Why increased cardiac output in anemia! - USMLE Forums
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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.


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  #1  
Old 09-25-2009
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Heart Why increased cardiac output in anemia!

A central concept in answering many "easy questions" in step 1.

Remember that hemoglobin carries oxygen and this O2 together with the dissolved O2 in plasma comprises the total oxygen content of the blood.

In anemia, there's no problem with the dissolved oxygen which determines the PO2 which in turn determines the saturation. All these three are normal in anemia.

What's decreased in anemia is the hemoglobin concentration. So we have a decreased oxygen content of the blood.

The oxygen delivery to the tissues is a measure of Cardiac Output and Oxygen content of the blood.

So here's why the cardiac output increases:

  • To compensate for the lowered oxygen content, this is done by increasing the heart rate and the stroke volume (hence the flow murmur in severe anemia).
  • On the other hand, RBC concentration is a major determinant of blood viscosity and so in anemia there's less viscosity and so less resistance to flow which translates to decreased peripheral vascular resistance.
  • The tissue hypoxia itself causes vasodilation
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  #2  
Old 03-29-2011
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Great explanation!
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  #3  
Old 11-16-2011
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Quote:
Originally Posted by step1addict View Post
A central concept in answering many "easy questions" in step 1.

Remember that hemoglobin carries oxygen and this O2 together with the dissolved O2 in plasma comprises the total oxygen content of the blood.

In anemia, there's no problem with the dissolved oxygen which determines the PO2 which in turn determines the saturation. All these three are normal in anemia.

What's decreased in anemia is the hemoglobin concentration. So we have a decreased oxygen content of the blood.

The oxygen delivery to the tissues is a measure of Cardiac Output and Oxygen content of the blood.

So here's why the cardiac output increases:

  • To compensate for the lowered oxygen content, this is done by increasing the heart rate and the stroke volume (hence the flow murmur in severe anemia).
  • On the other hand, RBC concentration is a major determinant of blood viscosity and so in anemia there's less viscosity and so less resistance to flow which translates to decreased peripheral vascular resistance.
  • The tissue hypoxia itself causes vasodilation
1. Can u please tell me why do they (heart rate and stroke volume) increase in decreased oxygen content? is it due to nervous phenomenon? obviously theres nothing with the chemoreceptor since u said PO2 remains same. than how are they increased? pls explain

2. I have concept regarding the high cardiac output in anemia. please correct me if i am wrong. 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. Does this sound alright?

Last edited by cadaver; 11-16-2011 at 11:07 PM. Reason: to add another question
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  #4  
Old 11-17-2011
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Great post cadaver
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  #5  
Old 11-19-2011
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Quote:
Originally Posted by cadaver View Post
1. Can u please tell me why do they (heart rate and stroke volume) increase in decreased oxygen content? is it due to nervous phenomenon? obviously theres nothing with the chemoreceptor since u said PO2 remains same. than how are they increased? pls explain

2. I have concept regarding the high cardiac output in anemia. please correct me if i am wrong. 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. Does this sound alright?
Your number 2 concept is the same that I mentioned in my post!!
Your number 1 question; answer is sympathetic activation.
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  #6  
Old 11-24-2011
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Great post. Thank you!
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  #7  
Old 08-24-2012
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FA 2012, page 281 says that contractility and stroke volume decrease with hypoxia and hypercapnia, any one has some explanation here?
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