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 USMLE Step 1 Bits & Pieces High yield short focused points, monographs, charts, illustrations, tables, and other stuff related to the USMLE Step 1 Exam.

#1
08-25-2009
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 3,316 Threads: 170 Thanked 4,921 Times in 1,809 Posts Reputation: 4974
Cardiac Output - Venous Return Curves

Cardiac output venous return curve is inevitable in USMLE step 1
Let's have a look

click image to enlarge

The Y axis represents the Cardiac output (CO) or Venous return (VR) in l/min
The X axis represents the right atrial pressure (RAP) in mm Hg

Curves that slope from Y to X represent venous return
Curves that slope from X axis and go up represent the cardiac output

The intersection of each two curves represent the actual status of a given case, for example the central point represent the normal physiological resting state with 5 l/min CO and VR and RAP of 2 mm Hg.

The intersection of VR curves with the X axis represent what's called the mean systemic filling pressure MSFP (the tightness with which the circulatory system is filled with blood) and the greater the MSFP the easier it is to push blood back into the heart, it's the pressure in the circulation when the heart stops and blood redistributes and equilibrate in all the vessels (classic USMLE question, is few seconds to one minute after ventricular fibrillation).

Higher VR curves represent higher volumes and and likewise lower VR curves represent hypovolemia, for CO it's the higher contractility that shift the curve up and heart failure that shift the curve down.

Examples:
Point H is a failed heart with increased preload
Point C can be seen in excercise where we have higher cardiac output because of sympathetic stimulation and higher venous retrun because of pressurized peripheral veins and dilated muscular arterioles

Last edited by Sabio; 08-25-2009 at 11:35 PM.
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#2
11-16-2009
 USMLE Forums Scout Steps History: 1 + CK Posts: 33 Threads: 3 Thanked 33 Times in 12 Posts Reputation: 43

That's a great explanation
Thank you Sabio
#3
11-21-2009
 USMLE Forums Scout Steps History: 1 + CS Posts: 95 Threads: 7 Thanked 67 Times in 21 Posts Reputation: 87
CO-VR curves

Good explanation

#4
04-07-2011
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Hi. If you may, can you clarify or expound on how this can be asked in the MLE? Because you said its a classic USMLE question.

Were you referring to what MSFP means or to the change in the CO-VR curve after ventricular fib.
#5
04-07-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 3,316 Threads: 170 Thanked 4,921 Times in 1,809 Posts Reputation: 4974

Quote:
 Originally Posted by msttelschmerz Hi. If you may, can you clarify or expound on how this can be asked in the MLE? Because you said its a classic USMLE question. Were you referring to what MSFP means or to the change in the CO-VR curve after ventricular fib.
The curve can come in several ways and in multiple possible scenarios. Therefore it's essential to understand how the curve works and what does MSFP means so that you can answer any question about it.
 The above post was thanked by: msttelschmerz (04-07-2011), NAZIAMK (08-09-2011)
#6
04-07-2011
 USMLE Forums Newbie Steps History: --- Posts: 2 Threads: 0 Thanked 1 Time in 1 Post Reputation: 11

It is an interesting subject but I cant understand why the point (H) correspond to heart failure, I learned that heart failure causes decreased contractility with decreased co and the curve of co shifts downward but the vr curve stays the same can someone explain this please thank u....
#7
04-07-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 82 Threads: 8 Thanked 114 Times in 49 Posts

I think, in Heart failure the contractility is decreased but the body tries to compensate it by increasing the Blood volume, that's the reason, why the pts with CHF are in volume overload.
So, the point which shows Heart failure should show two things, dec. contractility & inc. Blood volume or as Sabio explained the inc. MSFP (mean systemic filling pressure)
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#8
08-09-2011
 USMLE Forums Scout Steps History: Not yet Posts: 72 Threads: 8 Thanked 25 Times in 16 Posts Reputation: 35

Quote:
 Originally Posted by Sabio Cardiac output venous return curve ..... dilated muscular arterioles
Xcellent xplanation...this forum Rocks...!!!
#9
09-15-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 3,263 Threads: 147 Thanked 5,053 Times in 1,567 Posts Reputation: 5149

Quote:
 Originally Posted by zozolight It is an interesting subject but I cant understand why the point (H) correspond to heart failure, I learned that heart failure causes decreased contractility with decreased co and the curve of co shifts downward but the vr curve stays the same can someone explain this please thank u....
The venous return curve stays the same because you still have the same amount of blood in the circulation.
If you have blood loss for example, the venous return curve will go down.

-
 The above post was thanked by: gokorie (09-18-2011)
#10
12-14-2011
 USMLE Forums Veteran Steps History: Not yet Posts: 226 Threads: 60 Thanked 30 Times in 24 Posts Reputation: 40

thx alot for this curve but what is the effect of b agonist alone on the curve??
#11
12-18-2011
 USMLE Forums Guru Steps History: 1 + CS Posts: 388 Threads: 48 Thanked 237 Times in 128 Posts Reputation: 247
Cardiac output venous return curves

Can anyone clarify what happens in Cardiac Output-Venous Return curve in case of
a, anaphylactic shock
b, septic shock
c, AV shunts?

Would really appreciated if anyone can help in this matter.
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#12
12-18-2011
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 262 Threads: 6 Thanked 371 Times in 154 Posts Reputation: 381

Quote:
 Originally Posted by shyangel18 Can anyone clarify what happens in Cardiac Output-Venous Return curve in case of a, anaphylactic shock b, septic shock c, AV shunts? Would really appreciated if anyone can help in this matter.
In anaphylactic and septic shocks, you have vasodilation and therefore more vernous return coming back to the heart but you have decreased right atrial pressure and therefore it's point B in the graph posted above by Sabio.

#13
12-20-2011
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That's a great explanation
#14
01-04-2012
 USMLE Forums Newbie Steps History: Not yet Posts: 1 Threads: 0 Thanked 1 Time in 1 Post Reputation: 11

Quote:
 Originally Posted by Hokolesqua In anaphylactic and septic shocks, you have vasodilation and therefore more vernous return coming back to the heart but you have decreased right atrial pressure and therefore it's point B in the graph posted above by Sabio. I don't know about AV-shunt
Well, i think as for anaphylactic and septic shocks, although there is a widespread vasodilation of arterioles and small veins, there is also a third-spacing of fluid (interstitial etc.) due to increased blood vessel permeability. Actually the total blood volume drops dramatically. So the VR curve should move left parallely.

Is that right?
 The above post was thanked by: prep891 (03-27-2014)
#15
05-08-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10
Cardiac output venous return curve is inevitable in USMLE step 1

Quote:
 Originally Posted by Sabio Cardiac output venous return curve is inevitable in USMLE step 1 Let's have a look......peripheral veins and dilated muscular arterioles
Thank you for your answer! Just to clarify, the mean systemic filling pressure is the pressure of blood exerted on tissues by virtue of its tendency to fill spaces when the pumping action of the heart and muscles is taken away? This is relevant in something like cardiothoracic surgery, critical care of heart failure, right? Can it be tested in any particular way?

#16
05-08-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 2 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10
That's what I was thinking! Is that right?

Quote:
 Originally Posted by workhard Well, i think as for anaphylactic and septic shocks, although there is a widespread vasodilation of arterioles and small veins, there is also a third-spacing of fluid (interstitial etc.) due to increased blood vessel permeability. Actually the total blood volume drops dramatically. So the VR curve should move left parallely. Is that right?
That's what I was thinking! Is that right?
#17
05-11-2013
 USMLE Forums Newbie Steps History: Not yet Posts: 3 Threads: 0 Thanked 0 Times in 0 Posts Reputation: 10
Acute hemorrhage versus sudden drop in blood volume

Quote:
 Originally Posted by luschka I think this is such a high yield topic. ........ If you have blood loss for example, the venous return curve will go down."---
Does anyone know why sudden drop in blood volume is point A while acute hemorrhage is point D?

Aren't they the same things?

Why would sudden drop in blood volume cause a compensation by the sympathetic nervous system while hemorrhage won't according to luschka?
#18
06-12-2013
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thanks alot
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#19
08-06-2013
 USMLE Forums Scout Steps History: 1 + CS Posts: 43 Threads: 1 Thanked 40 Times in 20 Posts Reputation: 50

Chronic anemia . [choice B]; (increase cardiac output)
o causes an increase in cardiac output in an effort to meet the metabolic demands of the tissues. This causes an increase in the slope of the cardiac output graph.
o iron deficiency anemia for example. Deficient iron causing a decrease in the concentration of Hb, causing a decrease in the total O2 content and thus tissue hypoxia.
o Anemia is believed to cause increased cardiac output by (1) decreased blood viscosity; and (2) increased sympathetic tone
o The heart can respond to tissue hypoxia by increased cardiac output. The increased output is matched by decreased peripheral vascular resistance and decreased blood viscosity (thinner blood flows more freely than thick blood), so that cardiac output can rise without an increase in blood pressure. Generally, anemia must be fairly severe (hemoglobin < 7 g/dL) before cardiac output rises.
o Cardiac out put = heart rate x stroke volume
o In anemia, the cardiac output increases, and that allows more hemoglobin to be exposed to the peripheral tissues, making up for the decreased hemoglobin concentration. Accordingly, the heart rate increases, which gives us one of the cardinal clinical manifestations of anemia, tachycardia, or fast heart rate.

You didn't consider the decrease in viscosity in anemia
(Causing the venous return curve to shift clock wise)
#20
08-23-2013
 USMLE Forums Newbie Steps History: --- Posts: 2 Threads: 1 Thanked 0 Times in 0 Posts Reputation: 10
thank you

great explanation.
Thank you Sabio.
#21
09-02-2013
 USMLE Forums Scout Steps History: Not yet Posts: 21 Threads: 0 Thanked 12 Times in 8 Posts Reputation: 22

Quote:
 Originally Posted by shyangel18 Can anyone clarify what happens in Cardiac Output-Venous Return curve in case of a, anaphylactic shock b, septic shock c, AV shunts? Would really appreciated if anyone can help in this matter.
anaphylactic shock: fluid extravasation and vasodilation+normal heart contractibility. so third spacing(edema) occurs and causes Decreased Venous Return and so low cardiac output.(D point on the curve I think).

Septic shock: usually normal or high cardiac output, low peripheral vascular resistance and high venous return. so I guess its C point or E point (high CO or Normal CO respectively)

AV shunt: means increased venous blood so enhanced frank-starling force and increased CO. E point on curves

cardiogenic shock (pump failure): normal venous return and decreased myocardial performance. G point

Im not very sure about these, because the body dosent work as simple as this!...but I think its enough for passing exams.

 Tags Cardiovascular-, Figures-, Physiology-

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