Unlike PaO 2, PAO 2 is not measured but calculated by using the alveolar gas equation: The difference between alveolar and arterial oxygen tensions is due to other factors: (1) V/Q imbalance in various parts of the lungs, (2) small right to left shunt (bronchial vein, thebesian vein, and small pulmonary arteriovenous anastomosis), and (3) resistance to the diffusion of oxygen across the alveolar membrane. The word gradient is a misnomer, and ideally, it should be referred to as A-a oxygen difference as the difference between alveolar and arterial oxygen is not due to any diffusion gradient. Therefore, hypoxemia due to V/Q mismatch, diffusion limitation, and shunt will have widened gradient, whereas hypoxemia due to hypoventilation would have normal gradient. Pathology of the alveolocapillary unit widens the gradient. The A-a oxygen gradient indicates the integrity of the alveolocapillary membrane and effectiveness of gas exchange. It is the difference between alveolar oxygen level (PAO 2) and arterial oxygen level (PaO 2) and is represented by the following equation: Alveolar to arterial (A-a) oxygen gradient = PAO 2 – PaO 2.

Dead space ventilation ratio