We compared cerebrospinal fluid (CSF) drainage (Group D; n = 8) to neuroanesthesia adjunct therapy (hyperventilation and mannitol administration; Group N; n = 8) for the prevention of paraplegia using a canine model of descending thoracic aortic cross-clamping (AXC; 2.5 mm distal to the left subclavian artery for 30 min). We expected no difference in neurologic outcome between groups. After surgical preparation and a 30-min stabilization period, dogs in Group D had CSF drained prior to application of the AXC. During the period of AXC, CSF was allowed to drain freely in an attempt to have cerebrospinal fluid pressure (CSFP) no greater than central venous pressure (CVP). Dogs in Group N were hyperventilated (PaCO2 28-32 mm Hg) and received 2 g/kg of mannitol prior to AXC and then 1 g centered dot kg-1 centered dot hr-1 during clamping. Systemic hemodynamics, CSFP, and arterial blood gases were measured at 1) baseline, 2) 2 min after AXC, 3) 20 min after AXC, 4) 5 min after AXC release, and 5) 30 min after resuscitation. With release of the AXC, PaCO2 was not controlled in Group D; in Group N the minute ventilation was further increased to maintain PaCO2 constant. At precisely 24 h after AXC, the animals were assessed for incidence and severity of paraplegia, using the Tarlov score, by an observer unaware of the experimental protocol. The animals were then killed, and the entire spinal cord was removed for histologic assessment. Multiple sections of the lumbar spinal cord were processed and stained with hematoxylin and eosin, then examined by light microscopy for nonviable neurons in the anterior spinal cord. At baseline, CSFP was significantly higher in Group D before CSF drainage. The CSFP was significantly less in Group D for all subsequent time periods (treatment times time interaction; P = 0.0001). However, in Group N, CSFP was always less than baseline values in Group D. There was no difference in spinal cord perfusion pressure (SCPP; distal mean arterial pressure - CSFP) between groups during cross-clamping. Acidosis was significantly worse in Group D with AXC release. The neurologic outcome was the same in both groups; Tarlov score was 3 in seven animals and 4 in one animal in each group (P = 1.0; Mann-Whitney ranksum test). There was no difference in the ratio of dead: total lumbar anterior spinal cord neurons between groups (0.010 +/- 0.016 and 0.015 +/- 0.033 for Groups D and N, respectively; P = 0.720). We conclude that neuroanesthesia adjunct therapy is as effective as CSF drainage for the prevention of paraplegia in this canine model. Such therapy is less invasive than drainage of CSF and may be of use clinically to decrease the incidence and severity of paraplegia after surgery on the descending thoracic aorta. (Anesth Analg 1995;81:800-5)