Respiratory Medicine

Foundations of Respiratory Pathology

A 62-year-old woman presents with progressive breathlessness over six months, worse on exertion. She smokes 20 cigarettes daily and works as a cleaner. Her chest X-ray shows hyperinflation. This scenario exemplifies the critical challenge in respiratory medicine: distinguishing between overlapping presentations of asthma, COPD, pneumonia, and pulmonary embolism. Understanding the epidemiological patterns and pathophysiological foundations of these five major respiratory conditions is essential for accurate diagnosis and timely intervention.

Key Epidemiological Facts:

• Asthma: Prevalence 5.9% in UK adults; accounts for 1,200 deaths annually

  • Peak onset bimodal: childhood and late adulthood
  • Atopic phenotype dominates in younger patients

• COPD: Affects 1.2 million diagnosed patients in UK; true prevalence ~3 million

  • 90% attributable to smoking (>20 pack-years threshold)
  • Fourth leading cause of death globally

• Community-acquired pneumonia (CAP): Incidence 5-11 per 1,000 adults annually

  • Mortality 5-10% in hospitalized patients, up to 30% in severe cases
  • Streptococcus pneumoniae responsible for 40% of bacterial CAP

• Pulmonary embolism (PE): Annual incidence 60-120 per 100,000

  • 30-day mortality 10-15% if untreated; <3% with appropriate anticoagulation
  • Recurrence risk 5-10% in first year post-event

📌 Mnemonic for PE risk factors (Virchow's Triad): SSS - Stasis, Surface abnormality, Substances (hypercoagulable state)

Pathophysiological Mechanisms and Clinical Differentiation

The pathophysiology of obstructive airway diseases centres on airflow limitation, but the mechanisms differ fundamentally. In asthma , Type 2 inflammation driven by Th2 lymphocytes triggers mast cell degranulation, eosinophil recruitment, and IgE-mediated bronchial hyperresponsiveness. This process is reversible with bronchodilators. Conversely, COPD involves irreversible small airway fibrosis and emphysematous alveolar destruction from neutrophil elastase and oxidative stress, creating a fixed obstruction pattern.

Key Mechanistic Differences:

• Asthma pathophysiology:

  • Bronchial smooth muscle hypertrophy and hyperreactivity
  • Airway remodelling: subepithelial fibrosis, mucus gland hyperplasia
  • Peak expiratory flow (PEF) variability >20% diagnostic

• COPD mechanisms:

  • Protease-antiprotease imbalance (α1-antitrypsin deficiency)
  • Loss of elastic recoil reduces expiratory flow rates
  • FEV₁/FVC ratio <0.7 post-bronchodilator defines obstruction

• Pneumonia cascade:

  • Alveolar macrophage activation → neutrophil chemotaxis
  • Cytokine storm (TNF-α, IL-1, IL-6) causes systemic inflammatory response
  • Consolidation impairs gas exchange: V/Q mismatch

• PE pathophysiology:

  • Thrombus occlusion increases pulmonary vascular resistance
  • Right ventricular strain: RV dilatation, septal bowing
  • Acute cor pulmonale if >30-50% vascular bed occluded

Diagnostic Criteria and Investigation Selection

A 45-year-old presents with acute breathlessness and pleuritic chest pain 10 days post-knee surgery. Her Wells score is 6.5 (high probability). This scenario demands systematic application of diagnostic criteria . NICE NG158 mandates immediate CTPA for high-probability PE; delaying for D-dimer risks fatal outcome.

Investigation Strategy by Condition:

• Asthma diagnosis (NICE NG80):

  • FeNO ≥40 ppb supports eosinophilic inflammation (specificity 90%)
  • Spirometry: FEV₁/FVC <0.7 with ≥12% reversibility post-bronchodilator
  • Peak flow diary: ≥20% diurnal variation over 2-4 weeks

• PE diagnostic pathway (NICE NG158):

  • Wells score ≥4: immediate CTPA (sensitivity 83%, specificity 96%)
  • Wells <4: D-dimer first; if <500 ng/mL, excludes PE (NPV 99%)
  • V/Q scan alternative if renal impairment contraindicates contrast

• CAP severity assessment (NICE NG138):

  • CURB-65 score guides admission: ≥2 requires hospital assessment
  • Chest X-ray mandatory: consolidation confirms diagnosis (sensitivity 70%)
  • Blood cultures before antibiotics if sepsis suspected

Severity Stratification and Prognostic Analysis

Severity assessment determines disposition and treatment intensity . A patient with acute asthma, PEF 33% predicted, unable to complete sentences, and oxygen saturations 88% meets life-threatening asthma criteria (BTS/SIGN), mandating immediate ICU consideration.

Critical Severity Markers:

• Life-threatening asthma (BTS/SIGN):

  • PEF <33% predicted or <50 L/min absolute
  • SpO₂ <92% or PaO₂ <8 kPa
  • Silent chest, cyanosis, exhaustion, altered consciousness

• CURB-65 for CAP mortality prediction:

  • Score 0-1: 0.7% mortality (outpatient)
  • Score 2: 9.2% mortality (consider admission)
  • Score ≥3: 22% mortality (urgent hospital admission)

• PE risk stratification (ESC 2019):

  • High-risk: systolic BP <90 mmHg or shock (15% mortality)
  • Intermediate-risk: RV dysfunction on echo + troponin elevation
  • Low-risk: normal BP, no RV strain (1% mortality)

🚩 Red Flag: Silent chest in acute asthma indicates insufficient airflow to generate wheeze - imminent respiratory arrest.

Evidence-Based Treatment Strategies

Treatment algorithms integrate guideline recommendations with patient-specific factors . NICE NG80 advocates stepwise asthma therapy, initiating low-dose ICS (beclometasone 200-400 mcg/day) as first-line controller, escalating to ICS/LABA combinations if uncontrolled.

Key Treatment Protocols:

• Acute severe asthma management:

  • Oxygen: target SpO₂ 94-98%
  • Salbutamol 5 mg nebulized every 15-30 minutes
  • Prednisolone 40-50 mg oral or hydrocortisone 100 mg IV
  • Magnesium sulphate 2 g IV over 20 minutes if life-threatening

• COPD exacerbation (NICE NG115):

  • Increase bronchodilators: salbutamol + ipratropium nebulizers
  • Prednisolone 30 mg daily for 5 days
  • Antibiotics if ≥2 of: increased dyspnoea, sputum volume, sputum purulence

• CAP antibiotic selection (NICE NG138):

  • CURB-65 0-1: amoxicillin 500 mg TDS 5 days
  • CURB-65 ≥2: co-amoxiclav 1.2 g TDS + clarithromycin 500 mg BD

Complex Case Integration and Multimorbidity

Real-world respiratory patients rarely present with isolated pathology . A 75-year-old with COPD, atrial fibrillation on warfarin, and CAP requires careful antibiotic selection avoiding macrolides (QT prolongation) and balancing anticoagulation risks during acute infection.

Special Considerations:

• Pregnancy and PE: LMWH preferred (enoxaparin 1 mg/kg BD); avoid DOACs

  • CTPA safe with abdominal shielding; V/Q higher fetal radiation

• Renal impairment adjustments:

  • PE: apixaban contraindicated if CrCl <15 mL/min; use LMWH
  • CAP: reduce co-amoxiclav dose if eGFR <30

• Asthma-COPD overlap syndrome (ACOS):

  • 15-20% of obstructive disease patients
  • ICS/LABA combination beneficial; avoid ICS monotherapy in COPD

• Post-PE complications:

  • Chronic thromboembolic pulmonary hypertension (CTEPH) in 2-4%
  • Requires lifelong anticoagulation; consider pulmonary endarterectomy

⭐ Clinical Pearl: In elderly COPD patients with recurrent exacerbations, check α1-antitrypsin levels if age <45 or minimal smoking history - augmentation therapy available if deficient.

High Yield Summary

Key Take-Aways:

• Asthma reversibility: ≥12% FEV₁ improvement or ≥20% PEF variability; COPD shows fixed obstruction FEV₁/FVC <0.7 post-bronchodilator
• Life-threatening asthma: PEF <33%, SpO₂ <92%, silent chest - requires ICU consideration and MgSO₄ 2 g IV
• CURB-65 ≥2 mandates hospital assessment for CAP; score ≥3 predicts 15-40% mortality
• Wells score ≥4 for PE requires immediate CTPA; <4 use D-dimer (<500 ng/mL excludes PE, NPV 99%)
• PE anticoagulation: apixaban 10 mg BD 7 days then 5 mg BD; LMWH in pregnancy/renal failure
• COPD exacerbations: prednisolone 30 mg 5 days + antibiotics if ≥2 of increased dyspnoea/sputum volume/purulence
• CAP antibiotic choice: amoxicillin 500 mg TDS if CURB-65 0-1; co-amoxiclav + clarithromycin if ≥2

Essential Respiratory Medicine Numbers/Formulas:

Key Principles/Pearls:

• Silent chest in acute asthma indicates insufficient airflow for wheeze - pre-arrest state requiring immediate escalation
• D-dimer only useful when pre-test probability low (Wells <4); high false-positive rate in elderly, malignancy, pregnancy
• COPD patients with frequent exacerbations (≥2/year) benefit from prophylactic azithromycin 250 mg three times weekly
• PE anticoagulation duration: 3 months if provoked (surgery/immobility), 6-12 months if unprovoked, lifelong if recurrent

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