302.1 🎓 醫孞生版

302.1.0.1 📌 䞀頁重點

302.1.0.1.1 Respiratory Physiology Overview
302.1.0.1.1.1 Three Main Functions
  1. Ventilation: bulk movement of air
  2. Gas exchange: O2 + CO2 across alveolar-capillary membrane
  3. Acid-base regulation: via CO2 elimination
302.1.0.1.1.2 Anatomy Quick Review
  • Upper airway: nose, mouth, pharynx, larynx
  • Lower airway: trachea, bronchi, bronchioles (terminal → respiratory)
  • Alveoli (~ 300 million, 70-100 m² surface area)
  • Pulmonary capillaries
  • Pleura (visceral + parietal)
  • Diaphragm + accessory respiratory muscles
302.1.0.1.1.3 Ventilation Variables
  • Tidal volume (Vt): ~ 500 mL
  • Respiratory rate (RR): 12-20/min
  • Minute ventilation (VE): Vt × RR = 5-8 L/min
  • Alveolar ventilation (VA): (Vt - VD) × RR (VD = dead space ~ 150 mL)
  • PaCO2 ∝ 1/VA (alveolar gas equation)
302.1.0.1.2 Hypoxemia (↓ PaO2)
302.1.0.1.2.1 Five Mechanisms

1. V/Q mismatch (Most Common) - Areas with low V/Q (perfused but not ventilated) → blood doesn’t oxygenate well - Areas with high V/Q (ventilated but not perfused) → wasted ventilation (dead space) - Most lung diseases involve some V/Q mismatch - Examples: COPD, asthma, pneumonia, atelectasis, PE - Improves with supplemental O2 (high response)

2. Shunt - Blood passes through unventilated alveoli or A-V malformations - True anatomic shunt: AV malformations, congenital - Physiologic shunt: severe atelectasis, pneumonia, ARDS, severe pulmonary edema - DOES NOT improve with supplemental O2 (key feature) - Calculation: A-a gradient ↑

3. Alveolar Hypoventilation - ↓ VE → ↑ PaCO2 + ↓ PaO2 - Examples: drug overdose, neuromuscular weakness, central depression, obesity hypoventilation, OSA - A-a gradient normal (key distinguishing feature) - Improves with supplemental O2

4. Diffusion Impairment - Thickened alveolar-capillary membrane - Examples: ILD, pulmonary edema, severe emphysema - ↓ DLCO - Improves with supplemental O2

5. Low FiO2 - Altitude - Rare in clinical (unless mountain medicine)

302.1.0.1.2.2 A-a Gradient
  • A-a gradient = PAO2 - PaO2
  • PAO2 (alveolar) = FiO2 × (PB - PH2O) - PaCO2/0.8 ≈ 150 - 1.25 × PaCO2 at sea level (FiO2 0.21)
  • Normal A-a gradient: 5-15 mmHg (younger), increases with age (age/4 + 4 rule)
  • ↑ A-a gradient: V/Q mismatch, shunt, diffusion impairment
  • Normal A-a gradient + hypoxemia: hypoventilation or low FiO2
302.1.0.1.3 Hypercapnia (↑ PaCO2)
302.1.0.1.3.1 Three Mechanisms

1. Alveolar Hypoventilation - Reduced VA → ↑ PaCO2 - Examples: opioid overdose, neuromuscular disease (ALS, GBS), severe COPD with fatigue, OSA, central hypoventilation

2. Increased Dead Space - More wasted ventilation, less effective alveolar ventilation - Examples: COPD, PE, ARDS, pulmonary fibrosis

3. Increased CO2 Production (Rare cause alone) - Fever, sepsis, hyperthyroidism, high-carb diet - Usually compensated by ↑ ventilation unless coexisting lung disease

302.1.0.1.3.2 Acute vs Chronic Hypercapnia
  • Acute: pH falls (respiratory acidosis); renal can compensate slowly
  • Chronic: kidney compensates with ↑ HCO3 → pH near normal
  • Bicarbonate retention ~ 3.5 mEq/L per 10 mmHg ΔPaCO2 chronic
  • ABG with normal pH + ↑ PaCO2 + ↑ HCO3 = chronic hypercapnia
302.1.0.1.4 Common Symptoms
302.1.0.1.4.1 Dyspnea (Shortness of Breath)

Etiology: - Cardiac: HF, MI, valvular, arrhythmia, pericardial effusion - Pulmonary: COPD, asthma, pneumonia, PE, pleural effusion, pneumothorax, ILD, PH - Anemia - Deconditioning / obesity - Anxiety / panic - Hyperthyroidism - Neurologic: neuromuscular disease, MS

Distinguishing Features: - Cardiac dyspnea: orthopnea, PND, exertional, S3, edema - Pulmonary: cough, wheeze, sputum, position-independent - Anemia: pallor, fatigue, palpitations - Anxiety: tingling, paresthesias, sigh-like respirations

Assessment: - mMRC (modified Medical Research Council) dyspnea scale 0-4 - NYHA (cardiac) - Borg scale (effort, 6-20) - 6-minute walk test

302.1.0.1.4.2 Cough

Acute (< 3 weeks): - Viral URI (most common) - Acute bronchitis - Pneumonia - Asthma exacerbation - Pertussis (esp adults)

Subacute (3-8 weeks): - Post-infectious cough - Pertussis - UACS - Asthma

Chronic (> 8 weeks): - 80-90% of chronic cough: 1. UACS (upper airway cough syndrome): post-nasal drip, allergic rhinitis, sinusitis 2. Asthma / cough-variant asthma: methacholine challenge 3. GERD: acid + non-acid reflux, esophageal hypersensitivity - Other causes: - ACE inhibitor cough (5-20%) - Chronic bronchitis (smoker) - Bronchiectasis - Eosinophilic bronchitis - Tuberculosis (endemic areas) - Lung cancer - Interstitial lung disease - Refractory chronic cough: gefapixant (P2X3 antagonist, 2024 FDA), nalbuphine - Empiric trial approach

302.1.0.1.4.3 Hemoptysis

Volume: - Trivial: < 30 mL/day - Moderate: 30-200 mL/day - Massive: > 200 mL/day OR > 100 mL in single episode (life-threatening)

Causes: - Bronchitis (40%): bronchiectasis, infectious, eosinophilic - Lung cancer (15-20%) - Tuberculosis (esp endemic) - Pulmonary embolism - Pneumonia - AVM - Vasculitis (granulomatosis with polyangiitis, Goodpasture) - Foreign body - Coagulopathy - Cardiogenic (MS, severe HF)

Workup: - Chest CT (preferred over CXR for evaluation) - Bronchoscopy (rigid for massive) - Sputum for TB, malignancy - Coagulation studies

Massive Hemoptysis Management: - Position with bleeding lung down - Secure airway (selective intubation if known side) - Reverse anticoagulation - Interventional bronchoscopy (cold saline lavage, balloon tamponade, ICU clips) - Bronchial artery embolization (definitive for most) - Surgery for refractory

302.1.0.1.4.4 Chest Pain (Pulmonary)

Pleuritic: - Pleuritis (viral, autoimmune) - Pneumonia - PE - Pneumothorax - Pericarditis (Ch283)

Non-pleuritic: - Bronchitis - Tracheitis - Lung cancer - Mediastinitis

302.1.0.1.4.5 Other Symptoms
  • Sputum: clear, purulent, blood-tinged
  • Wheezing: asthma, COPD, foreign body, anaphylaxis, HF
  • Stridor: upper airway obstruction
  • Snoring: OSA
  • Daytime sleepiness: OSA
  • Night sweats: TB, lymphoma, HIV
  • Weight loss: malignancy, TB
  • Hoarseness: laryngeal pathology
302.1.0.1.5 Physical Examination
302.1.0.1.5.1 Inspection
  • Cyanosis (central / peripheral)
  • Clubbing (chronic hypoxia, lung cancer, ILD, CF)
  • Use of accessory muscles
  • Tracheal deviation (pneumothorax, large effusion)
  • Chest shape (pectus, kyphoscoliosis, barrel chest)
  • Respiratory rate, pattern
302.1.0.1.5.2 Palpation
  • Tactile fremitus: ↑ in consolidation, ↓ in effusion/pneumothorax
  • Tracheal position
  • Chest expansion
302.1.0.1.5.3 Percussion
  • Dull: consolidation, effusion, atelectasis
  • Hyperresonant: pneumothorax, emphysema
302.1.0.1.5.4 Auscultation
  • Normal breath sounds: vesicular (peripheral), bronchial (over bronchi)
  • Adventitious sounds:
    • Crackles (rales): fine (ILD, pulmonary edema), coarse (bronchitis, pneumonia)
    • Wheezes: asthma, COPD, foreign body
    • Rhonchi: large airway secretions
    • Stridor: upper airway obstruction
    • Pleural friction rub: pleuritis
  • Egophony: E sounds like A → consolidation
  • Whispered pectoriloquy: whispered voice clear → consolidation
302.1.0.1.6 Pulse Oximetry + ABG
302.1.0.1.6.1 Pulse Oximetry (SpO2)
  • Non-invasive, reliable in most
  • Limitations: dark skin (sometimes), nail polish, CO poisoning (false high), methemoglobinemia, peripheral perfusion
302.1.0.1.6.2 ABG
  • pH, PaO2, PaCO2, HCO3, lactate
  • Distinguishes hypoxemia mechanism
  • Acid-base assessment
  • Sample: radial > brachial > femoral
302.1.0.1.7 Diagnostic Tests Overview (Detailed in Ch302)
  • Pulmonary function tests (PFTs)
  • Chest X-ray
  • High-resolution CT (HRCT)
  • V/Q scan (PE workup)
  • CT pulmonary angiography (CTPA)
  • Bronchoscopy
  • Sputum analysis
  • Pleural fluid analysis (thoracentesis)
  • Echocardiogram (cor pulmonale, PH)
  • Sleep study (polysomnography)
  • Cardiopulmonary exercise testing (CPET)

302.1.0.2 🩺 床邊速查

  • 5 hypoxemia mechanisms: V/Q mismatch (most common), shunt, hypoventilation, diffusion, low FiO2
  • Shunt vs V/Q: shunt NO improvement with O2; V/Q improves
  • Hypoventilation: A-a gradient normal; PaCO2 ↑
  • Chronic hypercapnia: HCO3 ~ 3.5 mEq/L per 10 mmHg ΔPaCO2 (compensation)
  • Chronic cough (> 8 wk): UACS + asthma + GERD = 80-90%
  • ACE inhibitor cough: 5-20%; switch to ARB
  • Hemoptysis 200 mL/d = massive: ICU, bronchoscopy, bronchial artery embolization
  • Refractory chronic cough: gefapixant 2024 FDA (P2X3 antagonist)