Source:  Anesthesiology 2002; 97:183-91

Reviewer: John F. Heath, M.D.

Summary:

• looked at an experimental model of spinal cord ischemia (and motoneuron function) similar to that which might occur during prolonged operations on the aorta or spine (e.g. resection of a thoracoabdominal aneurysm.)
• 15 ketamine-sufentanil anesthetized pigs
• Evoked potential monitoring of the motor tract included:
• transcranial electrical stimulator attached to the scalp
• recording electrodes over the right triceps and bilateral quadriceps muscles.
• Group A: 5 pigs underwent short-term (< 10 min) reduction of transcranial myogenic motor evoked potentials (tcMEP's) secondary to temporary or permanent clamping of lumbar segmental arteries.
• Group B: 10 pigs underwent prolonged (> 60 min) tcMEP reduction by the same technique of arterial clamping.
• Data collected:
• Neurologic function evaluated every 24 hours for three days.
• At that time, the animals were killed, addtl data collected:
• Infarction volume (percent necrotic tissue of total gray matter volume)
• number of eosinophilic neurons (sign of selective neuronal necrosis)
• number of viable motoneurons in the lumbosacral spinal cord.

• In all but one animal, tcMEP's were reduced to less than 25% of baseline (accepted criteria for evidence of spinal cord ischemia).
• In all but one animal in Group A, tcMEP's were reduced for < 10 min, and the pigs nad normal neuronal function and no evidence of infarction at 72 hours after the initial tcMEP reduction.
• Seven animals in Group B had reduced tcMEP's for > 60 min. These pigs were paraplegic and demonstrated massive spinal cord infarction at 72 hours.
• Two animals (one in each group) had tcMEP reduction for 40 min with evidence of moderate spinal cord infarction but normal neurologic function.
• Positive predictive values of fast recovery of tcMEP's (< 10 min) for normal motor function and normal histologic findings were 1.00.
• Negative predictive values were 0.64 for normal motor function and 1.00 for absence of spinal cord infarction.

• In ketamine-sufentanil anesthetized pigs...
• intraoperative tcMEP's have good prognostic value for neurologic outcome during spinal cord ischemia
• recovery of depressed tcMEP's within 10 min after the initial decline is associated with both normal histopathologic findings and normal neurologic function.
• Since the spinal cord anatomy and physiology of pigs (vs. dogs) closely resembles that of humans, tcMEP's may prove useful in monitoring patients during operations where there is a significant risk of spinal cord ischemia