Spinal Cord Monitoring

The anaesthesia technique as a whole revolves around spinal cord monitoring used in the intraoperative period to assess the integrity of the dorsal and ventral columns of the spinal cord. Previously, subjective tests such as the wake-up test and clonus test were used, but now, when

TABLE 7.3

Predictors of Postoperative Mechanical Ventilation

a. Anterior spinal surgery [3]

b. Preoperative FVC <50% predicted [3]

c. Preoperative FEV1 <50% predicted [3]

d. Maximum inspiratory pressure of less than 40 cm H,0 [3]

e. Blood loss more than 30ml/kg [3]

f. Pre-existing neuromuscular disorders, congenital heart disease, right ventricular failure [3]

g. Cephalad location of the curve [3]

available at the facility, they have been replaced by more objective ways of measurement such as somatosensory-evoked potentials (SSEP) and motor-evoked potentials (MEP).

Wake-Up Test

The wake-up test includes waking up the patient during the surgery and observing the movement of lower limb fingers upon instruction. The patient must be counselled in the preoperative period itself and ensured that there will not be pain and he or she will not remember the procedure. The anaesthesiologist has to be notified at least 15 minutes before the wake-up test so that the muscle relaxant can be timed, inhala-tional agents discontinued, and haemodynamics ensured. The main disadvantages of this test included the inability to monitor at critical steps such as screw placement, the subjective nature of the test, and the risk of violent movements, disconnections of invasive lines, accidental extubation, etc. This has been replaced with more objective and sophisticated means of monitoring the integrity of columns of spinal cord such as evoked potentials. Evoked potentials are the discharges that are collected from a specific area after stimulating some other point.

Somatosensory Evoked Potential

As the name suggests, this involves stimulating a peripheral nerve with surface electrodes and recording the evoked potentials from the sensory cortex through scalp electrodes. The process is continued throughout the surgery and the amplitude and latency of the wave form is compared with the baseline. The most common nerves used for monitoring are posterior tibial nerve, peroneal nerve, and median nerve. The decrease in amplitude of more than 50% and increase in latency by 10% is considered significant [3]. Various anaesthetic agents interfere in the recording of SSEP, and the anaesthesiologist must optimally maintain anaesthesia with other agents. The effect of anaesthetic agents on SSEP is represented in Table 7.4.

Muscle relaxants decrease the background noise in SSEP and helps in monitoring [23]. The other factors effecting SSEP include the

TABLE 7.4

Effect of Anaesthetic Agents on SSEP [23]

Anaesthetic Agent

Amplitude

Latency

Inhalational agents

Decrease

Increase

Nitrous oxide

Decrease

No effect

Propofol

No effect

No effect

Thiopentone

Decrease

Increase

Ketamine

Increase

No effect

Etomidate

Increase

No effect

Opioids

No effect

No effect

Dexmedetomidine

No effect

No effect

temperature, mean arterial blood pressure, PO, PCO2, and haemoglobin concentration. The combination of opioid, muscle relaxant, and a sub MAC doses of inhalational agent or propofol infusion is considered optimal for SSEP monitoring [5]. The main advantage of SSEP is that there is no patient movement expected, the surgeon need not stop operating at critical steps; however, the inability to monitor motor tracts and the longer latency outweighs its benefits.

Motor Evoked Potentials

MEP is the compound muscle action potential (CMAP) recorded in various muscle groups after stimulating motor cortex through scalp electrodes. The muscle groups above the level of correction are considered the control group and are compared with the muscle groups below the level of correction and also with their corresponding baseline values. All anaesthetic agents that effect SSEP also effect MEP in a similar manner, but MEP are also affected with muscle relaxant [3]. A physiological and pharmacological steady state must be maintained during MEP monitoring [3]. A combination of a short-acting opioid and propofol total intravenous anaesthesia (TIVA) without muscle relaxant is considered optimal for MEP monitoring. The advantage with MEP monitoring is that it measures the integrity of motor tracts, and the latency is minimal [23]. The main disadvantage being movement of the patient every time the stimulus is given, leading to tongue lacerations, change in position of the patient causing compression of vital structures, such as the eyes, and damage to endotracheal tube. So, the position of the patient must be examined after each stimulus and a bite block has to be introduced before prone position to prevent injuries to tongue and damage to endotracheal tube.

 
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