ANS PHARMAC:: what is net effect on BP with epinephrine with alpha blocker?? - USMLE Forums
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Old 08-15-2011
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Default ANS PHARMAC:: what is net effect on BP with epinephrine with alpha blocker??

I am reading ANS pharmac and question is: what is the net effect on BP with the following drug combination: epinephrine with alpha blocker?

##Ans: will decrease BP

I know that epinephrine has alpha 1,2 and beta 1,2 action .. so when u give alpha blocker it will basically only act on Beta 1 and 2:

- beta 1 : results in postive inotropic and chromotropic effect:: so it should increase BP, right?

- beta 2 : results in vasodilation :: so it should decrease BP, right?

so how is it determined that the net effect is that it will lower BP?
could it be that epinephrine has a stronger effect on beta 2 receptors??

Ya , I am pretty confused, if u happen to know lemme know..
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Old 08-16-2011
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Default ANS Pharmac

1.Beta 2 receptors are more sensitive to epinephrine than alpa 1 receptors
2.alpha blockade will also cause vasodilatation .
Hence net effect shall be fall in BP.
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Old 08-16-2011
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Default

hi Preeti,

thx for the reply.. But my question is woudn't the beta 1 stimulation result in positive chronotropic and inotropic effect which would result in raised BP? The only explanation I can think of is that epinephrine has a stronger effect on beta 2 receptors than the beta 1.. Another thing could be that the total peripheral resistance is the main determinant for blood pressure.. So I guess for this question we should only look at alpha 1 vasoconstriction and beta 2 vasodilation, and alpha blocker will antagonize the alpha action, so you are left with beta 2 vasodation--- hence reduction in BP.. This is the explanation I can think of, but if anyone agrees or disagrees please post.. thx
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Old 08-16-2011
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Default the best explanation

Basically mean arterial pressure equals

MAP = [(2 x diastolic)+systolic] / 3

**Note that Diastole counts twice as much as systole because 2/3 of the cardiac cycle is spent in diastole.

The beta 1 activation will affect the systolic pressure.. Diastolic B.P is proportional to mean arterial resistance and beta 2 receptor action causes vasodilation...so there is decrease in diastolic B.P...overall there is decrease in mean arterial blood pressure...this phenomenon is known as epinephrine reversal or loss of pressor effect

Last edited by DOCSHO; 08-16-2011 at 09:03 PM.
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