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 evoked spinal cord potential (ESP) and the partial pressure of oxygen in tissue in the epidural space (E-pO2) during aortic clamping. Eighteen adult mongrel dogs were studied as follows. In group I (n = 6), the descending thoracic aorta was clamped partially at the proximal site for 15 min to maintain the distal arterial pressure at 60, 40, and 20 mmHg consecutively at 15 min intervals. In group II (n = 6), the descending 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 observed 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 III. The decrease of E-pO2 correlated well with the distal arterial pressure, and a rapid return to baseline of the E-pO2 was observed after declamping. The E-pO2 changed in response to spinal ischemia more rapidly than did ESP in all groups. The critical level of E-pO2 was 50 mmHg or a 40% decrease from baseline. Because the ESP reflects spinal function and the E-pO2 reflects spinal blood pressure, we propose that combined recording of ESP and E-pO2 would improve spinal monitoring during thoracic aortic surgery.