A 28-year-old male construction worker from Delhi presents to the emergency department with severe headache, dizziness, and confusion after being trapped in a poorly ventilated basement for 2 hours. His wife noticed cherry-red discoloration of his lips. Arterial blood gas shows PaO₂ 95 mmHg (normal), PaCO₂ 38 mmHg, pH 7.38. Carboxyhaemoglobin level is 32%. Despite high PaO₂, his oxygen saturation by pulse oximetry reads 88%. Which of the following best explains the discrepancy between his PaO₂ and oxygen saturation?

Explanation

## Understanding the Oxygen Dissociation Curve in CO Poisoning ### The Clinical Paradox This patient has a **normal PaO₂ (95 mmHg)** but **low SpO₂ (88%)** — a critical discrepancy that tests understanding of oxygen transport physiology. **Key Point:** PaO₂ measures dissolved oxygen in plasma (which is normal), while SpO₂ reflects haemoglobin saturation. Carboxyhaemoglobin (COHb) cannot carry oxygen and is counted as "saturated" by pulse oximetry but is functionally useless. ### Why This Happens | Parameter | Normal | This Patient | |-----------|--------|---------------| | PaO₂ (dissolved O₂) | 80–100 mmHg | 95 mmHg ✓ | | Functional Hb | 100% available | 68% available (32% is COHb) | | SpO₂ (pulse ox reading) | 95–100% | 88% | | Actual O₂ delivery | Adequate | **Severely compromised** | **High-Yield:** In CO poisoning: - Dissolved oxygen (PaO₂) remains normal because CO does not dissolve in blood - COHb has a cherry-red appearance and prevents oxygen binding - Pulse oximetry reads falsely *high* in some devices (COHb absorbs light similarly to oxyHb) - The discrepancy between PaO₂ and SpO₂ is pathognomonic for CO poisoning ### Oxygen Dissociation Curve Context The oxygen dissociation curve describes the relationship between PaO₂ and haemoglobin saturation. In CO poisoning: - The curve itself is **not shifted** - The problem is **reduced functional haemoglobin** (not a curve shift) - Even if the curve were shifted right (increasing O₂ unloading), it would not explain the normal PaO₂ with low SpO₂ **Clinical Pearl:** The treatment is **100% oxygen and hyperbaric oxygen therapy** — not because it increases PaO₂ (already normal), but because it: 1. Competitively displaces CO from haemoglobin 2. Increases dissolved oxygen to partially compensate for lost haemoglobin capacity [cite:Harrison 21e Ch 297]