Hypoxemic Respiratory Failure - Differential Diagnosis Framework
DECREASED INSPIRED OXYGEN
• Low PATM Can decrease first part of alveolar gas equation
- PATM 760 mm Hg at sea level
- PH20 47 mm Hg in average settings
• Causes: High altitude (eg, climbing the Andes or Mount Everest)
HYPOVENTILATION
• Rising PaCO2 decreases second part of alveolar gas equation
• Causes: Decreased respiratory drive due to neurologic injury or sedatives, obesity-hypoventilation syndrome, OSA
DIFFUSION DISORDER
• Difficulty diffusing oxygen across capillary-alveolar interface
• Less common than VIQ mismatch and shunt
• Improves with supplemental oxygen
• Causes: Interstitial lung diseases, pulmonary edema (most common cause of reversible diffusion defects)
VENTILATION/PERFUSION (V/Q) MISMATCH
• Most common cause of hypoxemic respiratory failure
• There are gravity-dependent gradients of both perfusion (Q) and ventilation (V) in the lungs
• Ideal gas exchange depends upon matching adequate ventilation with adequate perfusion
• Mismatching in either direction leads to hypoxemia
• Causes: Pneumonia, asthma, COPD, pulmonary embolus
SHUNT
• Occurs when blood is bypassing the lungs
• Causes: Intrapulmonary shunting (eg, pulmonary AVMs, hepatopulmonary syndrome), intracardiac shunting (eg, atrial or ventricular septal defects)
• Does not improve with supplemental oxygen
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