V/Q mismatch is the most common cause of hypoxemia. The arterial blood oxygen level does not rise to the same proportion as the alveolar oxygen level due to its admixing with unoxygenated blood coming from bronchial veins, mediastinal veins, and thebesian veins. The rise in gradient is due to disproportionate increase in alveolar oxygen level. High FiO 2 by increasing both the alveolar and arterial oxygen level widens the gradient. The drop in PaO 2 after 70 years is about 0.43 mmHg per year. It is primarily due to age-induced decrease in the PaO 2 level because of the rise in V/Q mismatch. The A-a oxygen difference increases with age. In young person, the A-a oxygen difference is <10 mmHg. R is the respiratory quotient and is approximately 0.8 at steady state on standard diet. It is assumed to be equal to arterial PCO 2.
PaCO 2 is the alveolar carbon dioxide tension. PH 2O is the water vapor pressure (47 mmHg at 37☌). Pb is the barometric pressure (760 mmHg at sea level). PAO 2 is the mean alveolar oxygen pressure.įiO 2 is the fractional concentration of inspired oxygen.
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.
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