board review question

[sublimaze]

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pt comes in for cataract extraction to be done under retrobulbar block.
pmhx sig for heart transplant 5 years ago, doing well on anti-rejection meds.
after propofol + block, pt's HR = 38.
answer choices are ephedrine, atropine, isoproterenol, and dobutamine.

i get why ephedrine and atropine are wrong - a direct acting agent is better and the denervated txplant heart lacks vagal tone/input.

the correct answer is isoproterenol.
why is dobutamine less effective in treating bradycardia in this setting?

 

[Gern Blansten]

I am going on the assumption that this is a question from a review book and not an actual exam remembered question, which we have been discouraged to post here.
This is from the open anesthesia keyword website, and I think it addresses your confusion.

Heart transplant - autonomic effect

The transplanted heart will not be able to respond to drugs that act by blocking the parasympathetic system because these connections were severed during the transplant. Therefore, treating bradycardia would have to be done with agents such as isoproterenol, glucagon, epinephrine, norepinephrine (drugs that have a direct effect on the heart). Isoproterenol is most commonly used for increasing heart rate in cardiac transplant recipients. Epinephrine/Norepinephrine may have exaggerated beta mimetic effects on the heart rate because the increase in blood pressure will not lead to a reflex slowing of the heart rate via the baroreceptor reflex (i.e., efferent vagus nerve). Implanted mechanical pacemakers work normally in heart transplant recipients since the cardiac leads are placed directly into myocardium.

Transplanted hearts have no baseline parasympathetic innervation and thus don't have the normal tachycardia and increased contractility response to hypotension/hypovolemia. Reflex venous contriction response to hypotension still exists. Thus intravascular volume is even more important. Circulating catecholes will have cause a delayed increase in rate and contractility. Indirect drugs that affect rate will not be effective (antimuscarinics, anticholinesterases, pancuronium, digoxin). Use direct acting agents (isoproterenol, epinephrine). Lack of vagal tone will result in baseline rate of 90-100. Vagal bradycardic reflexes will also be absent (laryngoscopy, hypertension, carotid sinus massage).

About 25% of patients will develop a bradycardia that will require implantation of a permanent pacemaker.
Sources

Barash: Clinical Anesthesia, ed 5, pp 311, 318, 1373.
Hines: stoelting's Anesthesia and Co-Existing Disease, ed 5, pp 21-22.

 

[pgg]

Dobutamine gives you its positive inotropic effects before its chronotropic effects.

Hopefully this hotlinks ... best graph I could find on GIS, even though it's got some extraneous crap in there.

 

[Idiopathic]

chronotropy is probably a side effect of dobutamine versus inotropy which is the main indication; the heart rate is usually a wanted byproduct of its administration. isoproterenol is simply a better choice.