Pathophysiology
II-14. COPD and pulmonary fibrosis: respiratory mechanics & blood gases
COPDと肺線維症の呼吸力学・血液ガスへの影響
Airway Mechanics in COPD
- Large airways: narrowed by bronchoconstriction (hyperactive SM), edema, mucus — reversible (drugs/quitting).
- Small airways (<2 cm): no cartilage; kept open by interstitial elastic tissue. Loss of elasticity (neutrophil elastase) = largely irreversible parenchymal damage.
- Expiration: ↑pleural pressure compresses small airways; without elastic support → narrow/occlude. Forced expiration → reduced peak flow then sharp drop (harder to exhale the more forced).
Lung Volumes in COPD
- ↑Residual volume (overinflated lungs); breathe on top of full lung → respiratory muscle exhaustion (common cause of death). ↓Forced expiratory volume, ↓vital capacity.
- Dynamic hyperinflation on exertion: forced expiration closes small airways → incomplete expiration → ↓end-expiratory volume → ↓inspiratory capacity.
- Smoking accelerates FEV1 decline. ↓FEV1 → ↓pO₂ (inverse to pCO₂ — CO₂ removable while pump function normal). Expiratory flow proportional to lung volume.
COPD Phenotypes
- Pink puffers (emphysema, no chronic bronchitis): 1st sign dyspnea; barrel chest, prolonged expiration, hunched; hyperventilate → adequate oxygenation (not hypoxemic); cachexia; normal RBC.
- Blue bloaters (chronic bronchitis): cough/wheeze; ↓respiratory drive → CO₂ retention → hypoxia + cyanosis; obese; secondary pulmonary hypertension; ↑RBC (↑viscosity); death from cor pulmonale (edema).
Erythrocytosis (Polyglobulia)
- Chronic ↓O₂ saturation → ↑EPO → ↑RBC (↑Hb/hematocrit). Higher Hb in COPD = good prognostic sign (live longer).
一問一答
▶How do lung volumes change in COPD?
↑Residual volume (overinflated lungs), ↓forced expiratory volume, and ↓vital capacity; patients breathe on top of a full lung, leading to respiratory muscle exhaustion.
▶Why is large-airway obstruction in COPD reversible but small-airway damage is not?
Large airways narrow from bronchoconstriction, edema, and mucus (reversible with drugs/quitting); small airways (<2 cm) lack cartilage and depend on interstitial elastic tissue, so elastin loss (neutrophil elastase) causes largely irreversible parenchymal damage.
▶Why does decreased FEV1 lower pO₂ but not raise pCO₂ early in COPD?
CO₂ is removable while pump function is normal, so pO₂ falls inversely while pCO₂ stays controlled; expiratory flow is proportional to lung volume.
▶What is dynamic hyperinflation on exertion in COPD?
Forced expiration closes small airways → incomplete expiration → ↓end-expiratory volume → ↓inspiratory capacity.
▶Why does forced expiration worsen airflow in COPD?
Increased pleural pressure compresses small airways that lack elastic support, so they narrow/occlude — the more forced the expiration, the harder it is to exhale.
▶What characterizes the "pink puffer" COPD phenotype?
Emphysema without chronic bronchitis: first sign is dyspnea; barrel chest, prolonged expiration, hunched posture; hyperventilation gives adequate oxygenation (not hypoxemic); cachexia; normal RBC.
▶What characterizes the "blue bloater" COPD phenotype?
Chronic bronchitis: cough/wheeze, ↓respiratory drive → CO₂ retention → hypoxia + cyanosis; obese; secondary pulmonary hypertension; ↑RBC (↑viscosity); death from cor pulmonale (edema).
▶Why does erythrocytosis (polyglobulia) develop in COPD?
Chronic ↓O₂ saturation → ↑EPO → ↑RBC (↑Hb/hematocrit); higher Hb is actually a good prognostic sign in COPD.
▶Why is respiratory muscle exhaustion a common cause of death in COPD?
The overinflated lungs (↑residual volume) force patients to breathe at high lung volumes, fatiguing the respiratory muscles.
▶What keeps small airways open and why does this matter in COPD?
They have no cartilage and are held open by interstitial elastic tissue; loss of this elasticity in COPD causes them to collapse during expiration.
▶What is the shape of the forced expiratory flow curve in COPD?
A reduced peak flow followed by a sharp drop, because forced effort compresses the unsupported small airways.
▶Why are pink puffers not hypoxemic despite emphysema?
They hyperventilate enough to maintain adequate oxygenation (at the cost of dyspnea and cachexia).
▶Why do blue bloaters develop secondary pulmonary hypertension and cor pulmonale?
Chronic hypoxia causes pulmonary hypoxic vasoconstriction → pulmonary hypertension → right heart strain (cor pulmonale) with edema.
▶How does smoking affect FEV1 over time?
It accelerates the decline of FEV1.
▶Why do blue bloaters become cyanotic while pink puffers do not?
Blue bloaters have a reduced respiratory drive causing CO₂ retention and hypoxia (cyanosis), whereas pink puffers hyperventilate to stay oxygenated.
▶What body habitus is typical of pink puffers vs blue bloaters?
Pink puffers are cachectic (thin); blue bloaters are obese.
▶Why does increased blood viscosity occur in blue bloaters?
Chronic hypoxia drives erythrocytosis (↑RBC), raising blood viscosity.
▶Which COPD component (large vs small airway) is targeted by bronchodilators?
Large airway bronchoconstriction — the reversible component; small-airway parenchymal destruction is largely irreversible.
▶What is the relationship between expiratory flow and lung volume in COPD?
Expiratory flow is proportional to lung volume — flow falls as lung volume decreases during expiration.
▶Why is a higher hemoglobin a favorable prognostic sign in COPD?
It reflects an adaptive EPO response to chronic hypoxia, and such patients tend to live longer.