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#1
05-19-2015
 USMLE Forums Addict Steps History: --- Posts: 137 Threads: 62 Thanked 3 Times in 3 Posts Reputation: 13
Q. PaO2

I tried to find about PaO2 and it is defined as the amount of O2 dissolved in blood. I am confused whether it also measures the amt. of o2 bound to hemoglobin or not? Can anybody Pls explain about this PaO2. Thanks.

#2
05-19-2015
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PaO2 is the partial pressure of the oxygen in gas form i.e. Unbound. Oxygen bound to hemoglobin in given by SaO2. The PaO2 plus the SaO2 plus the total Hb in g/dl make up the O2 CONTENT. Also given by formula content= (hb* 1.34 * Sao2) +PaO2
#3
05-19-2015
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 66 Threads: 0 Thanked 101 Times in 50 Posts Reputation: 111

Quote:
 Originally Posted by Aumsai I tried to find about PaO2 and it is defined as the amount of O2 dissolved in blood. I am confused whether it also measures the amt. of o2 bound to hemoglobin or not? Can anybody Pls explain about this PaO2. Thanks.
It does NOT measure the oxygen bound to Hgb. As Hassan says, it's merely measuring the unbound part.

Many people do confuse it with SaO2 (ie. oxygen saturation), but they are not the same. If anything, PaO2 is relatively meaningless in terms of oxygen content because of one thing that Hassan left out of the formula: the solubility constant of oxygen, so it's really:

O2 content=(SaO2 x Hbg x 1.34) + (PaO2 x 0.0031)

Where I find PaO2 helpful clinically are things like determining severity of ARDS (ie. P:F ratio -- will not be on step 1, but is fair game for 2CK), or for calculating the A-a gradient.
 The above post was thanked by: Aumsai (05-19-2015), gokulramani (05-19-2015)
#4
05-19-2015
 USMLE Forums Addict Steps History: --- Posts: 137 Threads: 62 Thanked 3 Times in 3 Posts Reputation: 13

Thanks young doc and Hassan for superb clarification.

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