Acute Respiratory Failure. David Aymond, PGY-II 4/16/2013. Disclaimer.
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David Aymond, PGY-II
PAO2= 150 – 1.2(PaCO2)
Remember, if an ABG is checked while the pt is on oxygen, this no longer holds true. This is only while breathing room air. If the ABG is checked while on oxygen, just simply google “MD-Calc=A-a gradient” and fill in the numbers.
Therefore its easy to see that the most important factors causing hypoxia are low Hgb and low SaO2. Therefore hypoxemia can cause hypoxia.
1. V/Q Abnormality: Shunts, Dead Space, and Mismatch
All of these can be separated based on the A-a gradient and Mixed Venous PO2. Remember these 2 major rules:
1. The only time we have a normal A-a gradient is when there is NO LUNG PATHOLOGY. Meaning the lungs have nothing to do with the respiratory failure. This is only the case in Alveolar Hypoventilation!
2. DO2/VO2 imbalance is the only situation where we have a decrease in the mixed venous PO2.
*What is the mixed venous PO2 and where do we find it?
Blood is shunted from the right to the left side of the heart as it passes alveoli without gas exchange
Dead space is the opposite of a shunt-the alveoli are ventilated but there is no perfusion
Notice the PaCO2
doesn’t rise until there
is 50% shunting
PaO2/FiO2 % Shunt
The PaO2 is found on the ABG and the FiO2 is 21% on room air, increases by 5% with each liter of O2 (variable), or is set on the ventilator. Remember to turn the FiO2 into a decimal (21%=.21)
Is the PaO2/FiO2 ratio used in any other settings?
that PEEP should
be used to
in high shunt