Chapter 20 · Respiratory Epidemiology, Physiology & Investigations

Epidemiology (resource-rich vs limited) · Fetal programming & genetics · Environmental risk factors · Lung development (fetal/perinatal/postnatal) · Respiratory physiology (PFTs, airway function, lung volumes, V/Q, gas transfer, breathing) · Imaging (X-ray, CT, MRI, ultrasound, nuclear medicine) · Airway endoscopy (indications, techniques)
📌 Core principles: Respiratory disease epidemiology differs greatly by region (asthma/CF in rich economies; pneumonia/TB in limited). Fetal programming (Barker hypothesis) links low birthweight to reduced lung function. Lung development continues postnatally (alveolarisation until ~8 years). PFTs: spirometry (FEV1, FVC, FEV1/FVC), body plethysmography (lung volumes), DLCO (gas transfer). Imaging: CXR first line; HRCT for ILD; V/Q scan for PE. Airway endoscopy (flexible vs rigid) for stridor, foreign body, persistent pneumonia.

📖 Respiratory Epidemiology, Development & Investigations – Key Concepts

🌍 Epidemiology
Resource-rich: asthma (10-15%), CF (1 in 2500), bronchiolitis. Resource-limited: pneumonia (leading cause child death), TB, pertussis. Environmental: air pollution, passive smoke, overcrowding.
🧬 Fetal programming & genetics
Barker hypothesis: low birthweight → reduced lung function, increased COPD risk. Genetic: CFTR (CF), alpha-1 antitrypsin (emphysema), surfactant proteins (ILD), ADAM33 (asthma).
🌱 Lung development
Embryonic (4-7wk), pseudoglandular (5-17wk), canalicular (16-26wk), saccular (24-38wk), alveolar (36wk-8yr). Surfactant appears ~24wk. Postnatal alveolarisation continues until ~8 years.
📊 Respiratory physiology & PFTs
Spirometry: FEV1, FVC, FEV1/FVC. Lung volumes: TLC, RV, FRC (plethysmography, helium dilution). Airway resistance (oscillometry, interrupter technique). V/Q mismatch (dead space, shunt). DLCO (gas transfer). Breathing control: central/chemoreceptors.
🖥️ Imaging (CXR, CT, MRI, US, nuclear)
CXR: first-line (infections, cardiac, foreign body). CT/HRCT: ILD, bronchiectasis, nodules, pE. MRI: vascular rings, chest wall. US: pleural effusion, lung consolidation, diaphragm. V/Q scan: pulmonary embolism (normal X-ray).
🔍 Airway endoscopy
Flexible bronchoscopy: dynamic airway, BAL, biopsy (indications: stridor, persistent wheeze, recurrent pneumonia, foreign body extraction, hemoptysis). Rigid bronchoscopy: foreign body removal, massive hemoptysis, laser. Safety: sedation/general anaesthesia, monitoring.

🔎 Symptom-based approach: using epidemiology & investigations

1️⃣
Infant with chronic cough, recurrent LRTI, FTT; born preterm, required oxygen – BPD (bronchopulmonary dysplasia). Lung function shows airflow obstruction, hyperinflation.
2️⃣
School-age child with exercise-induced wheeze, normal CXR – Asthma. Spirometry (reversible obstruction), FeNO, methacholine challenge if normal spirometry.
3️⃣
Infant with inspiratory stridor worse on feeding/crying, normal cry – Laryngomalacia (flexible bronchoscopy gold standard).
4️⃣
Unexplained hypoxaemia, normal CXR, no cardiac disease – Pulmonary embolism (V/Q scan or CT pulmonary angiography).
5️⃣
Child with suspected foreign body aspiration, normal X-ray – Rigid bronchoscopy (diagnostic and therapeutic).
📊 Red flags on PFTs: FEV1 <40% predicted (severe obstruction), FEV1/FVC <0.70 (obstruction), low DLCO (interstitial disease, pulmonary vascular).

📋 Respiratory investigation algorithm & imaging selection

🩻
Stepwise respiratory investigation
▪️ Step 1: History (wheeze, cough, SOB, exposures, prematurity).
▪️ Step 2: CXR – first-line for fever, tachypnoea, suspected pneumonia, foreign body.
▪️ Step 3: Spirometry (≥6 years) – FEV1, FVC, FEV1/FVC, bronchodilator reversibility.
▪️ Step 4: FeNO (eosinophilic airway inflammation).
▪️ Step 5: Lung volumes (plethysmography) for restriction or hyperinflation.
▪️ Step 6: CT/HRCT for ILD, bronchiectasis, pulmonary nodules.
▪️ Step 7: Flexible bronchoscopy + BAL for recurrent pneumonia, persistent atelectasis, suspected airway anomaly.
🩺
Indications for airway endoscopy in children
▪️ Stridor (evaluate dynamic airway: laryngomalacia, vocal cord paralysis, subglottic stenosis).
▪️ Persistent wheeze not responding to asthma therapy.
▪️ Recurrent or persistent pneumonia (BAL for microbiology, foreign body).
▪️ Hemoptysis (source localisation, biopsy).
▪️ Suspected tracheoesophageal fistula or H-type fistula.
▪️ Pre- and post-lung transplant assessment.
📊
Interpreting PFTs – obstructive vs restrictive
▪️ Obstructive: reduced FEV1/FVC (<0.70), FEV1 low, FVC normal/reduced, increased RV/TLC (air trapping).
▪️ Restrictive: FEV1/FVC normal or high, FEV1 and FVC reduced proportionally, TLC reduced.
▪️ Mixed: features of both.

💡 Reflex prompts – Respiratory foundations

🧬 Which genetic disorder is the most common cause of bronchiectasis in resource-rich countries?
Cystic fibrosis (CFTR mutation).
🌍 What is the leading cause of childhood mortality in resource-limited countries?
Pneumonia (S. pneumoniae, H. influenzae type b, RSV).
📉 The Barker hypothesis relates low birthweight to which adult lung disease?
Reduced lung function, increased COPD risk (fetal programming).
🌱 At what gestational age does surfactant production begin?
Approximately 24 weeks (canalicular/saccular phase).
🫁 Which lung function parameter defines obstructive ventilatory defect?
FEV1/FVC < 0.70 (or less than LLN).
🩻 First-line imaging for suspected pulmonary embolism in a child with normal CXR?
V/Q scan (low radiation) or CT pulmonary angiogram (if V/Q nondiagnostic).
🔍 Which bronchoscopy modality is preferred for removal of a peanut from the airway?
Rigid bronchoscopy – allows better airway control and foreign body forceps.
📊 A child with cystic fibrosis has FEV1 55% predicted. What does this indicate?
Moderate obstructive lung disease (CF lung disease progression).
🧪 What is the gold standard to diagnose bronchiectasis?
High-resolution CT chest (HRCT) – bronchial dilation, wall thickening.
🩺 Most common congenital airway anomaly causing inspiratory stridor in infants?
Laryngomalacia (floppy arytenoids, omega-shaped epiglottis).