Emergency Medicine Emergency Medicine · Respiratory · High Yield

Causes of Hypoxia

Hypoxia (SpO2 <94% or PaO2 <8 kPa) has five pathophysiological mechanisms. Identifying the mechanism directs treatment.

✦ The Mnemonic

"VVHDS — Five Mechanisms of Hypoxia"

V/Q mismatch · Ventilatory failure · Hypoventilation · Diffusion impairment · Shunt

VQ V/Q Mismatch Most common cause; PE, pneumonia, COPD, asthma — responds to supplemental O2
Sh Right-to-Left Shunt Blood bypasses ventilated alveoli; PFO, AVM, hepatopulmonary syndrome — does NOT respond to O2
H Hypoventilation Low respiratory rate — opioids, sedatives, neuromuscular disease; ↑CO2 + ↓O2; responds to O2 + ventilation
D Diffusion impairment Thickened alveolar membrane — fibrosis, pulmonary oedema — responds to supplemental O2
A Low inspired O2 (altitude) FiO2 normal (21%) but partial pressure low; high altitude; enclosed spaces

📚 Clinical Breakdown

The 100% oxygen test: give 100% O2 via non-rebreather mask. If PaO2 rises to >400 mmHg (53 kPa) — V/Q mismatch or hypoventilation. If PaO2 fails to rise appropriately — large right-to-left shunt. This test is used to diagnose intracardiac shunts (PFO, ASD) and hepatopulmonary syndrome.

Type 1 vs Type 2 respiratory failure: Type 1 (hypoxaemic) — PaO2 <8 kPa, PaCO2 normal or low; caused by V/Q mismatch, shunt, diffusion impairment. Type 2 (hypercapnic/ventilatory failure) — PaO2 <8 kPa + PaCO2 >6 kPa; caused by hypoventilation, neuromuscular disease, severe COPD.

CO poisoning: pulse oximetry is falsely normal — SpHb measures COHb as OxyHb. Diagnose with arterial blood gas + co-oximetry (measures COHb directly). Treat with 100% O2 (reduces COHb half-life from 4h to 1h) or hyperbaric oxygen for severe cases (COHb >25%, neurological features, pregnancy, cardiac involvement).

Most common mechanism V/Q mismatch
Does NOT respond to O2 Right-to-left shunt
Type 2 RF definition PaO2 <8 kPa + PaCO2 >6 kPa
CO poisoning — SpO2 Falsely normal — must use co-oximetry

⭐ Clinical Pearl

Permissive hypoxia in COPD: patients with chronic hypercapnia have blunted CO2 drive and rely partly on hypoxic drive. High-flow O2 can suppress this drive and worsen hypercapnia. Target SpO2 88–92% in known or suspected COPD (compared to 94–98% for most patients). Use Venturi masks for precise O2 delivery.

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