The effect of two low-dose propofol infusions on the relationship between six-pulse transcranial electrical...

Transcranial stimulation of the motor cortex using high-voltage electrical stimuli given in train is a method of monitoring the integrity of the motor pathways during thoracoabdominal aortic aneurysm surgery. The purpose of this study was to assess the relationship between the stimulus intensity and the corresponding amplitude of the myogenic motor evoked potential (tcMEP) in response to six-pulse transcranial electrical stimulation during two levels of low-dose propofol infusion and stable fentanyl/nitrous oxide anaesthesia.

Nine patients (37-78 yr) scheduled to undergo surgery on the thoracoabdominal aorta were studied. After achieving a stable anaesthetic state the output voltage was decreased with 50 V intervals from 350 V to 200 V during a target propofol infusion aimed at a plasma steady-state concentration of 0.7 microg x ml(-1) and increased with 50 V intervals from 200 V to 450 V during a target propofol infusion aimed at a plasma steady-state concentration of 1.4 microg x ml(-1). TcMEPs were recorded from the right tibialis anterior muscle.

Doubling the target propofol infusion to 1.4 microg x ml(-1) resulted in a 30-50% decrease in tcMEP amplitude. The largest tcMEP amplitude using the six-pulse paradigm was found during a propofol infusion aimed at a plasma concentration of 0.7 microg x ml(-1) and demanded a stimulus output of 350 V, corresponding to a charge density of 7.5 microC x cm(-2) per phase.

Doubling the target propofol infusion to 1.4 microg x ml(-1) provides less robust, but still recordable tcMEPs in response to six-pulse electrical stimulation. Safety guidelines are discussed.