Notes: Paraplegia as a consequence of spinal cord ischemia associated with procedures on the thoracic and thoracoabdominai aorta has been linked to the interaction of proximal hypertension with elevated cerebrospinal fluid pressure (CSFP) during aortic cross-clamping (AXC). CSFP reduction via cerebrospinal fluid (CSF) drainage is thought to significantly prolong the cord's tolerance to AXC. Likewise, partial exsanguination is reported to effectively reduce ischemic injury by controlling proximal hypertension. To evaluate the individual and collective efficacy of both techniques, 18 mongrel dogs (25 to 35 kg), divided into three equal groups, underwent a fourth interspace left thoracotomy AXC. Baseline proximal arterial blood pressure (PABP), distal arterial blood pressure (DABP), and CSFP were established and monitored at 5-minute intervals during 120 minutes of AXC, and for 30 minutes thereafter. Group I animals were partially exsanguinated prior to AXC to maintain PABP at a mean of 115 to 120 mmHg. Group II animals had sufficient (16 ± 5 cc) CSF withdrawn to maintain a DABP-CSFP gradient, i.e., spinal cord perfusion pressure (SCPP) of 20 mmHg. Group III animals were treated with both CSF drainage and partial exsanguination in the same manner as groups I and II, respectively. Periop-erative somatosensory evoked potential (SEP) monitoring evaluated cord function. Postoperative neurological outcome was assessed with Tariov's criteria. SEPs degenerated approximately 22 minutes following AXC for groups II and III. In contrast, group I exhibited rapid (10 ± 7 min) SEP loss. All five surviving group I animals displayed paralysis 48 hours postopera-tively. Mean PABP was significantly higher in group II (155 ±18 mmHg) than in either group 1 (117 ± 9 mmHg) or Ill (120 ± 14 mmHg) (p 〈 0.001). CSFP was significantly higher in group I (14 ± 4 mmHg) than in either group II or III (5 ± 2 mmHg) (p 〈 0.0001). The only parameter associated with neurological injury was low SCPP, which inversely correlated with CSF dynamics. Group I animals, with a mean SCPP of 4.6 mmHg, exhibited paraplegia, while groups II and Ill, with SCPP values above 20 mmHg, were free of neurological injury. Proximal hypertension did not play a role in cord injury. This study underscores the potential of CSF drainage to protect the ischemic spinal cord. (J Card Surg 7994;9:637–637)