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Assess efficiency of pulmonary gas exchange
Room air = 21%
PAO2 = FiO2 × (Patm - 47) - (PaCO2 / 0.8)
A-a = PAO2 - PaO2
PaO2 improves with supplemental O2. Most common cause.
PaO2 improves with supplemental O2.
PaO2 does NOT improve significantly with O2. Blood bypasses alveoli.
PaO2 improves with O2. PaCO2 remains elevated without ventilation support.
The difference between alveolar (calculated) and arterial (measured) oxygen partial pressures. It reflects how efficiently the lungs transfer oxygen to the blood.
Age-related changes in lung elasticity and increased V/Q mismatch. The formula (Age/4)+4 approximates the expected increase.
Higher FiO2 raises PAO2 but may not proportionally increase PaO2, especially with shunting. Also, high O2 can cause absorption atelectasis.
The ratio of CO2 produced to O2 consumed, typically 0.8 on a mixed diet. Varies with metabolism (0.7-1.0). We use 0.8 in the alveolar gas equation.
Most useful on room air to identify if hypoxemia is from lung pathology vs. hypoventilation. Less interpretable at high FiO2.
PaO2/FiO2 ratio is simpler and commonly used in ICU (e.g., ARDS criteria). A-a gradient is more precise for differentiating causes of hypoxemia.
A normal A-a gradient with hypoxemia points to hypoventilation (check PaCO2) or low FiO2. An elevated A-a gradient indicates a lung parenchymal or vascular problem.