A 28-year-old woman from Delhi presents to the emergency department with acute onset dyspnea and chest pain following a road traffic accident. On examination, she is tachypneic (RR 32/min), tachycardic (HR 118/min), and has a blood pressure of 88/54 mmHg. Arterial blood gas shows: pH 7.28, PaCO₂ 32 mmHg, PaO₂ 95 mmHg, HCO₃⁻ 14 mEq/L, lactate 6.2 mmol/L. Despite supplemental oxygen, her oxygen saturation remains at 92%. Which of the following best explains the discrepancy between her PaO₂ and oxygen saturation?

Explanation

## Analysis of Oxygen Dissociation Curve in Shock ### Clinical Context This patient is in hypovolemic shock (low BP, tachycardia, elevated lactate) with metabolic acidosis (pH 7.28, low HCO₃⁻, elevated lactate). Despite a PaO₂ of 95 mmHg (which is acceptable), her oxygen saturation is only 92% — this dissociation indicates a **rightward shift** of the oxygen-hemoglobin dissociation curve. ### Mechanism of Rightward Shift A rightward shift means hemoglobin has **decreased affinity for oxygen** — it releases oxygen more readily to tissues. This occurs with: 1. **Acidosis** (↓ pH) — Bohr effect 2. **Hypercapnia** (↑ PaCO₂) — Bohr effect 3. **Increased 2,3-DPG** — produced during anaerobic metabolism and shock 4. **Increased temperature** — from tissue inflammation/infection **Key Point:** In this case, metabolic acidosis (pH 7.28) + tissue hypoxia (lactate 6.2) + shock state → ↑ 2,3-DPG production → rightward shift. This is a **compensatory mechanism** that enhances oxygen unloading to ischemic tissues, even though arterial oxygen content is compromised. ### Why PaO₂ ≠ SaO₂ Here The PaO₂ of 95 mmHg would normally correspond to ~97–98% saturation on a normal (leftward) curve. But with rightward shift, the same PaO₂ yields only ~92% saturation — oxygen is being released from hemoglobin more aggressively. **Clinical Pearl:** In shock states, a rightward shift is **beneficial** — it prioritizes oxygen delivery to hypoxic tissues over maintaining high arterial saturation. Do not over-interpret the "low" SaO₂ in isolation. ### Bohr Effect Mnemonic **CADET, go and get some (2,3-DPG):** - **C**O₂ ↑ → rightward shift - **A**cid ↑ (pH ↓) → rightward shift - **D**PG (2,3-DPG) ↑ → rightward shift - **E**xercise / **T**emperature ↑ → rightward shift [cite:Guyton & Hall Textbook of Medical Physiology Ch 41]