The effect of acute traumatic brain injury on the performance of shock index

Shock index (SI) is recognized to be a more reliable early indicator of hemorrhage than traditional vital signs. Acute traumatic brain injury (TBI) can be associated with autonomic uncoupling and may therefore alter the reliability of SI in patients with combined TBI and peripheral hemorrhage. The aim of this study was to evaluate the performance of SI when acute TBI of mild and moderate severity were associated with progressive simple hemorrhage.

This study was undertaken in a laboratory setting. Brian injury was induced using the lateral fluid percussion model in anesthetized rats. The fluid percussion device delivered an applied cortical pressure of 1.2 atm and 1.8 atm, producing mild and moderate TBI, respectively. Control animals underwent identical procedures but with no applied cortical pressure. Hemorrhage was induced 10 minutes after brain injury, at a rate of 2% of blood volume per minute until 40% blood volume was withdrawn.

The SI response to increasing volume of hemorrhage was unaltered when control and mild TBI groups were compared (test of interaction p = 0.39). There was a 50% mortality rate observed 20 to 60 minutes after hemorrhage in the moderate TBI group. The SI response to hemorrhage in the moderate TBI group compared with the control group became significantly different at 40% blood volume loss (test of interaction p = 0.048). Comparison of the SI response with hemorrhage between survivors and nonsurvivors of moderate TBI revealed a significant difference (p = 0.007). SI was markedly attenuated in the presence of increasing hemorrhage in the nonsurvivor subgroup of moderate TBI.

SI significantly underestimated underlying hemorrhage in the presence of acute TBI of moderate severity where attenuation of the biphasic heart rate and blood pressure response was also most pronounced.