INITIAL ASSESSMENT OF AUTOMATED SSEP

FOR DETECTION OF INTRAOPERATIVE

POSITIONAL NEUROPRAXIA

IN CARDIAC SURGERY

John M. Murkin; T. Turkstra; J Chui; R. Mayer

Department of Anesthesiology and Perioperative Medicine,

London Health Sciences Center, Schulich School of Medicine,

University of Western Ontario,

London, Ontario, Canada.

•Neuropraxia: transient loss of neural conductivity due to nerve fibre compression without loss of neurofibrils;

•spontaneous recovery may occur in a few days to weeks, or may persist indefinitely

•In cardiac surgery the incidence of neuropraxia has been estimated to occur in up to 37.5% of cardiac surgical patients

• may occur due to stretch of brachial plexus over first rib due to sternal retraction and can exacerbate with IMA retractor

• may occur due to compression of peripheral nerve at ulnar groove or compression of arm during positioning

• somatosensory evoked potentials involve repetitive

stimulation (n = 300 Hz) of peripheral sensory nerve with recording of micropotentials at brachial plexus (Erb’s point) and spinal cord (C5)

•Signal averaging relative to reference electrode (forehead) is used to discriminate nerve transmission potentials from other biological signals ie. EEG, ECG potentials

• conventional SSEP requires use of needle electrodes and experienced operator to detect and analyze resultant waveforms

•By convention, an increase in latency of signal transmission of 10% or a reduction in amplitude of 50% has been used to detect nerve injury

Latency Amplitude

Repetition frequency

Trend over time showing stable SSEP signal over 1 hour

Monitor homonculus showing unchanged SSEP

Alert displaying significant decrease ampiitude

Waveform Display showing Ulnar Nerve SSEPs gone with Median significantly decreased in amplitude

Waveform Display at the End of the Case Showing Median Recovered but Still Decreased in Amplitude and Ulnar Also Decreased in Amplitude but still causing an alert.

Of 13 patients with complete SSEP date, persistent intraoperative alert was detected in 2 patients

Transient alert was detected in 5 patients but resolved with release of IMA retractor (n = 3) or repositioning of arm ( n = 2)

persistent alert (n = 2) was associated with clinical symptoms of tingling and weakness and EMG confirmed ulnar neuropathies

SSEP device demonstrated automated detection of neuropraxia

Device was easy to use and functioned in OR environment

Repositioning relieved SSEP changes in 5 cases but persistent change correlated with clinical symptoms confirmed with EMG

An intervention protocol and prospective study is planned

CONCLUSION