Pulmonary — gas exchange, spirometry, obstructive vs. restrictive
**Gas exchange:** Fick's law — diffusion ∝ (ΔP × area) / (thickness). At sea level, alveolar PAO₂ ≈ 100 mmHg, PACO₂ ≈ 40 mmHg. **A-a gradient** = PAO₂ − PaO₂; normal <10 + age/4. Elevated A-a: V/Q mismatch, shunt, diffusion impairment.
**Lung volumes:** TV (500 mL), IRV, ERV, RV. Capacities: IC, FRC (ERV+RV), VC (IRV+TV+ERV), TLC.
**Spirometry (FEV1/FVC):** • **Obstructive** — FEV1/FVC < 0.7 (adults). Asthma (reversible), COPD (chronic bronchitis + emphysema; not reversible), bronchiectasis, CF. • **Restrictive** — FEV1/FVC normal/↑ but both ↓. Parenchymal (IPF, pneumoconioses, sarcoidosis), extrapulmonary (kyphoscoliosis, obesity hypoventilation, neuromuscular).
**Asthma treatment (GINA 2024):** SABA alone no longer recommended. Step 1-2: ICS-formoterol as needed. Step 3+: daily low/medium-dose ICS-LABA. Add-on: LAMA, biologic (omalizumab = anti-IgE; mepolizumab = anti-IL-5).
**COPD (GOLD 2024):** bronchodilators (LAMA ± LABA) foundation. ICS when exacerbations or eosinophils ≥300. Smoking cessation, vaccines (flu, pneumococcal, COVID, RSV per ACIP), pulmonary rehab, LTOT if PaO₂ ≤55 mmHg or SpO₂ ≤88% at rest.
**PE:** Wells score, D-dimer for low-risk; CTPA for intermediate/high. Anticoagulate (DOAC preferred over warfarin per CHEST 2021) for 3 months provoked, ≥3 unprovoked. Massive PE with shock → thrombolysis.
**ARDS (Berlin 2012):** acute onset, bilateral infiltrates, not explained by volume overload, PaO₂/FiO₂ ≤300. Low tidal volume 6 mL/kg IBW, plateau <30, prone position for PaO₂/FiO₂ <150.