolam, 10 mcg/kg of injection glycopyrrolate and induction will be with 2 mcg/kg of injection fentanyl intravenously and injection propofol at 2 mg/kg, along with muscle relaxation using injection Atracurium 0.5 mg/kg given intravenously.

• Post induction (prior to surgical stimulus) values of SPI, HR and MAP will be noted.

• Anaesthesia will be maintained with air, oxygen, IPPV, inhalational agent targeting a MAC 1-1.2 and top-up doses of injection Atracurium (0.1 mg/kg

• At the end of the surgery, once there is no surgical stimuli - SPI, HR and MAP values will be noted every 1 minute for 5 minutes until reversal. The residual neuromuscular blockade will be reversed at the end of surgery by using an injection of neostigmine 0.05–0.07 mg/kg and an injection of glycopyrrolate 10 mcg/kg. All values observed will be on the continuous scale.

• Post tracheal extubation, Emergence Agitation will be assesed using the modified Riker Sedation Agitation Scale every 10 minutes until discharge from the PACU.

• Total intra operative opiod usage will be noted at the end of every surgery

Postoperative:

• After extubation, patient will be admitted in the post operative room. Once able to communicate, Post operative pain will be assessed using Numeric Rating Scale(0-10) every 3 minutes for 15 minutes

• Emergence agitation will be assessed using the Modified Riker Sedation Agitation Scale (RSAS) post extubation and every 10 mins till discharge from PACU