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 effect of B1 agonist on preload??  



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Author12 Posts
  #1

B1 agonist increases contractility and vent systolic pressure...fair enuff...however im confused how it decreases preload(this is wat kaplan says)....can anyone explain this plz???




  #2

i think due to the heart rate increase, though i am not sure.


  #3

ISOPRETERENOL- B1=B2 so,, inc HR & dec PVR (dec preload)...

it could be the ans,, not sure.



  #4

I agree with Geroo


  #5

if we administer alpha 1 agonist, there is a compensatory decrease in heart rate right? this is due to reflex parasymp on the heart to maintain bp to constant level.

so does the reverse hold true too? if we administer beta 1 agonist there is a decrease in sympathetics to the periphery with vasodilation to decrease PR and maintain BP?

thanks in advance to anyone who can help!


  #6

I don't know. I think that increased heart rate is enough to decrease preload


  #7

i agree with u all....Beta agonists act on the heart with respect to increasing its inotropy, chronotropy and dromotropy, tht wud be- contractility, heart rate and conduction velocity, respectively.

the effects of inc heart rate wud be decreased ventricular filling time in diastole resulting in a dec preload. after all ventricular end-diastolic volume is an index of ventricular preload.

dharjma, u ask whether the ANS reflexes wud reflexively cause vasodilation in the peripheral vessels when a beta1 agonist is given. can someone shed some light on this?



  #8

ANS to Djharma

when we give aa alpha1 agonist, it acts on the vessels and constricts it, hence this will be detectesd by the baroreceptors in the aortic arch and they will activate the parasymapatheticas and the heart rate will decrease

now suppose u give an alpha1 antagonist then it will dilate the vessels, and this will again be detected by the baroreceptors and they will increase the heart rate,...

now ans to ur question when u give a beta agonist, it will increase the heart rate, and the ionotropicity and the chronotropicity, but it will have no effect on the vessels, hence the baroreceptor will not tdetect the changes, which it does by distention or relaxation of vessels, i mean the stretch on the vessels, so it will not have a compensatory change...

hope it helps



  #9

agree with sturge,

vessels contain beta-2 receptors, so beta-1 agonist (at low doses atleast) will not elicit a compensatory response.



  #10

Well...can't resist adding to the confusion here..grin

Here's my 2-cents on this

Since

MAP= HR*SV* TPR

if you give a B1 agonist drug...it would increase the MAP (by increasing the HR and SV)...baroreceptor reflex would kick in and decrease the sympathetic and increase the parasympathetic discharge...

Decrease in the sympathetic tone should then DECREASE the preload to the heart and NOT increase it by causing pooling of blood in the capacitance vessels (a.ka. the veins)...

grin



  #11

guyz lemme conclude:

Different methods of decrease in preload by a beta 1-agonist:

a) increase in contractility : dec. end systolic volume.

b) inc. in heart rate : dec. in diastolic interval --> dec. preload

c) capacitance veins open up (good point doc)



  #12

Would increase in contractility decrease EDV? If so, preload will be decreased as well.





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