Can nitrous oxide (N2O) displace oxygen in the blood?

Nitrous Oxide Can Displace Oxygen in the Blood and Cause Hypoxemia

Yes, nitrous oxide can displace oxygen in the blood, potentially causing hypoxemia, especially when administered without supplemental oxygen. 1, 2

Mechanism of Action and Risk

Nitrous oxide can cause hypoxemia through several mechanisms:

1. Direct displacement of oxygen:

   • The FDA drug label explicitly states that nitrous oxide "MAY DISPLACE OXYGEN AND CAUSE RAPID SUFFOCATION" 1
   • When inhaled in high concentrations without adequate oxygen supplementation, nitrous oxide can reduce the partial pressure of oxygen in the alveoli

2. Diffusion hypoxia:

   • After discontinuation of nitrous oxide, it rapidly diffuses from the bloodstream into the alveoli, potentially diluting alveolar oxygen 3
   • This phenomenon, known as "diffusion hypoxia," can occur during or immediately after nitrous oxide administration 3

Clinical Guidelines and Safety Measures

The American Thorax Society recommends several safety measures when using nitrous oxide 2:

• Always co-administer with supplemental oxygen (30-50%) to prevent hypoxia
• Monitor patients with pulse oximetry during administration
• Avoid use in patients with emphysema or those at risk of hypercapnic respiratory failure

Evidence on Hypoxemia Risk

Research findings on diffusion hypoxia are somewhat mixed:

• Some studies show significant decreases in arterial oxygen saturation after nitrous oxide administration, particularly following hyperventilation 4
• Other studies with healthy volunteers found no clinically significant hypoxemia when using a 50:50 mixture of nitrous oxide and oxygen 5, 6

Practical Administration Considerations

When administering nitrous oxide:

• Different oxygen delivery devices produce varying concentrations of nitrous oxide (6.5-34.3%) when used with a 1:1 nitrous oxide-oxygen mixture 7
• The therapeutic range for nitrous oxide is 20-30% 7
• After discontinuation, consider administering 100% oxygen to prevent potential diffusion hypoxia 3

Additional Safety Concerns

Beyond oxygen displacement, nitrous oxide has other important physiological effects:

• It can increase pressure in air-containing body cavities as it diffuses into these spaces faster than nitrogen diffuses out 3
• Long-term exposure can inactivate vitamin B12-dependent enzymes, affecting DNA production and potentially causing neurological damage 3, 2

Contraindications

Nitrous oxide should be avoided in patients with:

• Pneumothorax
• Bowel obstruction
• Middle ear and sinus disease
• Recent cerebral air-contrast studies
• Pregnancy (first two trimesters)
• Immunosuppression 3

Always ensure adequate oxygen supplementation when administering nitrous oxide to prevent hypoxemia and potential tissue hypoxia.