The Influence of Induced Hypotension and Spine Distraction on Canine Spinal Cord Blood Flow

Abstract

Deliberate hypotension is used in scoliosis surgery to reduce the need for blood transfusion and to improve operating conditions. There are concerns, however, that hypotension may decrease spinal cord blood flow (SCBF) and thereby predispose the spinal cord to ischemic injury, particularly when it is distracted during Harrington instrumentation. In a canine model, the mean arterial pressure (MAP) was reduced to 50% of the normotensive value with sodium nitroprusside and halothane to study its effects, with and without spinal distraction, on spinal cord blood flow measured by the hydrogen clearance technique. The induction of systemic hypotension resulted in a significant decrease in spinal cord blood flow from 15.7 ± 1.1 ml/min/100g (control) to 10.7 ± 4.7 ml/min/100g. This initial decrease in spinal cord blood flow returned to normotensive values by 35 minutes following the induction of hypotension, suggesting an autoregulatory effect. This indicates that the induction of deliberate hypotension to half its normotensive mean arterial pressure is associated with a significant decrease in spinal cord blood flow that returns to normotensive levels by 35 minutes. One and two centimeters of longitudinal distraction applied during systemic hypotension did not reduce spinal cord blood flow when it was applied at least 45 minutes after the hypotension was induced. Thus, when longitudinal stretch of a magnitude approximating that used clinically during Harrington instrumentation is applied in the presence of systemic hypotension, the normal SCBF is not reduced when the autoregulating system is functioning. Cautiously extrapolating these findings clinically suggests that nitroprusside-induced hypotension does decrease spinal cord blood flow, but this effect is not sustained, and spine distraction applied during hypotension induced to 50% of MAP does not reduce SCBF once it has returned to normotensive flows.