SN5 SHORT NOTE 5M

Describe Entonox in detail.

General Surgery · Final Professional

Definition

Entonox is a ready-mixed 50:50 mixture of nitrous oxide and oxygen in a single cylinder, used as an inhaled analgesic. It provides analgesia with retained consciousness and is self-administered by the patient through a demand valve.

Uses & Cautions

It is widely used for analgesia in labour, in ambulances and emergency departments, and for short painful procedures (dressing changes, manipulations). Its advantages are a rapid onset and offset, patient control, and the guaranteed 50% oxygen. As it contains nitrous oxide, it is avoided where N₂O is contraindicated (pneumothorax, bowel obstruction, air embolism, decompression sickness, middle-ear disease), and prolonged use carries the vitamin-B₁₂ risk. The cylinder must be kept above a certain temperature to prevent the gases separating (lamination).

CLINICAL PEARL

Entonox = 50% N₂O + 50% O₂, self-administered for rapid, controllable analgesia (labour, ambulances, procedures) — but it carries all the contraindications of nitrous oxide, chiefly closed air spaces.

Practical Points

In use, Entonox is patient-controlled: the patient holds the mask or mouthpiece and triggers a demand valve, which provides a built-in safety feature, since an over-sedated patient drops the mask and stops inhaling. Its rapid onset and offset suit intermittent pain such as contractions or a brief procedure, but it takes a minute or so of breathing to become effective, so it is started a little ahead of an anticipated painful moment, and it is stored and used above its pseudocritical temperature to keep the two gases mixed.

CLINICAL PEARL

Think of Entonox as ‘nitrous oxide in a bottle, at a safe oxygen concentration’: excellent for rapid, self-controlled analgesia, but carrying every one of nitrous oxide’s contraindications, above all the presence of a closed air space such as a pneumothorax.

DANGER / REMEMBER

A safety feature worth stating is that Entonox is self-administered: if the patient becomes drowsy they release the mask and stop inhaling, so the technique is inherently self-limiting when used as intended.

Lamination separation below −6°C — store and transport above it.

KEY POINT

Key points TO remember

Entonox = 50:50 nitrous oxide/oxygen; self-administered inhaled analgesic (conscious).
Uses: labour analgesia, pre-hospital/ED, short painful procedures; rapid on/off.
Contraindicated wherever N₂O is (pneumothorax, bowel obstruction, air embolism, middle-ear, decompression sickness).
Keep cylinder warm to prevent gas separation (lamination).

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

All questions in Inhalational Anaesthetic Agents

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