Angina and beta blockers

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Wahoo07

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Ok, so here might be a basic/dumb question for y'all.

In first aid, in the antianginal therapy section, it says that B-blockers help angina by affecting afterload. Is this right, and if so what's the mechanism? I know that B-blockers can decrease BP via it's negative inotropic/chronotropic effects leading to decreased cardiac output, but what about afterload? As I understand it, afterload is a measure of aortic or peripheral resistance, so how are B-blockers affecting this? If anything, a nonspecific beta-blocker like propranolol would cause an INCREASE in TPR via blocking beta-2 mediated vasodilatation. Can someone please clarify this issue for me? Thanks! :)

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The mechanism is indirect, as far as I can ascertain. I agree that, taken on its own, blockage of B-2 would raise BP, since it has a slight vasodilatory effect. However, if your pressure is 160/100, and you take a B-blocker and it goes down to 135/80, did you not decrease afterload? I agree that the primary mechanism is the reduction of CO, but eventually CO stabilizes, and that is the mechanisms by which afterload must be reduced, in compensation. However, it is poorly worded if it explains it like that, as if the afterload reduction is the primary effect. I believe it is definitely a secondary (if not tertiary) one.

Directly, B-blockers decrease angina by decreasing the amount of work done by the heart and decreasing its oxygen demand, obviously.
 
What idiopathic said.

First Aid is useful as a checklist of things to learn, but if you try and glean conceptual information from it directly you will get tripped up by the shoddy explanations.
 
Beta blockers' mechanisms are not completely worked out but there are some good hunches. Beta blockers do 2 main things: they cause a decrease in C.O. by their negative inotropic and chronotropic effects on the heart and they cause a blockade of renin release from the kidneys. They are actually increasing preload by their effect of decreasing HR (with fewer beats, the heart has more time in diastole for LV filling). When a patient first takes a beta blocker, they might see a transient worsening of heart failure but they are one of the three drug classes known to increase survival (or decrease mortality) in heart failure. You can see worsening due to an increase in peripheral vascular resistance upon initial taking the drug but it will fall due to the decreased sympathetic outflow (the improvement could take a while, i.e. months).
 
Thanks for your replies everyone! What Idiopathic said was what we were taught, but I was going through my first aid for finals review and when I ran across that, I was just :confused: . Thanks again! :)
 
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