302.3 🏥 內科專科考前版

302.3.1 Mechanistic Deep Dive

302.3.1.1 Alveolar Gas Equation

  • PAO2 = FiO2 × (PB - PH2O) - PaCO2/R
  • R (respiratory quotient) ≈ 0.8
  • Sea level: PAO2 = 0.21 × (760 - 47) - PaCO2/0.8 = 150 - 1.25 × PaCO2
  • Important for A-a gradient calculation

302.3.1.2 Diffusion Capacity (DLCO)

  • Measures CO transfer
  • ↓ in: ILD, emphysema, pulmonary HTN, anemia
  • ↑ in: asthma, polycythemia, alveolar hemorrhage
  • Corrected for hemoglobin

302.3.1.3 Ventilation Control

  • Central chemoreceptors (medulla): respond to ↑ PaCO2 / ↓ pH in CSF
  • Peripheral chemoreceptors (carotid + aortic bodies): respond to ↓ PaO2 (mainly)
  • Mechanoreceptors (lung stretch, J-receptors)
  • Central pattern generator (medullary respiratory centers)

302.3.1.4 Cough Reflex

  • Sensory: TRPV1, TRPA1, P2X3 receptors on vagal sensory nerves
  • Central processing
  • Motor: inspiration → glottic closure → forceful expiration
  • P2X3 inhibition (gefapixant) blocks reflex

302.3.2 Recent Trials & Updates

302.3.2.1 COUGH-1 + COUGH-2 (2022) — Gefapixant

  • Refractory chronic cough
  • ↓ Cough frequency
  • FDA approved 2024
  • Side effect: dysgeusia (taste alteration)

302.3.2.2 MK-7264 (Gefapixant) + Other P2X3 Inhibitors

  • Sivopixant, eliapixant in trials
  • Refractory cough treatment

302.3.2.3 2024 ATS/ERS/JRS/ALAT Dyspnea Update

  • Refined definition
  • Mechanism-based assessment
  • Patient-reported outcome integration

302.3.2.4 CIRCO (2024) — Long COVID Dyspnea

  • Microvascular dysfunction in long COVID
  • Specific testing protocols

302.3.3 High-Yield Specialist Points

302.3.3.1 Refractory Chronic Cough

  • Failed UACS + asthma + GERD trials
  • Consider:
    • Eosinophilic bronchitis (sputum eosinophils, ICS responsive)
    • Bronchiectasis (HRCT)
    • Chronic infection (NTM, fungal)
    • Habit cough / tic
    • Bordetella infection
    • Neurogenic cough (laryngeal sensitivity)
  • Treatment: gefapixant, nalbuphine, gabapentin, amitriptyline

302.3.3.2 Cough Hypersensitivity Syndrome

  • Neurogenic etiology
  • Triggered by mild stimuli (perfumes, temperature)
  • Central sensitization
  • Treatment: speech therapy, neuromodulators, gefapixant

302.3.3.3 Bronchodilator Reversibility Testing

  • FEV1 improvement > 12% AND > 200 mL post-bronchodilator
  • Asthma typically reversible
  • COPD: < 12% reversibility

302.3.3.4 DLCO Pearls

  • Corrected for Hgb
  • ↓ DLCO alone: pulmonary HTN, anemia
  • ↓ DLCO + obstruction: emphysema (>> COPD)
  • ↓ DLCO + restriction: ILD
  • ↑ DLCO: alveolar hemorrhage, polycythemia, asthma

302.3.3.5 Exercise-Induced Bronchospasm

  • 70-80% of asthmatics
  • Cold/dry air, exercise
  • ↑ FEV1 reduction after exercise
  • Treatment: pre-exercise SABA, ICS daily

302.3.3.6 Pulse Oximeter Pitfalls

  • Race-related inaccuracies (recently recognized)
  • Dark skin → may overestimate SaO2
  • 2024 FDA looking at this
  • ABG remains gold standard

302.3.3.7 Capnography

  • End-tidal CO2 (EtCO2)
  • Useful for ventilation assessment
  • Often used in procedural sedation, ICU
  • ↓ EtCO2: ↓ cardiac output, hyperventilation
  • ↑ EtCO2: hypoventilation, ↑ CO2 production

302.3.3.8 Six-Minute Walk Test (6MWT)

  • Functional capacity
  • < 300 m severely limited
  • Distance + SpO2 + Borg dyspnea + HR
  • Useful for ILD, PAH, COPD, HF, pre-transplant

302.3.3.9 Cardiopulmonary Exercise Testing (CPET)

  • VO2 max gold standard for fitness
  • Distinguishes cardiac vs pulmonary limitation
  • Anaerobic threshold
  • Wasserman 9-panel plot

302.3.4 Pearls

  • 5 hypoxemia mechanisms; V/Q most common
  • Shunt: no O2 response (key distinguisher)
  • Hypoventilation: normal A-a gradient
  • Chronic cough: UACS + asthma + GERD = 80-90%
  • ACE inhibitor cough: 5-20%
  • Refractory cough: gefapixant 2024 FDA
  • Massive hemoptysis: bronchial artery embolization (definitive)
  • DLCO: ↓ in ILD, emphysema, PH; ↑ in alveolar hemorrhage