The Impact of an Epidural Anesthetic on the Consumption of Sevoflurane in Major Abdominal Surgery

Major abdominal surgery - like non-laparoscopic bowel resection, liver, gastric, or pancreas surgery - is performed under general anesthesia. Pain control for after the surgery can be achieved purely with intravenous and oral pain medication or in combination with freezing of the nerves. Nerve freezing (nerve block) placed before surgery has the potential to substantially reduce the amount of inhaled anesthetic given to the patient during surgery. This can benefit the patient with being more awake and crisp more quickly after surgery. It can also reduce cost to the system. A further benefit which has received very little attention so far, is that reducing the amount of inhaled anesthetic given also lowers the environmental footprint created by the anesthetic. The investigators of the proposed study plan to quantify the amount of inhaled anesthetic used for each case and will compare how the consumption is affected by whether the epidural block is applied before or after surgery. Patients will have a nerve block catheter (epidural catheter) placed prior to the induction of general anesthetic by an experienced regional anesthesiologist. The epidural catheter will be bolused with a solution to which the anesthesiologist is blinded which will either be local anesthetic or dextrose (sham). The general anesthetic will be conducted according to a the protocol with the aim of maintaining a standard anesthetic depth monitored by patient state index (PSI). Measurements of the MAC-Value (minimum alveolar concentration) of inhaled anesthetic will be recorded every five minutes and will be noted down by a blinded observer. At the end of the case the anesthesiologist blinded to the solution will inject another solution (now a dextrose (sham) or local anesthetic before the patient is woken up.

Following ethics approval, eligible patients meeting the inclusion/exclusion criteria will be consented in pre-assessment unit or day surgery ward at least 2 hours prior to their surgery. Patients will then be randomized into two groups: Epidural catheter insertion with 14 mL of a solution containing 0.125% bupivacaine and 2 mcg/ml sufentanil injection (Treatment group) Epidural catheter insertion with 14 mL normal saline injection (Sham group) The patient, anesthesia provider, data recorder, OR staff and Post Anaesthetic Care Unit (PACU) nurses, will be blinded at the beginning of the case. After randomization, each patient will receive a clear 20 mL syringe containing either the Treatment or the Sham solution. Prior to induction of general anesthesia, an epidural catheter will be inserted by an experienced acute pain physician who has been performing epidural blocks for at least 5 years. Catheter placement will be verified by a single test dose of 60 mg lidocaine to exclude subarachnoid tip location. Induction of general anesthesia will follow a standardized protocol with the intravenous administration of fentanyl (2 mcg/kg), propofol (2 mg/kg), and rocuronium (0.6 mg/kg). General anesthesia will initially be maintained with sevoflurane at 1.0 age-adjusted Minimal Alveolar Concentration (MAC). Subsequently, the age-adjusted MAC will be titrated to achieve an intraoperative PSI target of 25-50 (Sedline, Masimo®), and intraoperative heart rate (HR) and mean blood pressure (MBP) target of +/-30% baseline values. The anaesthesiologist will have the ability to administer IV boluses of analgesic (remifentanil 0.5mcg/kg) at if the PSI is <50 and the MBP or HR is above 30% of baseline. Vasopressors such as phenylephrine (100 mcg IV bolus) and ephedrine (5 mg IV bolus) can be used as last-line therapy to treat hypotension that is unresponsive to MAC adjustments. The age-adjusted MAC value will be recorded every 5 minutes starting at the time of skin incision until the time of skin closure. In addition, intraoperative analgesic and vasopressor usage are recorded. The epidural catheter will be run continuously throughout the surgery with 4-6 ml/h with study drug number 1 as per randomization - so either treatment or sham. After the conclusion of MAC recording and prior to emergence from general anesthesia, the anaesthesiologist will be unblinded to the randomized groups, and the patients who received Sham solution will be given 20 mL of the treatment solution via the interscalene catheter to ensure patients receive adequate analgesia postoperatively. Postoperatively, patients will be transported to PACU where the distribution of the sensory or motor block will then be checked and recorded 30 minutes after arriving in PACU in both groups. The NRS for pain will also be recorded in PACU at 0, 15, 30 and 45 minutes.

14 mL injection of a solution containing 0.125% bupivacaine and 2 mcg/ml sufentanil before the start of surgery and 14 ml of 5% dextrose at the end of surgery into the epidural catheter

14 mL injection of 5% dextrose before the start of surgery and 14 ml of a solution containing 0.125% bupivacaine and 2 mcg/ml sufentanil at the end of surgery into the epidural catheter

The average age-adjusted end- tidal minimum alveolar concentration (MAC) will be calculated by averaging the end tidal MAC required to maintain the depth of anesthesia as measured by PSI between 25-50 intra-operatively. This will be compared between the two groups.

Intraoperative heart rate (in beats/minute) will be documented at 5 minute interval and compared between the two groups

Intraoperative use of short acting opioid (in microgram/Kg) as per the study protocol will be recorded and compared between the two groups

Total amount of opioids (in mg of morphine equivalents) in the PACU and over the first 24 postoperative hours will be recorded and compared between the two groups

Maximum pain scores will be recorded using numerical rating scale (NRS) of 0-10 (where 0=no pain and 10= maximum pain) on arrival to PACU and at 0,15,30, 45 minutes and at 24 postoperative hours and compared between the two groups

Nausea scores (on a Likert scale of 1-4 where 1 = no nausea, 2= mild nausea, 3= moderate nausea and 4= severe nausea) and incidence of vomiting will be recorded on arrival to PACU and at 0,15,30, 45 minutes and at 24 postoperative hours and will be compared between the two groups

Frequency and total consumption of opioids (in morphine equivalents) over the first 24 postoperative hours will be recorded and compared between the two groups

Sedation scores will be recorded at 15 minute intervals based on Ramsay sedation scale (1-6) and will be compared between the two groups

Sensory testing of the abdominal dermatomes will be performed in the PACU at 30 minutes of arrival to PACU to document the success of the block

Amount and frequency of the intraoperative usage of vasopressor (phenylephrine in micrograms) will be documented and compared between the two groups

Intraoperative mean blood pressure (in mm Hg) will be documented at 5 minute interval and compared between the two groups

Inclusion Criteria: Patients undergoing elective reconstructive foot and ankle surgery Patients eligible for popliteal and saphenous nerve block All adults 18 years of age or older Capable to give consent Exclusion Criteria: Patients who are unable to give consent Local anaesthetic allergy Hemidiaphragm paresis on the contralateral side to the block/surgery site Bleeding diathesis Coagulopathy Pre-existing neurological deficits Patients with a Body Mass Index >35 Patients with significant comorbidities, physiological limitations, and allergies that are unable to tolerate the protocolized induction and maintenance of anesthesia.