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NBME 23 Answers
A 45-year-old man is brought to the emergency ...
I had the idea that this was a neurogenic shock and increasing intracranial pressure could affect the vagus too. I think the question really wants us to go that direction.
The Cushing reflex leads to bradycardia!
Wait I'm confused. I thought hypovolemic shock leads to an increased SVR?
apparently, there's a thing called sympathetic escape that can happen after a while (i.e. he's been out for 24 hours): Accumulation of tissue metabolic vasodilator substances impairs sympathetic-mediated vasoconstriction, which leads to loss of vascular tone, progressive hypotension and organ hypoperfusion.
also also if he hit his head he could have loss of sympathetic outflow from a hypoxic medulla which could lead to vasodilation, which further reduces arterial pressure, but this was a hard one for me lol. I also put increased ICP wah.
Any lack of sympathetic outflow/increased vagal outflow should reduce HR, not increase it. Further, you would expect brainstem signs if there was hypoxia to the brainstem. For example, if you had damage to the solitary nucleus, you wouldn't be able to regulate your HR in response to reduced BP. Since this patient has reduced BP and increased HR, this indicates that the primary disturbance is likely the reduced BP.
He's also been in a desert for 24+ hours so.
In a patient who develops hypotension following high-energy trauma, neurogenic shock is a diagnosis of exclusion that is made after hypovolemic and obstructive cardiogenic shock have been ruled out!
Plus Absent Bradycardia rules it out