Type I (hypoxemic) respiratory failure is defined by PaO2 < 8.0 kPa with normal or low PaCO2, caused by V/Q mismatch, shunt, diffusion impairment, or low FiO2. Common causes include pneumonia, ARDS, PE, and pulmonary edema. Type II (hypercapnic) respiratory failure involves PaCO2 > 6.0 kPa (with or without hypoxemia), caused by alveolar hypoventilation from reduced respiratory drive (opioids, CNS depression), neuromuscular weakness (GBS, MG crisis, ALS), chest wall restriction (obesity hypoventilation, kyphoscoliosis), or severe airflow obstruction (COPD, asthma). The A-a gradient distinguishes between these mechanisms: normal A-a gradient (< 15 in young adults, increases with age) with hypercapnia suggests pure hypoventilation; widened A-a gradient suggests parenchymal or vascular disease. The formula PAO2 = (FiO2 × 713) - (PaCO2/0.8) calculates the expected alveolar oxygen tension for comparison with measured PaO2.