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Old 06-11-2011
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Question PaO2 decreases as Hb Falls

In FA 2010, Page 506, "Oxygen Content of Blood"

It states: "O2 content of arterial blood decreases as Hb falls, but O2 saturation and arterial Po2 Do not"

Now my understanding is that blood PaO2, is the amount of O2 in the blood. O2 that is carried by Hb = O2 Saturation.

How is the O2 content of arterial blood falling if Hb decreases? I mean, if there is less Hb to bind to, shouldn't the arterial O2 be increased?

Additionally, O2 Saturation should call, and PaO2 should rise?
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Old 06-11-2011
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Quote:
Originally Posted by patelMD View Post
In FA 2010, Page 506, "Oxygen Content of Blood"

It states: "O2 content of arterial blood decreases as Hb falls, but O2 saturation and arterial Po2 Do not"

Now my understanding is that blood PaO2, is the amount of O2 in the blood. O2 that is carried by Hb = O2 Saturation.

How is the O2 content of arterial blood falling if Hb decreases? I mean, if there is less Hb to bind to, shouldn't the arterial O2 be increased?

Additionally, O2 Saturation should call, and PaO2 should rise?
There are a couple of issues we have to address in this question.

O2 content = Hb x 1.34 x SaO2 + 0.003 x PaO2

I assume you are familiar with this equation (one of the most important in medicine).

Blood PaO2 is NOT the amount of O2 in the blood, O2 content is. Essentially the equation is saying that you can carry O2 in the blood in one of 2 ways: 1) Bound to Hb [Hb x 1.34 x SaO2] or 2) Freely dissolved [0.003 x PaO2].

The key is to understanding what we mean by partial pressure of a gas in a solution. When you really consider this idea, it makes no sense. The whole point of gas pressure is the free movement of gas colliding with container walls which is something it can't do in solution. So partial pressure of a gas in solution really means the equilibrium pressure of the gas in the air with it's solution form (simple physics idea). In the case of PaO2 it really refers to the partial pressure of O2 in the lungs during exchange (around 100mmHg dropping to 95mmHg in the arterial system due to lung's own O2 consumption).

So as long as you have intact respiratory function your PaO2 should be the same. But from the equation you can easily see that when Hb drops O2 content also drops. SaO2 should be the same because while Hb is low they are still normal and thus bind O2 normally. PaO2 should be normal as your respiratory function is intact.
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Old 06-11-2011
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In easy words..
pO2 is the dissolved oxygen
while O2 content is both the dissolved O2 and O2 attached to Hb so it would be decreased in anemias..
O2 saturation is O2 per gm of Hb ..(so it would be normal in anemia because although the Hb is dec,the O2 per gm of Hb will be same)

Source..UW
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