Clinical meaning
Type I (hypoxemic) respiratory failure is defined by PaO2 < 60 mmHg 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 > 45 mmHg (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.
Diagnosis & workup
Diagnostics & workup: - ABG: PaO2 < 60 mmHg (Type I criterion) and/or PaCO2 > 45 mmHg (Type II criterion) - A-a gradient calculation: normal (< 15 + age/4) vs widened; guides differential diagnosis - SpO2 continuous monitoring: unreliable in severe anemia, carbon monoxide poisoning, poor perfusion, dark skin pigmentation (may overestimate) - CXR: identifies cause — consolidation (pneumonia), bilateral infiltrates (ARDS/edema), effusion, pneumothorax - PFTs (when stable): obstructive vs restrictive pattern; DLCO for diffusion impairment - Capnography (ETCO2): non-invasive CO2 monitoring; useful for trending in Type II - VBG as screening: pH and PCO2 correlate well with ABG values (VBG PaCO2 ~3-5 mmHg higher than arterial) - Point-of-care ultrasound: pleural effusion, consolidation, B-lines (pulmonary edema), pneumothorax, diaphragm excursion