Gordon Tait

Intraoperative Detection of Spinal Cord Ischemia Using Somatosensory Cortical Evoked Potentials during Thoracic Aortic Occlusion

Paraplegia remains a devastating and unpredictable complication of surgical procedures requiring temporary occlusion of the thoracic aorta, interruption of important spinal radicular vessels, or both. Intraoperative monitoring of the physiological integrity of the spinal cord should permit the early detection of spinal cord ischemia, the judicious and timely institution of corrective measures, including bypass or shunting, and the preservation of important intercostal arteries in appropriate circumstances. A model of spinal cord ischemia was created by temporary proximal and distal occlusion of the canine thoracic aorta. Serial measurement of somatosensory cortical evoked potentials (SCEP) generated by peripheral nerve stimulation, reflecting the status of long-tract neural conduction, was used to monitor alterations in spinal cord function during ischemia. Twelve animals subjected to aortic occlusion demonstrated a characteristic time-related deterioration of the SCEP with virtual extinction of the signal at a mean interval (+/- standard error of the mean) of 12.4 +/- 1.5 minutes. Six animals in which reperfusion was established immediately following the loss of the SCEP (Group 1) demonstrated complete recovery without neurological sequelae, as assessed by clinical and histological criteria. In 6 animals (Group 2), the period of aortic occlusion was extended for an additional 15 minutes following loss of the SCEP (27.3 +/- 2.3 minutes); postoperatively, 4 of 6 animals sustained major neurological lesions characterized by spastic paraplegia and histological evidence of spinal cord infarction (Group 1 versus Group 2, p less than 0.05). We conclude that distinctive alterations in the SCEP are indicative of reversible ischemic spinal cord dysfunction. On-line monitoring of spinal cord function using the technique of SCEP provides a rational basis for determining of SCEP provides a rational basis for determining operative strategy during surgical procedures on the thoracic aorta.

Comparison of a Porous‐Surfaced with a Totally Porous Ventricular Endocardial Pacing Electrode

Porous endocardial ventricular electrodes, re‐cent innovations in cardiac pacing technology, have been shown to perform in a su‐perior manner compared to conventional solid electrodes with respect to such parameters as stimulation threshold, sensed R‐wave amplilude, and source im‐pedance, This experimental study compared the electrophysiological performance of a porous‐surfaced and a totally porous electrode, two fundamental design vari‐ations of the porous eJectrode concept which are in current clinical use. Six porous‐surfaced (1 mm length, 2.3 mm diameter, 8.8 mm2 outer surface area, pores < 25 μn) and six totally porous (1.3 mm length, 2.0 mm diameter, 7,5 mm‐ outer surface area, pores 100 to 150 μm) flanged ventricular endocardial eleclrodes were implanted into the right ventricular apex of 12 dogs. Stimulation threshoids under constant current and constanl voltage conditions at pulse durations of 0.1, 0.25, 0.5, 0.75, 1.0, 1.5, and 2.0 milliseconds, sensed peak‐to‐peak R‐wave amplitudes, and source impedance ivere measured at implant and at l, 4, 8, 12,16, 24, and 30 weeks (explant) thereafter. Analyses of voriance on the data for the strength‐durolion curves at expiant and the threshold‐time curves at l ms pulse duration indicated highly significant differ‐ences between the performance of the two types of electrodes, the porous‐surfaced electrodes displaying average Stimulation threshoids approximateiy 30% lower than the totally porous electrodes. Students t tests indicated the magnitude of the sensed R‐wave to be maintained over the 30 week period for the porous‐surfaced electrodes but to decrease by about 20% for the totally porous electrodes. In addition, the chronic source impedance of the porous‐surfaced electrodes was significtmtly Jess (about 257%) than that of the totally porous electrodes. These differences indi‐cate better overall electrophysiological performance for the porous‐surfaced electrodes as compared with the totally porous electrodes. Both types of electrodes, however, operate well within the limits of highly acceptable function and therefore rep‐resent attractive designs for clinical use in endocardial pacing.