Question

what is the difference between PaO2 and PAO2?

Answer 1

In particular, the greater the imbalance of ventilation-perfusion ratios the more PaO2 tends to differ from the calculated PAO2.

- (The difference between PAO2 and PaO2 is commonly referred to as the 'A-a gradient.' However, 'gradient' is a misnomer since the difference is not due to any diffusion gradient, but instead to V-Q imbalance and/or right to left shunting of blood past ventilating alveoli. Hence 'A- a O2 difference' is the more appropriate
term.)
b) The alveolar-arterial PO2 difference, notated P(A-a)O2, varies normally with age and FIO2. Up to middle age, breathing ambient air, normal P(A-a)O2 ranges between 5 and 20 mm Hg. Breathing anFIO2 of 1.0 the normal P(A-a)O2 ranges up to about 110 mm Hg. If P(A-a)O2 is increased above normal there is a defect of gas transfer within the lungs; this defect is almost always due to V-Q imbalance.
c) Because of several assumptions in clinical use of the alveolar gas equation, precision in
calculating PAO2 is not achievable. Fortunately an estimate of P(A-a) O2 is usually sufficient for clinical purposes.
d) Since oxygen enters the pulmonary capillary blood by passive diffusion, it follows that in a
steady state the alveolar PO2 must always be higher than the arterial PO2. This fact is useful to spot 'garbage' blood gas data, a not infrequent problem. For example, a PaO2 of 150 mm Hg in a patient breathing 'room air' at sea level (FIO2 = .21) must represent some kind of error, since at all conceivable.
PaCO2 values the P(A-a)O2 would have a negative value; even with extreme hyperventilation (PaCO2 10 mm Hg) the alveolar PO2 would be no higher than 140 mm Hg. A moment's reflection will reveal several possible explanations for the apparently negative alveolar-arterial PO2 difference: the patient was in fact breathing supplemental oxygen during or just prior to the sample drawing; an air bubble in the arterial sample syringe; a quality control or reporting error from the lab; a transcription error - someone
wrote down the wrong number; etc.