How many of you see pets with head trauma in your practice? Okay, that’s good. There are several ways to look at prognosis following head trauma but the key feature of any of the measurements is serial evaluations. One singular measurement doesn’t seem to correlate well with prognosis in such a dynamic disease. A recent retrospective study evaluated the shock index (heart rate divided by systolic BP) to determine the correlation with mortality in a population of dogs with head trauma. A second part of the study was to determine if it was predictive of survival to discharge or improvement in signs during hospitalization.
Results
A total of 86 cases and an unknown number of control dogs (normal dogs) were included. There was quite a list of possible ways these poor dogs were traumatized and an even longer list of breeds affected. See the study for full details on that. The mean SI for the control group was 0.75 (range 0.62-0.92 with not normal distribution). The mean SI for dogs with head trauma was 0.91 (range 0.34-3.33, also not normally distributed). SI was significantly (0.0014) higher in the trauma group compared to the control group. However, the SI was not significantly different between dogs that diet or were euthanized compared to those that lived until discharge. There was also no significant difference in SI between dogs with a normal neurologic examination at discharge and those that were improved or static, but not normal, at discharge.
So, what does this tell us? My take away is that the SI is higher in dogs with head trauma, but it doesn’t prognosticate (using the data from this cohort of dogs) regarding survival or neurologic outcome. Why is SI higher in pets with head trauma? Following trauma, if the intracranial pressure (ICP) goes UP (hemorrhage, edema) the mean arterial BP goes up to keep blood flowing to the brain. The HR will concurrently drop due to a feedback loop. This SHOULD result in a lower SI (HR/systolic BP). The authors noted that in one case they had a dog with significant hypovolemia at the initial evaluation which resulted in a very high SI but due to a swing in pressure dynamics in the brain, the dog oscillated between hypo and hypertension over time. The authors suggest that SI may be an unreliable measurement in post-head trauma patients for this reason. So, I return to all of you smart people out there. Why is the SI higher in head trauma than in control dogs? This has also been reported in humans post head trauma so there must be a very good reason, but I can’t figure it out and would love your thoughts!
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Reference: McConnell BM, Cortes Y, Bailey D. Retrospective evaluation of shock index and mortality in dogs with head trauma (2015-2020): 86 cases. DOI: 10.1111/vec.13411