Background: Somatosensory- (SSEPs) and motor-evoked potentials (MEPs) are commonly used as an intraoperative neurophysiologic monitoring tool to detect aberrations to the spinal cord integrity during spinal surgery. Inhalational anaesthetic agents have a significant influence on evoked potentials by suppressing the amplitude and prolonging the latency. Evidences suggest that total intravenous anaesthesia (TIVA) is superior to inhalation anaesthesia for neuromonitoring in spinal surgery, and support the use of up to 0.5 minimum alveolar concentration (MAC) of inhalational anaesthetic agents in these procedures.

Methods: Patients undergoing spinal surgeries will be prospectively recruited and be induced with TIVA at baseline of the evoked potentials. They will be randomized to receive balance anaesthesia with A) Desflurane MAC 0.5+remifentanil, or B) Desflurane MAC 0.75+remifentanil until the end of surgery. Influence of 1.0 MAC desflurane on the evoked potentials after the completion of surgery will also be studied.

Desired Results: Endpoints include the changes to the amplitude and latency of SSEPs and MEPs with 0.5 and 0.75 MAC desflurane. The findings will indicate the safety of desflurane at the studied concentrations for spinal surgeries, and substantial savings with the use of desflurane instead of TIVA.

Recording of Evoked Potentials characteristics:

The following characteristics of the evoked potential signals will be recorded:

• Amplitude: Size of the response, measured in microvolts (μV)
• Latency: Speed of the response, measured in milliseconds (ms)
• Morphology: Appearance of the response - Simple/Complex wave shape Baseline evoked potentials will be recorded at various points of surgery.

Anaesthesia variables:

At the time of recording of evoked potentials:

- Intraoperative temperature, blood pressure, ETCO2, anaesthesia depth - bispectral index monitoring (BIS), remifentanil TCI, MAC of desflurane, Train of Four, volume of propofol and remifentanil until randomization, total volume of desflurane and remifentanil used, anaesthetic medications used, FiO2, SpO2

Patient characteristics:

- Age, gender, race, weight, BMI, medications used, haematocrit, preop blood glucose (for Diabetics)

Safety considerations:

'Recommended standards for intraoperative monitoring of somatosensory evoked potentials', Guideline 11B of the American Clinical Neurophysiology Society (2009) and 'Intraoperative motor evoked potential monitoring - A position statement by the American Society of Neurophysiological Monitoring' will form the basis of the safety aspects of the study.

Conduct of Anaesthesia:

• The induction of anaesthesia will be achieved using targeted propofol and remifentanil infusions (TIVA). Muscle relaxants atracurium, rocuronium or suxamethonium will be used to facilitate intubation.
• Anaesthesia will be maintained on TIVA until the patient has recovered from muscle relaxant. Baseline characteristics (BC1) of the evoked potentials in TIVA will be obtained at this stage.
• Once the BC1 has been obtained, the patients will be randomized to two groups:

Group A- Anaesthesia maintained with 0.5 MAC desflurane/remifentanil infusion Group B- Anaesthesia maintained with 0.75MAC desflurane/remifentanil infusion

• Baseline characteristics 2 (BC2) will be obtained and the decision to proceed with 0.5 or 0.75 MAC desflurane made.
• On completion of the neuromonitoring phase of surgery, as defined by completion of instrumentation and start of closure, all patients in Group A and B will be maintained with 1.0 MAC desflurane with remifentanil infusion. Baseline characteristics 3 (BC3) will be recorded at this stage.
• The remifentanil infusion will be titrated at various stages of surgery to maintain a stable BIS and will also be based on haemodynamic variables.

All patients will get additional analgesics and antiemetics as deemed necessary. Post-operative recovery will follow the standard industry norms.

Inclusion Criteria:

• Patients aged ≥ 21 years old, scheduled for spinal surgery.
• Patients who require SSEP/MEP and with no neurological deficits.
• Patients of American Society of Anesthesiologist I-III physical status.
• Consenting patients.
• Elective procedures.

Exclusion Criteria:

• Patients with sensory or motor deficits preoperatively.
• Patients with significant cardiovascular and/or respiratory disease.
• Emergency procedures.
• Cord injury secondary to trauma.
• Non-consenting patients.
• Category A patients (Prisoners).
• Patients with previous stroke.
• Patients with Hep C, HIV.