A fire victim rescued from a closed space house fire is drowsy with cherry red skin discolouration. Pulse oximetry reads 99 percent saturation on room air. Arterial blood gas carboxyhaemoglobin is measured. At what level does carboxyhaemoglobin indicate severe poisoning requiring consideration of hyperbaric oxygen therapy?

• A Greater than 5 percent
• B Greater than 10 percent
• C Greater than 25 percent ✓
• D Carboxyhaemoglobin level is irrelevant once oximetry exceeds 95 percent

Explanation

Levels above 20 to 30 percent cause serious neurological impairment, and levels above about 25 percent, especially with altered consciousness, metabolic acidosis, or pregnancy, are indications for hyperbaric oxygen. Standard pulse oximetry cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and therefore reads falsely normal, which explains the misleading 99 percent value and makes option D dangerously wrong.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.