Nitrous Oxide

Introduction

Nitrous oxide is used in combination with other inhalation agents. It has a low potency meaning it is not suitable for use as the sole anaesthetic agent. It is used as part of a balanced anaesthetic technique as it has been shown to reduce the required concentration of volatile agent, but also shows some analgesic properties. Care must be taken when using nitrous oxide as it reduces the inspired oxygen availability, so should be closely monitored.

Pharmacokinetics

Nitrous oxide is both nonirritant and nonflammable. The blood:gas partition coefficient is very low. This means that speed of induction can be increased when nitrous oxide is used in combination with a volatile agent. It also exhibits the second gas effect. This occurs as the volume of nitrous oxide present in the alveoli is high, meaning it has a concentrating effect on the second agent present, increasing the uptake of the second agent. However, patients are at risk of diffusional hypoxia when nitrous oxide is used. This is due to the diffusion of nitrous oxide into the alveoli from the blood is faster then that of nitrogen from the alveoli to blood, making the alveolar oxygen concentration lower. To help prevent this, the nitrous oxide should be turned off a short while before the oxygen. The MAC of nitrous oxide is extremely high, meaning it has a low potency and so cannot be used as the single anaesthetic agent, and must be used in combination. Due to its stability, nitrous oxide undergoes hardly any metabolism.

Adverse Effects

Central Nervous System

• Shown to be an N-methyl-D-aspartate receptor anatagonist, and so may reduce the development of enhanced pain sensitivity.
• Increases intracranial pressure

Cardiovascular System

• Direct myocardial depression, but controlled by activation of the sympathetic nervous system, making any effects small.
• Risk of arrhythmia development.

Respiratory System

• Lowers alveolar oxygen partial pressures so should be closely monitored.

Other Systems

• Prolonged exposure may lead to bone marrow suppression via vitamin B12 dependent enzyme inactivation.
• Possibly teratogenic so should be avoided in pregnant patients.

Contraindications

• Increased intracranial pressure.
• Pneumothorax and lung pathology.
• Gastric Dilation and volvulus and intestinal obstruction.
• Anaemia.