This study was a prospective non-randomized and comparative study (quasi-experimental) conducted in the neurosurgery operation theatre of a tertiary care institute. Adult patients of the age group 18–60 years presenting for any spinal surgery requiring MEP monitoring as part of their operating procedure were considered for recruitment in a consecutive manner between January 2020 and March 2021. After getting approval from the institutional ethics committee (IEC No. 64/19), informed consent was taken from patients satisfying the inclusion criteria in the study. Patients who met the recruitment criteria were assigned into two groups of 64 patients each, labelled as the propofol group (group P) and dexmedetomidine group (group PD).

Inclusion criteria 1.

ASA physical status I and II

2.

Age between 18 to 60 years of either sex of GCS 15

3.

Elective spine surgery under GA

4.

Patients who have given valid informed consent

Exclusion criteria 1.

Patient not satisfying inclusion criteria

2.

Baseline heart rate < 60 beats/min

3.

Patient with OSA and morbid obesity and on chronic opioid analgesic.

4.

Patient on beta-blockers or sick sinus syndrome.

5.

Having contraindications of MEP monitoring—epilepsy, cortical lesion, raised ICT, devices like pacemakers, vascular clips and shunts.

6.

Patient posted for an emergency procedure

7.

Skull defects at the region where electrodes need to be placed.

All patients underwent a pre-anaesthetic check-up which consisted of a detailed history taking regarding present complaints, past medical history, personal history, and general physical and systemic examination. Patient preparation started on the day before surgery and was kept nil per oral (NPO) for 8 h and premedicated with tab ranitidine 150 mg and alprazolam 0.5 mg.

Baseline parameters like heart rate ( HR), mean arterial pressure( MAP), SpO2, and bispectral ( BIS) index were noted. After preoxygenation for 3–5 min general anaesthesia was induced with fentanyl (2 μg/kg) and propofol (titrated to loss of verbal response). After the adequacy of mask ventilation was ensured, vecuronium (0.1 mg/kg) was given. After 3 min, patients were intubated with appropriate-size endotracheal tubes. Gas sampling was done through the side port attached to the ventilator circuit to monitor the end-tidal carbon dioxide and anaesthetic gas levels. A nasopharyngeal temperature probe was placed (body temperature was maintained throughout the procedure between 36 and 37 C). Heart rate, mean arterial pressures and BIS values were recorded just Before induction, after intubation, on starting propofol /dexmedetomidine then 10 min, 20 min, 30 min, 60 min, 120 min, 180 min and at the end of the surgery to study the effect of the two modalities of anaesthesia on hemodynamic and depth of anaesthesia. Just after induction anaesthesia was maintained with air, oxygen and Isoflurane (MAC 0.4–0.5) and intermittent doses of fentanyl (50 μg). Wearing off the effect of Vecuronium was confirmed with the ulnar nerve stimulation and baseline transcranial motor-evoked potentials were recorded. After a satisfactory MEP response with either propofol or dexmedetomidine, a BIS value of 40–60 was kept constant. Anesthesia was maintained in group P using injection propofol infusion at 50–150 μg/kg/min. In group PD, anaesthesia was maintained using dexmedetomidine with a loading dose of 1 μg/kg injected over 10 min and infusion at 0.4 μg/kg/h along with injection propofol infusion at 50–150 μg/kg/min. with 50% oxygen and air. Additional drugs administered in group PD were the same as in group P and muscle relaxant was not administered in either of the groups. All of the patients were subjected to controlled ventilation at a frequency of 14–16/min. A bite block was placed between the jaws. Ventilation was adjusted to obtain a stable airway pressure with end-tidal carbon dioxide levels between 30 and 40 mmHg (adjusted after obtaining an arterial blood gas to correlate with a partial pressure of carbon dioxide between 35 and 45 mmHg). In all cases, the BIS was used to monitor the depth of anaesthesia, with the BIS maintained between 40 and 60 by titrating the level of propofol infusion in both groups.

Needle electrodes were placed over the scalp for electrical stimulation of the motor cortex. MEPs were recorded from bilateral upper and/or lower extremities (according to the requirement of the case) using needle electrodes. The subdermal EEG needle electrodes used for the study purpose were 1.5 mm long and were of 27 G (Medtronic). The equipment used for stimulating and recording MEP is Medtronic NIM—Eclipse TM system 68L2128 neuro-physiological detector. The needle electrodes were placed after positioning and proper cleaning of the local site with chlorhexidine and 70% ethyl alcohol solution and then were secured using waterproof adhesive plasters. The stimulus intensity is to be kept between 200 and 350 V. The stimulus parameters were kept the same as those used for obtaining the baseline for all the subsequent stimulations. The MEPs were recorded simultaneously from muscles bilaterally. The MEP waveform's latency and amplitudes were analysed on the left and right sides to determine the comparative study in both groups.

For placement of the stimulating electrodes, a 10–20 montage system was used. For recording MEP in the upper limb; stimulating electrodes were placed at C3 and C4 and MEPs were recorded in the bilateral abductor pollicis brevis muscle (innervated by median nerve; C8, T1) using bipolar needle electrodes. For recording MEP in the lower limb; stimulating electrodes were placed at C1 and C2 and recorded in the bilateral abductor hallucis muscle (innervated by medial plantar nerve; L4, L5) using bipolar needle electrodes.

To study the effect of anaesthetic agents on the motor-evoked potential waveforms the amplitude and latency of the waveforms were measured. At the time of skin closure, the anaesthetic agents were stopped. The total amount of propofol and dexmedetomidine was recorded. Inhalational anaesthetics are to be stopped at about 10 min prior to the end of surgery and to be ventilated with 100% oxygen. Reversal of residual neuromuscular block will be done with Neostigmine 50 μg/kg IV with Glycopyrrolate 10 μg/kg IV.

Sample size and data analysis

Sample size estimation was performed using power and sample size calculation software (version 3.1.2, DuPont and Plummer November 2021). Assuming the power of the study to be 90% and the probability of type 1 error to be 5%, a total of 128 patients were found to be required to detect a statistically significant difference in the mean dose of propofol consumption. Hence, a total of 128 patients were incorporated into the study and were distributed randomly into two study groups, groups PD and P, each consisting of an equal number of patients (n = 64).

The nominal variables would be measured as proportions, ordinal variables would be measured as median and IQR while continuous variables would be measured as mean ± SD. The association between nominal variables will be tested using the chi-square test. the difference in ordinal variables would be tested using a non-parametric test. While continuous variables would be measured using a t-test.