Preparatory Mindset

Hypoxia is not just low oxygen — it's INADEQUATE OXYGEN AT THE TISSUE LEVEL. There are FOUR types, each with different mechanisms. The key exam skill is distinguishing them based on ABG results and clinical context. Remember: hypoxic hypoxia = low PaO₂; anemic = normal PaO₂ but low O₂ content; circulatory = normal blood gases but poor delivery; histotoxic = cells can't use oxygen.

Core Concepts

• Hypoxia = inadequate O₂ at tissue level.


• Four types: (1) Hypoxic: ↓ PaO₂ (lung disease, high altitude) — correct with O₂; (2) Anemic: ↓ O₂-carrying capacity (anemia, CO poisoning) — PaO₂ normal; (3) Circulatory/stagnant: ↓ blood flow (shock, heart failure); (4) Histotoxic: cells can't use O₂ (CN poisoning).


• O₂-Hb dissociation curve shifted RIGHT by ↑H⁺, ↑CO₂, ↑temperature, ↑2,3-BPG → O₂ unloads more easily..

High-Yield Points

• Four types: hypoxic (↓ PaO₂), anemic (↓ Hb), circulatory (↓ perfusion), histotoxic (cells can't use O₂)


• O₂ content = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)


• Cyanosis appears when deoxygenated Hb >5 g/dL


• CaO₂ difference: mixed venous O₂ reflects tissue extraction

Topic Summary

The four types of hypoxia are: hypoxic (low PaO₂), anemic (low Hb), circulatory (poor perfusion), and histotoxic (impaired utilization). Treatment depends on the underlying cause.