MONITORING NEWS
Welcome to ORIMtec’s online newsletter, Monitoring News, which features news and publications related to intraoperative neurophysiological monitoring (IONM) and the surgical monitoring field. Contents are listed chronologically, with the most recent entry listed first.

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“Lower Extremity Pulse Oximetry for Anterior Lumbar Procedures”
Presented at the American Society of Neurophysiological Monitoring - May 2003

Authors: Ken S. Sato, MD; Daniel Slepian, PA, CNIM; and Douglas Enoch, MD, MS, FACS.

Presentation: American Society of Neurophysiological Monitoring (ASNM) Conference, 2002.

Keywords: Intraoperative, Ischemia, Oxygen Saturation, Somatosensory Evoked Potential

Introduction: The anterior approach is widely used for stabilization/decompression of the lumbar spine. This approach involves manipulation of the arteries overlying the spine, which can lead to vascular compromise. While intraoperative tibial somatosensory evoked potential has been used to detect this compromise the addition of intraoperative pulse oximetry of the left great toe provides a more rapid warning of limb ischemia.

Methods: An unselected series of 34 anterior lumbar procedures were monitored between October 1998 and August 2000. Tibial somatosensory evoked potentials (SSEPs) were recorded in all 34 procedures. A Nicolet Biomedical System (Bravo) was used to record intraoperative SSEPs from subdermal electrodes inserted after induction and positioning. Tibial nerves were simulated posterior to the medial malleolus at 2.7-4.7 Hz and 35ma and recorded from needle electrodes placed at the C7-T1 interspinous space, sagittal and parietal scalp, (Fpz-Cz), (Fpz-C3’, C4’). Continuous stimulation with multiple repetitions was averaged with alteration between both legs. Oxygen saturation was monitored in addition to SSEPs in 11 procedures. A digital oxygen saturation monitor was attached to a left toe and measurements were recorded throughout the procedure. Serial blood pressures, core temperatures and anesthetic variations were also recorded throughout the procedure.

Results: Of the 23 procedures without oxygen saturation monitoring, nine cases had significant (increased latency of 10 percent and/or an amplitude reduction of at least 50 percent) changes in the left tibial SSEPs, representing 26 percent. In all nine of these cases, only the left tibial SSEP showed a significant change from baseline. The surgeon was immediately notified when a significant change occurred. In one case, the left SSEP was completely obliterated while the right tibial SSEP showed no significant latency or amplitude abnormality. It was subsequently found that the patient had lost both dorsalis pedis and posterior tibial pulses. Occluding emboli was found distal to the operative site during endarterectomy; the patient suffered no vascular or neurologic deficit.
Of the eleven procedures monitored with both oxygen saturation and tibial SSEPs, none had a significant change in the left or right tibial evoked response. The remaining four procedures monitored with both modalities had a rapid decrease followed by a complete loss in measurable oxygen saturation from the left great toe, representing 36 percent. In each procedure, compression of the vascular structures was released with immediate return of measurable oxygen saturation and no postoperative deficit.

Conclusions: The data strongly suggest that pulse oximetry has a greater sensitivity for iliac artery compromise than SSEPs. The cases monitored with pulse oximetry showed a loss of signal before any significant change occurred in the SSEPs. Furthermore, based on our SSEP data, when the vascular structures are compromised, this primarily affects the left leg.

MONITORING NEWS Q&A

November 2004

Anterior Lumbar Surgery
Question: What affect will partial or complete occlusion of the common iliac or femoral artery have on evoked potentials during anterior lumbar surgeries?

Answer: The evoked potential will show a gradual deterioration of the amplitude and an increase in latency when stimulating the tibial nerve. Recently we have applied a pulse oximeter to the great toe to track O2 saturation and pulse in conjunction with the evoked potentials. Our data suggests, when a significant reduction in the O2 saturation and loss of a detectable pulse is observed, a degrading evoked potential is inevitable.

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