Somatosensory evoked potentials

In a patient having cortical somatosensory evoked potentials monitoring of spinal cord integrity during spinal surgery, the following may produce important inaccuracies EXCEPT which one?

a) Blood pressure variations

b) Temperature variations

c) Neuromuscular blocking drugs

d) Haemorrhage down to a haemoglobin of 5.5 g/dL

e) Maintenance of anaesthesia with sevoflurane

Answer: c

Explanation

Somatosensory evoked potentials (SSEPs) are monitored during spinal surgery and in many centres they have replaced wake-up testing during spinal surgery. The technique involves the analysis of recorded signals picked up by recording electrodes sited over the spine or scalp and caused by stimulating peripheral nerves. The signal may be affected by temperature variation and hypotension. Anaemia to a haematocrit of <15% may also affect SSEPs. The motor tracts are not monitored, so SSEPs are unaffected by neuromuscular blocking drugs and may actually be enhanced as muscle blockade decreases the ‘noise’ from muscle tissue. Volatile anaesthetic agents increase SSEP latency and decrease amplitude. SSEPs use usually involves an anaesthetic with targetcontrolled propofol infusion, which has little significant affect.

Reference

Boisseau N, Madany M, Staccini P, et al. Comparison of the effects of sevoflurane

and propofol on cortical somatosensory evoked potentials. Brit J Anaesth 2002; 88(6):

785–9.