Respiratory · Year 1 · from Respiratory
Case 1: Carbon Monoxide Poisoning
Clinical Image
Source: Wikimedia Commons - Pulmonary Embolism - Public Domain
Case Presentation
A 32-year-old man is brought to the emergency department by paramedics after being found unresponsive in his garage with his car engine running. His wife called 911 when he did not answer his phone for several hours. On arrival, he is confused and complaining of severe headache. Vital signs show heart rate 110 bpm, blood pressure 145/88 mmHg, respiratory rate 24/min, and pulse oximetry reading 98% on room air. Physical examination reveals cherry-red skin coloration and no focal neurological deficits.
Arterial blood gas on room air shows pH 7.32, PaCO2 32 mmHg, PaO2 95 mmHg, calculated SaO2 97%. However, co-oximetry reveals carboxyhemoglobin (COHb) level of 35% (normal <3%). The calculated A-a gradient is normal at 12 mmHg.
This case illustrates a critical limitation of standard pulse oximetry and ABG analysis. The patient's PaO2 is normal because dissolved oxygen in plasma is unaffected by carbon monoxide. However, CO binds hemoglobin with 200-250 times greater affinity than oxygen, forming carboxyhemoglobin that cannot carry oxygen. Additionally, CO shifts the oxygen-hemoglobin dissociation curve leftward, impairing oxygen release to tissues. The pulse oximeter reads falsely normal because it cannot distinguish oxyhemoglobin from carboxyhemoglobin.
The patient is immediately placed on 100% oxygen via non-rebreather mask to accelerate CO displacement from hemoglobin (reducing half-life from 4-5 hours on room air to 60-90 minutes). Given his altered mental status, he is transferred for hyperbaric oxygen therapy, which further reduces CO half-life to 20-30 minutes and increases dissolved oxygen delivery. His symptoms resolve over 24 hours, and follow-up COHb levels normalize.
Key Learning Points
- Carbon monoxide binds hemoglobin with 200-250 times greater affinity than oxygen, forming carboxyhemoglobin
- CO causes a leftward shift of the oxygen-hemoglobin dissociation curve, further impairing tissue oxygen delivery
- Standard pulse oximetry and ABG-calculated oxygen saturation are falsely normal in CO poisoning; co-oximetry is required
- PaO2 reflects dissolved oxygen only and may be normal despite severely reduced oxygen content
- Oxygen content equation: CaO2 = (1.34 x Hgb x SaO2) + (0.003 x PaO2) - CO reduces the first term dramatically