Effect of Phenylephrine (alpha agonist) on Systolic BP

[justadream]

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Why does phenylephrine (an alpha agonist) increase systolic BP?

My understanding (quite possibly wrong) was that:

Systolic BP is mostly determined by cardiac output (so heart rate + contractility of the heart are the main factors)

Diastolic BP is mostly determined by peripheral resistance


Thus, I can see why an alpha-agonist (phenylephrine) increases diastolic BP (alpha1-mediated vasoconstriction) but I cannot see why it would increase systolic BP (and in fact, does so drastically - in FA 2016 page 247, 248 you can see that phenylephrine increases systolic BP just about as much as does norepinephrine*)


*Norepinephrine acts on both alpha and beta (remember that beta1 increases heart rate and contractility) whereas phenylephrine only acts on alpha yet the two seem to have the same effect on systolic BP...


Here's my guess at the answer:
The increase in vasoconstriction is so large that it causes increased venous return which then leads to increased cardiac output (which leads to increased systolic BP)?

 

[ASM student]

P = Q x R
Systolic blood pressure determined mainly but not only by the cardiac output (Q)
Diastolic pressure is determined mainly but not only with TPR
This never rules out an increase in systolic blood pressure with phenylephrine as the heart is pumping against increased resistance .

Sent from my PLK-L01 using Tapatalk

 

[Phloston]

Why does phenylephrine (an alpha agonist) increase systolic BP?

My understanding (quite possibly wrong) was that:

Systolic BP is mostly determined by cardiac output (so heart rate + contractility of the heart are the main factors)

Diastolic BP is mostly determined by peripheral resistance


Thus, I can see why an alpha-agonist (phenylephrine) increases diastolic BP (alpha1-mediated vasoconstriction) but I cannot see why it would increase systolic BP (and in fact, does so drastically - in FA 2016 page 247, 248 you can see that phenylephrine increases systolic BP just about as much as does norepinephrine*)


*Norepinephrine acts on both alpha and beta (remember that beta1 increases heart rate and contractility) whereas phenylephrine only acts on alpha yet the two seem to have the same effect on systolic BP...


Here's my guess at the answer:
The increase in vasoconstriction is so large that it causes increased venous return which then leads to increased cardiac output (which leads to increased systolic BP)?

You definitely don't want to over-think this. No, an alpha-1 agonist will not increase BP through increased preload/venous return. Arteriolar vasoconstriction simply increases afterload against the left ventricle, so the pressure required to pump against narrower vessels is greater during systole. Diastolic should also increase in the setting of arteriolar vasoconstriction.

 

[justadream]

You definitely don't want to over-think this. No, an alpha-1 agonist will not increase BP through increased preload/venous return. Arteriolar vasoconstriction simply increases afterload against the left ventricle, so the pressure required to pump against narrower vessels is greater during systole. Diastolic should also increase in the setting of arteriolar vasoconstriction.

So to clarify, are you saying the phenylephrine does the following:

Arteriolar vasoconstriction ==> increased afterload ==> increased systolic BP

Arteriolar vasoconstriction ==> increased preload/venous return ==> increased diastolic BP

 

[Phloston]

So to clarify, are you saying the phenylephrine does the following:

Arteriolar vasoconstriction ==> increased afterload ==> increased systolic BP

Arteriolar vasoconstriction ==> increased preload/venous return ==> increased diastolic BP

You definitely don't want to over-think this. No, an alpha-1 agonist will not increase BP through increased preload/venous return. Arteriolar vasoconstriction simply increases afterload against the left ventricle, so the pressure required to pump against narrower vessels is greater during systole. Diastolic should also increase in the setting of arteriolar vasoconstriction.

 

[justadream]

Sorry for my apparent ignorance about this subject but are you saying that "Arteriolar vasoconstriction ==> increased afterload ==> increased systolic BP" is correct but the latter statement (about vasoconstriciton leading to increased diastolic BP via increased preload/venous return) is incorrect?

If so, then I have my answer for how/why systolic BP increases after phenylephrine but now I'm struggling to understand the mechanism by which vasoconstriction leads to increased diastolic pressure.

 

[Kendragon]

Instead of blood vessels filled with fluid, think of a balloon filled with some air.

If you squeeze the balloon in your hands, the pressure in the balloon will grow up. This is like how vasoconstriction (squeezing), increases diastolic pressure.

Now, you exhale into the balloon to inflate it. This is like systolic pressure, since you're pumping matter into the balloon. When you exhale (systole), the pressure in the balloon goes up. Now, squeeze the balloon as you exhale into it. The pressure in it will be even higher. This is why vasoconstriction (squeezing), also increases systolic pressure.

 

[poohbear12345]

Yea, you are probably over thinking this one remember Flow (Q) = P/R, if we are keeping flow constant, increasing Resistance such as an alpha constrictor means pressure will increase. Remember the body regulates to a certain pressure at the baroreceptors (like how baroreceptors reset the setting of chronic hypertension https://www.ncbi.nlm.nih.gov/pubmed/3042363)and flow indirectly (Via sympathetic tone to the heart) and (vasodilation secondary to ischemia in poor perfusion states). But that makes it more complex than it needs to be. Just remember Flow (Q ) = P/R your body will tolerate increased pressures to maintain flow. Diastolic BP is more often reflective of sympathetic tone where Systolic BP is a function of pump function and the capacitance/elastance of blood vessels (think of how Isolated Systolic Hypertension- is secondary to calcification/atherosclerosis). If im wrong here or this doesnt make sense, pleaes let me know, always looking to refine my understanding.

 

[A$AP]

Why does phenylephrine (an alpha agonist) increase systolic BP?

My understanding (quite possibly wrong) was that:

Systolic BP is mostly determined by cardiac output (so heart rate + contractility of the heart are the main factors)

Diastolic BP is mostly determined by peripheral resistance


Thus, I can see why an alpha-agonist (phenylephrine) increases diastolic BP (alpha1-mediated vasoconstriction) but I cannot see why it would increase systolic BP (and in fact, does so drastically - in FA 2016 page 247, 248 you can see that phenylephrine increases systolic BP just about as much as does norepinephrine*)


*Norepinephrine acts on both alpha and beta (remember that beta1 increases heart rate and contractility) whereas phenylephrine only acts on alpha yet the two seem to have the same effect on systolic BP...


Here's my guess at the answer:
The increase in vasoconstriction is so large that it causes increased venous return which then leads to increased cardiac output (which leads to increased systolic BP)?

A1 receptors are also located on veins. Phenylephrine will activate those, increase preload, and thus systolic pressure.
Unless I am missing something I'm not sure why all the complex discussion is needed

 

[ChessMaster3000]

Agree with phloston, DONT overthink this. The only thing I can think of off the top of my head where diastolic pressure goes down and systolic pressures stays normal or increases is aortic regurgitation