alveolararterial

Alveolar-arterial, in medical terminology, commonly refers to the alveolar-arterial (A-a) gradient, a measure of the difference between the oxygen pressure in the alveoli and the oxygen pressure in arterial blood. It is a useful indicator of the efficiency of pulmonary gas exchange and helps distinguish different causes of hypoxemia.

Calculation and measurement: The A-a gradient is calculated from the alveolar gas equation. Alveolar oxygen tension (PAO2) is estimated as PAO2 = FiO2 × (Pb − PH2O) − PaCO2 / R, where FiO2 is the fraction of inspired oxygen, Pb is barometric pressure, PH2O is water vapor pressure, PaCO2 is arterial CO2, and R is the respiratory quotient (about 0.8). The gradient is then A-a gradient = PAO2 − PaO2, where PaO2 is obtained from an arterial blood gas (ABG). Normal values vary with age and FiO2; on room air at sea level, about 5–15 mmHg in young adults is typical, increasing roughly with age (approximately (age/4) + 4 is a commonly cited approximation).

Clinical interpretation: A normal A-a gradient suggests hypoxemia due to hypoventilation or low inspired oxygen rather than a gas-exchange problem. An elevated A-a gradient points to impaired alveolar-capillary gas exchange, which can result from ventilation–perfusion (V/Q) mismatch, diffusion limitation, shunt, or significant age-related changes. The gradient tends to widen with higher FiO2 and at higher altitudes.

Uses and limitations: The A-a gradient is widely used in evaluating respiratory failure and guiding diagnostic reasoning. It requires accurate FiO2 and ABG data, and its interpretation should consider patient age, altitude, and clinical context.