Respiratory Failure

Definitions & Types - Gasping for Definitions

• Respiratory Failure (RF): PaO2 < 60 mmHg (hypoxemic) OR PaCO2 > 50 mmHg (hypercapnic) on room air.

⭐ Normal A-a gradient in Type II RF = pure hypoventilation.

Pathophysiology - Why Lungs Stumble

Mechanisms of Impaired Gas Exchange:

• V/Q Mismatch: Most common. Ventilation/perfusion imbalance. Responds to O2.
  • Causes: Pneumonia, PE, Asthma, COPD. (📌 P-P-A-C)
• Shunt: Extreme V/Q mismatch; blood bypasses alveoli. Refractory to O2.
  • Causes: ARDS, severe pneumonia, cardiac shunts.
• Hypoventilation: ↓Alveolar ventilation → ↑PaCO2, ↓PaO2. Normal A-a gradient.
  • Causes: CNS depression, neuromuscular weakness.
• Diffusion Impairment: Thickened alveolar-capillary membrane. Exercise-induced hypoxemia.
  • Causes: ILD, emphysema.
• Low Inspired O2: e.g., high altitude. ↓PaO2, normal A-a gradient.
• Alveolar Gas Equation: $P_A O_2 = F_i O_2 (P_B - P_{H_2O}) - P_a CO_2 / R$

⭐ Normal A-a gradient in hypoxemia suggests hypoventilation or low inspired FiO2.

Clinical Features & Diagnosis - Symptom Sleuthing

• Symptoms: Dyspnea, tachypnea, cyanosis. Altered sensorium (agitation, confusion).
  • Type I (Hypoxemic): Restlessness.
  • Type II (Hypercapnic): Headache, asterixis.
• A-a Gradient: $P_A O_2 - P_a O_2$. Normal: <15 mmHg.
  • ↑ A-a causes: 📌 VSD (V/Q mismatch, Shunt, Diffusion impairment).
• ABG Interpretation:

Management Principles - Airway Allies

• Oxygen Therapy:
  • Goal: $PaO_2$ >60 mmHg, SpO2 >90%.
  • COPD: Target SpO2 88-92%.
  • Devices:
    • Nasal Cannula (NC): Low flow, FiO2 24-44%.
    • Venturi Mask: Precise FiO2 (24-60%).
    • Non-Rebreather Mask (NRBM): High FiO2 (up to 90%).
• Non-Invasive Ventilation (NIV):
  • Indications: 📌 COPD (pH <7.35, $PaCO_2$ >45 mmHg), ACPE, Immunosuppressed, Post-extubation.
  • Contra: Resp arrest, unstable, no airway protection, facial trauma, ↑secretions.
• Mechanical Ventilation (MV):
  • Indications: NIV failure/contra, severe hypoxemia ($PaO_2/FiO_2$ <150), resp arrest, airway protection, ↓GCS.

⭐ ARDS: Lung-protective ventilation (LPV) with low tidal volumes (4-6 mL/kg PBW) & $P_{plat}$ <30 $cmH_2O$ improves survival.

Spotlight Cases - ARDS & COPD Alarms

• ARDS (Acute Respiratory Distress Syndrome)
  • 📌 BERLIN criteria: Acute onset, bilateral opacities (non-cardiac edema), P/F ratio $\le \textbf{300}$ mmHg with PEEP $\ge \textbf{5}$ cmH2O.
  • Severity (P/F mmHg): Mild ($\textbf{201-300}$), Mod ($\textbf{101-200}$), Sev ($\le\textbf{100}$).
  • Mgmt: Lung Protective Ventilation (LPV), optimal PEEP, prone positioning.
• COPD Exacerbation with Respiratory Failure
  • Triggers: Infections (viral/bacterial), pollutants, non-compliance.
  • Mgmt: Controlled O2 (target SpO2 88-92%), nebulized SABA/SAMA, systemic corticosteroids, antibiotics (if indicated). Consider NIV (BiPAP) for RF.

⭐ COPD + Hypercapnic RF: NIV is 1st line ventilation (reduces intubation/mortality).

High‑Yield Points - ⚡ Biggest Takeaways

• Respiratory Failure: PaO₂ < 60 mmHg (Hypoxemic) or PaCO₂ > 50 mmHg (Hypercapnic).
• Type I (Hypoxemic): V/Q mismatch (e.g., Pneumonia, PE); ↑ A-a gradient.
• Type II (Hypercapnic): Alveolar hypoventilation (e.g., COPD, Myasthenia); normal A-a gradient.
• ARDS: Acute onset, bilateral opacities, P/F ratio ≤ 300 mmHg, non-cardiogenic origin.
• Management: O₂ therapy (cautious in Type II), treat underlying cause, mechanical ventilation.
• P/F ratio (PaO₂/FiO₂) assesses hypoxemia severity, crucial in ARDS diagnosis and management.