THE EFFECTS OF ARTERIAL BLOOD-GAS VALUES ON LESION VOLUMES IN A GRADED RAT SPINAL-CORD CONTUSION MODEL

The detrimental effects of extreme blood gas values are well documented. However, the range of normal values has not been rigorously defined. There is an ongoing debate concerning the need for ventilation and tight control of blood gas values in spinal cord injury models. Consequently, we performed a retrospective study of 84 rats using a graded rat spinal cord contusion model. Spinal cord ionic lesion volumes were calculated from Na and K shifts at 24 h after injury. Blood gas measurements were obtained 5 min before contusion. For pH values of 7.31-7.46, systemic acidosis was associated with a small but significant decrease in ionic lesion volumes in the 12.5 and 25 g.cm contusion groups (p < 0.05 and p < 0.03, respectively). pH had no effect on ionic lesion volumes in the 50 g.cm contusion group (p > 0.5). PaO2 values from 23 to 53 mm Hg showed an effect only at 25 g.cm (p < 0.05). PaO2 values of 46-138 mm Hg and calculated HCO3 values of 13-28 mEq/L had no effect on ionic lesion volumes. Two conclusions may be derived from these data. First, mild systemic acidosis is associated with a small reduction in ionic lesion volumes after mild and moderate injury but not after severe injury. This suggests that secondary mechanisms play a greater role in mild injuries. Second, variations in arterial blood gases within clinically normal ranges do not strongly influence 24-h ionic lesion volumes in a graded spinal cord injury model. The effects of blood gas values on ionic lesion volumes are not statistically significant unless the data are adjusted for injury severity. Although blood gas values must be carefully monitored, ventilation may not be needed routinely in rat spinal cord injury models. We recommend maintaining pH values between 7.35 and 7.40, PaCO2 between 35 and 41 mm Hg, and PaO2 greater than 71 mm Hg.