View Full Version : why cardiac output increases in anemia?

11-17-2011, 02:18 AM
Why there is increased cardiac output in anemia?

somebody says it is due to decrease oxygen content (notice not the PO2 which remains normal). Can anyone explain me how decreased oxygen content can increase heart rate and stroke volume ?

others explained that in the following way :
In anemia, since blood viscosity decreases, so the total resistance decreases. And we know Mean Arterial Pressure (MAP)= Cardiac output X Total Resistance. So according to this equation MAP also decreases since the total resistance decreases. To compensate this, Sympathetic neurons fire and increase the heart rate and stroke volume. And so cardiac output increases.

I noticed people emphasizes on 1st explanation. I dont know what is the correct answer and the explanation. Please help.

11-22-2011, 08:37 PM
Anemia is a very broad term and is used generally. Anemia can refer to the reduction in the number of erythrocytes, reduction in the concentration of oxygenated hemoglobin, and reduction in hematocrit. Then there's acute or chronic anemia, as well as different causes or types. Examples are decrease in Hb concentration, or altered quality of hemoglobin.

Let's look at 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.

In this case there is only a slight decrease in the number of erythrocytes and is not enough to cause erythropoiesis which would increase not decrease the viscosity of the blood toward normal levels. However, in the case of chronic iron deficiency, that's usually due to chronic blood loss, the amount of blood loss can surpass the necessary amount of circulating erythrocytes that's maintained by erythropoiesis and at which point can lead to a decrease in blood viscosity.
I would assume that CO can increase by decreased viscosity and TPR, but more importantly, what is the status of the blood loss? Is the patient having trouble maintaining their BP by having a decreased circulating volume due to the amount of blood lost? In this case surely CO would be increased largely due decreased circulating volume, decreased stretch of vessel walls, increased firing of sympathetics, decreased firing of parasympathetics, and increased HR and CO.

As far as total O2 content or hypoxia and CO is concerned I have yet to come across a direct or indirect relationship. There is angiogensis in response to hypoxia, and don't forget the effect of hypoxia on Hb where it can affect the affinity of Hb to oxygen (right and left curve shifts). Does anyone know of a direct or indirect relationship?

Also chronic anemia can lead to increased cardiac mass because of volume overload which can be due to excessive erythropoiesis. Not to mention in chronic blood loss kidneys are compensating over time by increasing blood volume.

Hope this helps.

11-13-2013, 03:58 AM
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