Phenylephrine's effects on pulse pressure First Aid 237 page 237

[as90]

High,
I missed a question on UWSA #2, qid 6811. I understand the answer the question, but I don't quite understand the first aid diagram.

I would see phenylephrine (pure alpha agonist) as causing an decrease in pulse pressure as it increases afterload (increases diastolic) with a decrease in systolic bp, thus pulse pressure would decrease.

In first aid it seems that both systolic and diastolic bp increase. I would imagine that systolic should decrease due to reflex bradycardia from vagal tone. Perhaps I am not interpreting the graph correctly. I am referring to the graph that is listed on page 237 before the alpha block.

Thanks in advance

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[IntelInside]

Since phenylephrine increases TPR and TPR increases both SBP and DBP, but DBP to a larger degree, PP decreases

What was the answer?

 

[xeroun]

Since phenylephrine increases TPR and TPR increases both SBP and DBP, but DBP to a larger degree, PP decreases

What was the answer?

Hmmm. I thought since Phenylephrine is an alpha 1 agonist it would cause vasoconstriction of BOTH arteries and veins. This would cause an increase in systolic and diastolic pressure hence pulse pressure stays same.

 

[IntelInside]

Hmmm. I thought since Phenylephrine is an alpha 1 agonist it would cause vasoconstriction of BOTH arteries and veins. This would cause an increase in systolic and diastolic pressure hence pulse pressure stays same.

I am fairly certain that the increase in SBP and DBP wont be equal. If you think about the histology of veins vs arteries. Looking at it from your perspective, which one will contract more? Because of the muscular nature of arteries the TPR increase, I believe, is much larger than the increase in stroke volume due to increased venous return from the vasoconstriction of veins.

Also I would check this out: http://books.google.com/books?id=1k...wAw#v=onepage&q=svr systolic pressure&f=false

Look on pg 270 on the graph and read the paragraph before and after it.

Here is another thread related to it: http://forums.studentdoctor.net/showthread.php?t=903059

Anyone else have a thought on this?

 

[IntelInside]

Does anyone have the answer to the question?

 

[MDSlacker]

Phenylephrine is acting on the alpha 1 receptors and there are 2 major locations of alpha 1 receptors that relate to blood pressure.

One place they are located is in arteries and specifically the arteries of the skin, GI, etc (basically think about where you would not want blood to go in fight/flight response) and then are also located in veins

The effect of phenylephrine, and alpha 1 receptors, is to constrict in both of these locations. By constricting veins you are effectively increasing venous return. By increasing venous return you are also increase CO (frank starling).

CO is one of the major determinants of systolic blood pressure so by increasing the cardiac output you are increasing the systolic blood pressure. The arteries are also being contracted and therefore there is an increase in the peripheral resistance. When you increase peripheral resistance you are in effect increasing the diastolic blood pressure. The diastolic pressure increases because the arteries are constricted so even during diastole there is more pressure on them because that blood is being squeezed.

So far I explained why systolic and diastolic both increase. Now why systolic increases more (to explain the increase in pulse pressure). The systolic blood pressure is increasing more than diastolic b/c there is constriction of the veins (increased VR and increased CO) and there is arterial constriction (increased TRP). By increasing both the volume of blood circulating and the amount of blood being pumped out by the heart you are greatly increasing the SBP and therefore increased pulse pressure.

I don't have any sources for any of this information just went based off memory and reasoned through this as I wrote it out so if I'm wrong in regards to any of this just let me know.

 

[as90]

Phenylephrine is acting on the alpha 1 receptors and there are 2 major locations of alpha 1 receptors that relate to blood pressure.

One place they are located is in arteries and specifically the arteries of the skin, GI, etc (basically think about where you would not want blood to go in fight/flight response) and then are also located in veins

The effect of phenylephrine, and alpha 1 receptors, is to constrict in both of these locations. By constricting veins you are effectively increasing venous return. By increasing venous return you are also increase CO (frank starling).

CO is one of the major determinants of systolic blood pressure so by increasing the cardiac output you are increasing the systolic blood pressure. The arteries are also being contracted and therefore there is an increase in the peripheral resistance. When you increase peripheral resistance you are in effect increasing the diastolic blood pressure. The diastolic pressure increases because the arteries are constricted so even during diastole there is more pressure on them because that blood is being squeezed.

So far I explained why systolic and diastolic both increase. Now why systolic increases more (to explain the increase in pulse pressure). The systolic blood pressure is increasing more than diastolic b/c there is constriction of the veins (increased VR and increased CO) and there is arterial constriction (increased TRP). By increasing both the volume of blood circulating and the amount of blood being pumped out by the heart you are greatly increasing the SBP and therefore increased pulse pressure.

I don't have any sources for any of this information just went based off memory and reasoned through this as I wrote it out so if I'm wrong in regards to any of this just let me know.

Thanks, the answer was decrease pulse pressure, it asked for which sympathomimetic would have the effects (decrease PP, etc.). Epinpehrine, phenylephrine were answer choices.

I don't have it in front of me. but what confused me was the graph in First Aid 2013 for the sympathomimetic because I thought systolic would not be increased only diastolic.