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
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.
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.
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)
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.
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?
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)
Happy-2
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