2.2 Respiratory Failure, Asthma, COPD & Infiltrative Lung Diseases

1. Respiratory Failure

Respiratory failure is defined as the inability of the respiratory system to maintain adequate gas exchange. It is clinically divided based on arterial blood gas (ABG) derangements.

• Type I (Hypoxemic) Respiratory Failure: Defined as PaO2 < 60 mmHg with a normal or low PaCO2. The primary pathophysiologic mechanism is ventilation/perfusion (V/Q) mismatch or right-to-left shunting. Common causes include pneumonia, pulmonary edema, acute respiratory distress syndrome (ARDS), and pulmonary embolism.
• Type II (Hypercapnic) Respiratory Failure: Defined as PaCO2 > 50 mmHg, usually accompanied by hypoxemia. The mechanism is alveolar hypoventilation. Causes include severe COPD exacerbations, neuromuscular disorders (e.g., Guillain-Barré syndrome, ALS), central respiratory depression (e.g., opioid overdose), and chest wall deformities.

Management is tailored to the underlying cause but generally requires supplemental oxygen, non-invasive ventilation (e.g., CPAP for Type I, BiPAP for Type II), or invasive mechanical ventilation.

2. Obstructive Lung Diseases

Obstructive diseases are characterized by a limitation of expiratory airflow. Pulmonary function tests (PFTs) classically show a decreased FEV1/FVC ratio (< 0.70).

Asthma

Asthma is a chronic inflammatory disorder of the airways characterized by reversible airway obstruction and bronchial hyperresponsiveness.

Diagnosis: Clinically suspected by a history of episodic wheezing, cough, and chest tightness, particularly at night or early morning. Diagnosis is confirmed by spirometry showing an obstructive pattern that reverses completely (FEV1 increases by ≥ 12% and 200 mL) post-bronchodilator administration.

Management Updates (GINA Guidelines): A major paradigm shift in asthma care has occurred. Short-acting beta-agonists (SABA) alone are no longer recommended due to the risk of severe exacerbations. The preferred reliever therapy across all asthma steps (mild to severe) is now as-needed low-dose Inhaled Corticosteroid (ICS) + Formoterol (a fast-acting LABA). For persistent asthma, maintenance therapy relies on daily ICS-formoterol, with dose escalation or addition of Long-Acting Muscarinic Antagonists (LAMA) or biologics (e.g., Omalizumab for IgE-mediated asthma) for severe cases.

Chronic Obstructive Pulmonary Disease (COPD)

COPD involves chronic bronchitis and emphysema, primarily driven by long-term tobacco smoke exposure or biomass fuel inhalation. It is characterized by progressive, irreversible airflow limitation.

Assessment and Management (GOLD Criteria): COPD management relies on symptom burden (assessed by CAT or mMRC scores) and exacerbation history.

• Group A (Low symptoms, low risk): Treat with a bronchodilator (SABA, SAMA, LABA, or LAMA).
• Group B (High symptoms, low risk): Treat with LABA + LAMA combination.
• Group E (High exacerbation risk, regardless of symptoms): Treat with LABA + LAMA. Consider adding ICS if blood eosinophils are ≥ 300 cells/µL.

Acute Exacerbations: Managed with targeted oxygen therapy (aiming for SpO2 88-92% to avoid worsening hypercapnia), short-acting bronchodilators, systemic corticosteroids (e.g., oral prednisone for 5 days), and antibiotics if the patient has increased sputum purulence plus increased volume or dyspnea.

3. Infiltrative and Restrictive Lung Diseases

Restrictive lung diseases present with decreased total lung capacity (TLC) and a normal or elevated FEV1/FVC ratio. They encompass intrinsic parenchymal diseases (Interstitial Lung Diseases) and extrinsic causes (kyphoscoliosis, obesity).

• Idiopathic Pulmonary Fibrosis (IPF): Occurs in older males. High-resolution CT shows a "usual interstitial pneumonia" (UIP) pattern with peripheral honeycombing and traction bronchiectasis. Antifibrotic agents (Pirfenidone, Nintedanib) can slow disease progression.
• Sarcoidosis: A multisystem granulomatous disease affecting young adults (especially African descent and specific genetic cohorts). It presents with bilateral hilar lymphadenopathy and non-caseating granulomas on biopsy. Asymptomatic patients are observed; symptomatic patients are treated with systemic corticosteroids.

4. Pulmonary Vascular Disease & Specific Infections

Pulmonary Embolism (PE)

PE is a potentially fatal condition where a thrombus (usually from a Deep Vein Thrombosis in the lower extremities) occludes the pulmonary arterial system.

Diagnosis: The approach requires clinical probability assessment using the Wells Criteria.

• Low Probability: Use the Pulmonary Embolism Rule-out Criteria (PERC). If PERC-positive, order a highly sensitive D-dimer. A negative D-dimer confidently rules out PE.
• High Probability: Skip D-dimer and proceed directly to CT Pulmonary Angiography (CTPA). If the patient has severe renal impairment or contrast allergy, a V/Q scan is the preferred alternative.

Management: Stable patients are treated with anticoagulation (DOACs or LMWH transitioned to DOAC/Warfarin). Hemodynamically unstable patients (massive PE with hypotension) require immediate reperfusion via systemic thrombolysis or surgical embolectomy.

Middle East Respiratory Syndrome Coronavirus (MERS-CoV)

A critical regional topic for the SMLE. MERS-CoV is a zoonotic betacoronavirus highly prevalent in the Arabian Peninsula. Dromedary camels are the primary animal host.

• Clinical Features: Patients present with severe acute respiratory illness, fever, cough, and shortness of breath. Gastrointestinal symptoms (diarrhea) and acute kidney injury are prominent differentiating features from other respiratory viruses.
• Diagnosis and Management: Diagnosis is via reverse-transcriptase PCR (RT-PCR) from lower respiratory tract specimens. Strict airborne, contact, and standard precautions are mandatory in healthcare settings. Management is entirely supportive, focusing on mechanical ventilation and ECMO for severe ARDS.