Comparison of TIVA with Propofol versus Sevoflurane and Dexmedetomidine on reliability of intra-operative MEP monitoring in spine surgical patients having pre-operative paresis.

Patients  are  allocated  the  study  group based  on  a  computer generated random number sequence. Patients in Group D will receive    dexmedetomidine    and    inhaled    sevoflurane    for anaesthetic maintenance, while group P will receive Propofolinfusion for maintenance of anaesthesia.

The patients will be maintained NPO (nil per orally) for 6 hours (solids) and 2 hours (clear fluids) prior to surgery. On reaching the operating room, patients in Group D will be induced with fentanyl 2 μg.kg-1, a sleep dose of propofol intravenously and muscle relaxation achieved with atracurium 0.5 mg.kg-1. Patients in group P will receive fentanyl 2 μg.kg-1, propofol to achieve plasma level of 3 μg.ml-1 and atracurium 0.5 mg.kg-1. The patient will be intubated and positive pressure ventilation maintained with oxygen-air mixture at a fresh gas floe of 1 litre per minute in a closed circuit to maintain normocapnia. Fentanyl infusion will be initiated for all patients at a dose of 1μg.kg-1.hour-1, which may be increased up to 3 μg.kg-1.hour-1 based on the managing anaesthetist’s judgement of the patients’ analgesic requirements.

Vitals monitoring will include invasive blood pressure, electrocardiogram, heart rate, oropharyngeal temperature, urine output and end tidal carbon dioxide concentration in all the patients. Use of a forced air warmer and maintaining the room temperature at 20°C, will aid in reducing the extent of patient hypothermia. All patients will have a BIS electrode placed for assessment of anaesthetic depth and to facilitate titration of anaesthetics.

Patientsin group D will receive a dexmedetomidine bolus of 0.5 μg.kg-1over 10 minutes followed by an infusion of 0.5   μg.kg-1.hr-1which will becombined with inhaled sevoflurane titrated to maintain an end-tidal concentration of 0.5 MAC.   Anaestheticdepth will be titrated with the sevoflurane to ensure that the BIS value stays below 60 and above 40.

Patients in Group P will be maintained on a propofol infusion based on the Eleveld-2.1 PKPD model targeting an effect site plasma concentration of 3 μg.ml-1. Anaesthetic depth will be adjusted    by    increasing    or    decreasing    the    effect    site concentration of propofol by 0.5 μg.ml-1, in order to maintain the BIS below 60 and above 40.

Motor evoked potential (MEP) monitoring with trans- cranial  electrical  stimulation  of motor cortex with  electrodes C3,  C4  placed  according  to  the  international   10-20  EEG system.  Stimulus will be applied at a  strength  of 300-500V, with  train  stimulus  reserved  for  inability  to  elicit  evoked potentials with a single twitch.

Muscle relaxants will not be use intraoperatively during tcMEP recording  and  subsidence  of  action  of  atracurium  will  be ascertained by ensuring all four twitches on train-of-four, with absence of fade. For the purpose of our comparison, we will record the amplitude of cMAPs recorded from the all groups ofmuscles  of the  patients  (side  with  the  higher  amplitude  in baseline recording will be taken). cMAP baseline  amplitude will be noted before the commencement of surgery and at time of dural opening / beginning of procedure.

Variables noted will include:

1. Reliability of the MEP recorded- proportion of muscles with recordable cMAPs to the total number of muscles monitored. We defined reliability of the MEP as a proportion of the muscles from which a measurable MEP signal / cMAP was achieved, to the total number of muscles for which measuring electrodes were placed, expressed as a percentage.
2. Number of stimuli required to elicit recordable cMAP.
3. Increase in stimulus strength (voltage).
4. Episodes of MEP suppression- global or focal.

Any patient in whom the baseline MEP amplitude is less than 50 V after reasonable escalation of stimulation strength and frequency will be considered to have absent evoked potential. The anaesthetic team will then have the option to cross-over the anaesthetic technique to the alternative group and attempt to record the MEP.