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Old 05-28-2015
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Default Diastolic/Systolic Afterload/Preload

Ok i am kind of confused about the relationship b/w Dias.BP/Syst.BP and Pre/Afterload. Can someone please correct me if this is wrong:

diastolic pres.= pressure in aorta in between ventricular contractions, and is proportional to the afterload (TPR) which the heart must over come during systole to eject the blood(CO), and the blood that must be pumped out is the preload???

I'm not sure if thats right but thats what i have come to and i'd appreciate it if someone can correct any errors and further explain anything else missing that would help me understand this concept. Any input is much appreciated.

Thanks!
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Old 05-29-2015
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Quote:
Originally Posted by sm3356 View Post
Ok i am kind of confused about the relationship b/w Dias.BP/Syst.BP and Pre/Afterload. Can someone please correct me if this is wrong:

diastolic pres.= pressure in aorta in between ventricular contractions, and is proportional to the afterload (TPR) which the heart must over come during systole to eject the blood(CO), and the blood that must be pumped out is the preload???

I'm not sure if thats right but thats what i have come to and i'd appreciate it if someone can correct any errors and further explain anything else missing that would help me understand this concept. Any input is much appreciated.

Thanks!
Technically diastolic BP is not BP between contractions, it is the lowest pressure in the arteries that can be recorded during the cardiac cycle and it happens to occur when the ventricles are relaxed. It is one of the indices for afterload. It is created by the volume of blood remaining in the aorta. After the aortic valve has closed, blood is going down its pressure gradient, perfusing the capillaries, getting to the venous side of the circulation, diastolic pressure is the smallest amount of blood remaining in the arteries. Relationship with TPR is that higher TPR means lower flow and more blood will stay in the aorta (any artery). Ventricles must develop pressure higher than diastolic BP. I think of it in two ways, (1) for blood to flow there must be a pressure gradient, so LV pressure must be higher than pressure in the aorta for blood to flow from the LV to the aorta, (2) diastolic BP is the pressure that is keeping aortic valve closed, blood is kind of pushing the valve in the direction of the ventricle and since the valve does not open in the backwards direction it is kept closed, heart has to push the valve with a higher force than diastolic BP to open the valve.

Amount of blood ejected during 1 cycle is the stroke volume (SV), cardiac output (CO) is the volume of blood ejected per unit time (usually per minute, which is why we use the formula SVxHR). Preload is not the amount of blood that must be pumped out. Preload is the volume of blood in the LV before ventricles start contracting, in other words after the ventricles are done filling completely (end-diastolic volume). Not all of the blood in the ventricle is ejected. The fraction of the blood ejected is ejection fraction (EF). About 55-60% of the end-diastolic volume is ejected.

Now I know I might not have explained it in a "scientifically" correct way (especially the blood pressure part) but that is how I think about that stuff.

Last edited by Nodo; 05-29-2015 at 08:30 AM.
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Old 05-29-2015
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wow great explanation, definitely clarified a lot for me.

another question in regards to TPR, so as u said the higher the TPR the more blood remains in the artery, so does that mean the diastolic pressure goes up when TPR goes up? and then that means the systolic pressure must also go up to over come the "pressure gradient" ? i just want to make sure i am understanding correctly, i'm still kinda confused about the TPR and afterload.

thanks so much for ur time!
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Originally Posted by sm3356 View Post
wow great explanation, definitely clarified a lot for me.

another question in regards to TPR, so as u said the higher the TPR the more blood remains in the artery, so does that mean the diastolic pressure goes up when TPR goes up? and then that means the systolic pressure must also go up to over come the "pressure gradient" ? i just want to make sure i am understanding correctly, i'm still kinda confused about the TPR and afterload.

thanks so much for ur time!
Yes. You can think of TPR as the "degree" of arteriolar constriction. If we go back to hemodynamics, Q=dP/R, where Q is flow (mL/min), dP is pressure difference, and R is resistance to flow. Resistance is inversely related with flow. Higher the resistance, lower the flow. A few things determine resistance, one of the most important ones is the vessel radius (important because body can change it very quickly and easily and it has a great effect). If you remember, there is a fourth power relationship between vessel radius and resistance. So if you decrease radius by two (constrict the arterioles) you will increase resistance by a factor of 16. More blood on the arterial side equals a higher arterial pressure. To add to that, arteries are not as compliant as veins, so increased blood volumes cause a greater change in pressure in the arteries as compared to veins.

Diastolic pressure goes up when TPR goes up (again, more blood on the arterial side). It is not systolic pressure that has to go up to overcome the increased TPR, heart has to develop more pressure to open the valve. It takes a greater pressure to open the valve and since you actually want to eject some blood, ejecting blood will further increase the pressure. So systolic pressure will have to go up as well. Increased afterload will also decrease stroke volume (less blood will be ejected). I will try to attach a graph to illustrate whats going on. I do not know if you have seen pressure-volume loops before but it is a nice way to demonstrate pressure and volume changes during a cardiac cycle. I suggest you take a look at those, plus the word on the street is that USMLE loves to ask about those. Anyways, here is the graph, try to see whats going on and I will help you read it if you have not seen it before. Cheers.
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Diastolic/Systolic Afterload/Preload-screen-shot-2015-05-29-8.24.26-pm.png  
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Some great replies here already, but wanted to jump in to add a bit more. I like to keep things very simple because that's what I can remember. You're basically correct, although physiologists could probably quibble with terminology, but they'll do that no matter you say

1) DBP is kinda like afterload, but beware of examples where it is NOT. Aortic Stenosis pops into mind; in that case, you have MUCH higher afterload than your DBP because the resistance of the valve is the issue. On step 1, make sure you're always thinking through the mechanism of what's happening because the love to write distractors that will get you.

2) Generally TPR and diastolic pressure are related, but again keep in mind disordered states where it is not. Hypovolemia is a big one -- think of the trauma victim who rolls in with a BP of 60/40 -- they are probably as vasoconstricted as possible, yet their DPB is dropping because they have no volume. I could imagine a step 1 question where they ask you about next step in management to bring up their BP -- you would have to give fluids, not a vasopressive drug. OF course they could just as easily give you someone in septic shock; you would still give fluids (lots), but you would also want to consider vasopressors. Either of these would make a great arrows question too.

Again, always know the mechanisms and they can't trick you. They will have to tell you enough information to make the correct decision.

I'll leave you with two of my favorite simple equations because I think they are helpful because they are always true.

CO = Heart Rate x Stroke Volume

MAP = CO x SVR [remember this is basically Ohms law: V=IR]

3 components of Stroke volume: preload, afterload, contractility

There's really no CV physiology question you can't answer with those, or at least eliminate a LOT of wrong answer choices. Cardiac output low? Then either HR or SV is low. Period. Always. CO low and HR high/normal? MUST be SV. Now you just have to figure out why: low/high preload, high afterload, or impaired contractility. MUST be one of those. Cannot be anything else.

So for your question, that equation can be combined:

MAP = HR x SV x SVR

So MAP (most of which is DBP) is related to SVR, but there are other things as well. MAP is always related to afterload (as part of Stroke volume), but remember to keep it in its broader context.
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Originally Posted by Young Doc View Post
Some great replies here already, but wanted to jump in to add a bit more. I like to keep things very simple because that's what I can remember. You're basically correct, although physiologists could probably quibble with terminology, but they'll do that no matter you say

1) DBP is kinda like afterload, but beware of examples where it is NOT. Aortic Stenosis pops into mind; in that case, you have MUCH higher afterload than your DBP because the resistance of the valve is the issue. On step 1, make sure you're always thinking through the mechanism of what's happening because the love to write distractors that will get you.

2) Generally TPR and diastolic pressure are related, but again keep in mind disordered states where it is not. Hypovolemia is a big one -- think of the trauma victim who rolls in with a BP of 60/40 -- they are probably as vasoconstricted as possible, yet their DPB is dropping because they have no volume. I could imagine a step 1 question where they ask you about next step in management to bring up their BP -- you would have to give fluids, not a vasopressive drug. OF course they could just as easily give you someone in septic shock; you would still give fluids (lots), but you would also want to consider vasopressors. Either of these would make a great arrows question too.

Again, always know the mechanisms and they can't trick you. They will have to tell you enough information to make the correct decision.

I'll leave you with two of my favorite simple equations because I think they are helpful because they are always true.

CO = Heart Rate x Stroke Volume

MAP = CO x SVR [remember this is basically Ohms law: V=IR]

3 components of Stroke volume: preload, afterload, contractility

There's really no CV physiology question you can't answer with those, or at least eliminate a LOT of wrong answer choices. Cardiac output low? Then either HR or SV is low. Period. Always. CO low and HR high/normal? MUST be SV. Now you just have to figure out why: low/high preload, high afterload, or impaired contractility. MUST be one of those. Cannot be anything else.

So for your question, that equation can be combined:

MAP = HR x SV x SVR

So MAP (most of which is DBP) is related to SVR, but there are other things as well. MAP is always related to afterload (as part of Stroke volume), but remember to keep it in its broader context.
Wow. Great points!
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Old 05-29-2015
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@nodo & @youngdoc
thanks for the great explanations, appreciate ur time!
both of your explanations helped clarify a lot.
i'm sure many other students will find these very helpful as well!!
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