Neurophysiological monitoring

In an anaesthetised, intubated patient various neurophysiological monitors may be

used. Of the following monitors, which one is LEAST LIKELY to be affected by a

concurrent remifentanil infusion?

a) Auditory evoked potentials

b) Electroencephalography

c) Bispectral index

d) Spectral entropy

e) Somatosensory evoked potentials


 

Answer: E

Explanation

The search for a monitor with good sensitivity and specificity to measure the depth

of an anaesthetic has been one of the great challenges in academic anaesthesia that

has thwarted many great minds. The electroencephalograph (EEG), auditory evoked

potentials (AEP), the bispectral index (BIS) and spectral entropy have all been

proposed and used with varying degrees of success and adoption. The electroencephalograph

was the first to be investigated. With increasing anaesthesia there is a

reduction in the activity of the EEG, there is a shift from higher frequency signals to

lower frequency and correlation between signals from different parts of the cortex

becomes more random. The multiple leads, bulky equipment and complicated readout

stopped it from ever being a practical candidate for monitoring depth of anaesthesia.

A number of commercial products employing BIS, AEPs or spectral entropy

are currently available. Generally these involve a simplified set of electrodes attached

to the patient connected to a small unit that measures electrical waveforms produced

within the central nervous system. These are then analysed and usually reduced

down to a linear dimensionless scale from 0 to 100 with the lower scores indicating

deeper levels of anaesthesia. Bispectral index and entropy monitors use specific

analysis of the EEG, and AEP interprets the EEG following an auditory stimulus.

All of these four monitors have been the subject of studies demonstrating an alteration

in reading when a remifentanil infusion is used. Somatosensory evoked potentials

(SSEPs) have substantially taken the place of wake-up tests in spinal surgery and

are adversely affected by volatile anaesthetics. As a result, target controlled anaesthesia

with propofol, usually with supplementary remifentanil, tends to be the

anaesthetic of choice when SSEPs are used.