Use of neuroanesthesia adjuncts (hyperventilation and mannitol administration) improves neurological outcome after thoracic aortic cross-clamping in dogs.

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Abstract
Using a canine model of thoracic aortic cross-clamping, we compared the incidence and severity of paraplegia with and without standard neuroanesthesia adjuncts (mannitol administration and deliberate hyperventilation). Better outcome was predicted for animals treated with mannitol and hyperventilation. Nineteen dogs (mean +/- SD weight, 21 +/- 3 kg) were anesthetized with methohexital to an isoelectric electroencephalogram. Animals were randomized to group C (control; n = 9) or group M (mannitol administration and deliberate hyperventilation; n = 10). In group C, animals were maintained normocapnic (PaCO2, 38 to 42 mm Hg). In group M, animals were hyperventilated to a PaCO2 of 28 to 32 mm Hg and received mannitol 2 g.kg-1 during surgical preparation, then 1 g.kg-1.h-1 by continuous infusion. The thoracic aorta was cross-clamped for 30 minutes. Systemic hemodynamics, cerebrospinal fluid pressure, and arterial blood gases were measured at (1) baseline, (2) 2 minutes after cross-clamp, (3) 20 minutes after cross-clamp, (4) 5 minutes after cross-clamp release, and (5) 30 minutes after resuscitation. No attempt was made to control the hemodynamic consequences of cross-clamping in either group. With release of the cross-clamp, PaCO2 was not controlled in group C; in group M the minute ventilation was further increased to maintain PaCO2 constant. At precisely 24 hours after cross-clamp 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 killed, and the entire spinal cord was removed for histological assessment. Multiple sections of the lumbar spinal cord were processed and stained with hematoxylin and eosin. With application of the cross-clamp, cerebrospinal fluid pressure and central venous pressure increased significantly in both groups. However, in group M the maximal mean cerebrospinal fluid pressure never exceeded baseline values in group C. With cross-clamp release, spinal cord perfusion pressure (distal mean aortic pressure minus mean cerebrospinal fluid pressure) was significantly greater in group M (86 +/- 23 vs 65 +/- 17 mm Hg; P = .0017 between groups). Acid-base balance was better maintained in group M. The incidence and severity of paraplegia were significantly lower in group M (P = .043; Mann-Whitney rank-sums test, two-tailed). In this group 10 of 10 animals could walk and 4 of 10 had complete recovery. In group C 4 of 9 animals were paraplegic. There was a strong negative correlation between the Tarlov score and the ratio of dead to total anterior spinal cord neurons in the lumbar region as assessed by light microscopy (P = .0004; Spearman's rank test). We conclude that a protocol using standard neuroanesthesia adjuncts (mannitol administration and deliberate hyperventilation) is associated with improved neurological outcome after thoracic aortic cross-clamping of 30 minutes' duration in dogs anesthetized with methohexital.

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