Preparatory Mindset

Respiratory failure is the INABILITY of the respiratory system to maintain adequate gas exchange. Type I = hypoxemic (PaO₂ <60) with normal/low PaCO₂ — caused by V/Q mismatch, shunt, diffusion impairment. Type II = hypercapnic (PaCO₂ >50) — caused by hypoventilation. ABG interpretation is essential for diagnosis.

Core Concepts

• Respiratory Failure = inability to maintain adequate gas exchange.


• Type I (hypoxemic): PaO₂ <60 mmHg, PaCO₂ normal or low.


• Causes: V/Q mismatch (most common), shunt (pneumonia, ARDS), diffusion impairment (ILD), hypoventilation (rarely).


• Type II (hypercapnic): PaCO₂ >50 mmHg.


• Causes: ↓ respiratory drive (opioids), neuromuscular disease (MG, ALS), airway obstruction (COPD, asthma), chest wall restriction.


• A-a gradient = PAO₂ − PaO₂.


• Normal <10-15, increases with age.


• ↑ A-a = V/Q mismatch, shunt, diffusion impairment.


• Normal A-a = hypoventilation..

High-Yield Points

• Type I (hypoxemic): PaO₂ <60, PaCO₂ normal/low — V/Q mismatch, shunt, diffusion impairment


• Type II (hypercapnic): PaCO₂ >50 — hypoventilation


• A-a gradient helps distinguish cause of hypoxemia


• Acute vs chronic: chronic hypercapnia → renal compensation (↑ HCO₃⁻)


• Oxygen therapy: target SpO₂ 92-96% (88-92% if at risk of CO₂ retention)

Topic Summary

Type I respiratory failure: low PaO₂, normal/low PaCO₂ (V/Q mismatch, shunt). Type II: low PaO₂, high PaCO₂ (hypoventilation). Treatment addresses the underlying cause with oxygen therapy and/or ventilation.