## Rightward Shift of the Oxygen Dissociation Curve **Key Point:** A rightward shift of the oxygen-haemoglobin dissociation curve indicates *decreased* oxygen affinity — haemoglobin releases oxygen more readily to tissues. ### Physiological Basis of Rightward Shift The most common cause in COPD is **increased 2,3-DPG (2,3-diphosphoglycerate)** levels. This occurs as a compensatory response to chronic hypoxaemia. 1. Chronic hypoxaemia in COPD stimulates increased glycolysis in red blood cells 2. Glycolysis produces more 2,3-DPG 3. 2,3-DPG binds to the central cavity of deoxyhaemoglobin 4. This stabilizes the T (tense) state, reducing oxygen affinity 5. Result: rightward shift, facilitating oxygen unloading to tissues ### Mnemonic for Rightward Shift (Bohr Effect + Others) **"CADET, face Right!"** - **C**O₂ (increased PaCO₂) - **A**cid (decreased pH / increased H⁺) - **D**PG (increased 2,3-DPG) - **E**xercise (increased metabolic demand) - **T**emperature (increased) All shift the curve **rightward** → oxygen release ↑ ### High-Yield Clinical Pearl In chronic hypoxaemia (COPD, high altitude, chronic anaemia), the body compensates by increasing 2,3-DPG production. This is a **beneficial adaptation** that enhances oxygen delivery to tissues despite lower PaO₂. ### Why Each Factor Matters in COPD | Factor | Effect on Curve | Mechanism in COPD | | --- | --- | --- | | ↑ 2,3-DPG | Rightward | Chronic hypoxaemia → ↑ glycolysis | | ↓ PaCO₂ | Leftward | Hyperventilation would shift left (not the primary driver) | | ↑ pH | Leftward | Alkalosis shifts left (opposite of COPD's usual state) | | ↓ Temperature | Leftward | Hypothermia shifts left (not typical in COPD) | **Clinical Pearl:** This rightward shift in COPD is why patients with chronic hypoxaemia can maintain reasonable tissue oxygen delivery despite lower arterial oxygen saturation. [cite:Guyton & Hall 13e Ch 41]
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