SPINAL EPIDURAL OXYGEN PARTIAL-PRESSURE AND EVOKED SPINAL-CORD POTENTIAL IN RELATION TO THE SEVERITY OF SPINAL ISCHEMIA DURING CROSS-CLAMPING OF THE THORACIC AORTA

Experiments were undertaken to determine the relationship between evok ed spinal cord potential (ESP) and the partial pressure of oxygen in t issue in the epidural space (E-pO(2)) during aortic clamping. Eighteen adult mongrel dogs were studied as follows. In group I (n = 6), the d escending thoracic aorta was clamped partially at the proximal site fo r 15 min to maintain the distal arterial pressure at 60, 40, and 20 mm Hg consecutively at 15 min intervals. In group II (n = 6), the descend ing thoracic aorta was clamped proximally for 30 min. In group III (n = 6), the descending thoracic aorta was cross-clamped at proximal and distal sites for 30 min. Postoperative complete paraplegia was observe d in 4 of 6 dogs in group III, but none in group II. The change in ESP with aorta cross-clamping was very mild in groups I and II. Transient increases and decreases in the ESP amplitude were observed in group I II. The decrease of E-pO(2) correlated well with the distal arterial p ressure, and a rapid return to baseline of the E-pO(2) was observed af ter declamping. The E-pO(2) changed in response to spinal ischemia mor e rapidly than did ESP in all groups. The critical level of E-pO(2) wa s 50 mmHg or a 40 % decrease from baseline. Because the ESP reflects s pinal function and the E-pO(2) reflects spinal blood pressure, we prop ose that combined recording of ESP and E-pO(2) would improve spinal mo nitoring during thoracic aortic surgery.