The Impact of a Preoperative Nerve Block on the Consumption of Sevoflurane

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:

Interscalene nerve block catheter insertion with 20 mL 0.5% ropivacaine injection (Treatment group) Interscalene nerve block catheter insertion with 20 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. The patients consenting for the study and meeting the eligibility criteria for the study will be randomized to either group-1 (PRE-GA) or group-2 (POST-GA) . Prior to induction of general anesthesia, all patients will receive an interscalene nerve block catheter (Pajunk e-cath) inserted under ultrasound guidance using the catheter-over-needle technique by an acute pain physician who has been performing interscalene blocks under ultrasound guidance for at least 5 years. Successful catheter placement will be verified by ultrasound visualization of the injectate spread. Based on the randomization, each patient will receive a clear 10 mL syringe containing either the Treatment or the Sham solution. Patients in group PRE-GA will receive 10 ml of 0.5% ropivacaine through the catheter before the start of surgery and 10 ml of saline (0.9%) at the end of surgery. The patients in group POST-GA will receive 10 ml of saline (0.9%) before the start of surgery and 10 ml of 0.5% ropivacaine at the end of surgery. All study medications will be prepared by the principal investigator who will be unblinded to the patient allocation. All other team members will be blinded to the group allocation.

Following the interscalene nerve block catheter insertion, all patients will receive general anesthesia with a standardized protocol using 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) if the Patient State Index (PSI) is >50 and/or 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. 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 transferred to recovery area where the distribution of the sensory or motor block will then be checked and recorded 30 minutes after arriving in PACU to document the success of the block in both groups by the nurses blinded to the group allocation. The Visual Analogue Pain Scale for pain will also be recorded in PACU at 0, 15, 30 and 45 minutes. Postoperative opioid consumption, nausea and vomiting scores and pan scores over the first 24 postoperative hours will also be collected for comparison.

Inclusion criteria

• Patients undergoing elective shoulder arthroscopy
• Patients eligible for interscalene brachial plexus block
• All adults > 18 years of age
• 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.