Entonox® is a colourless, odourless gas, supplied piped or in cylinders, the cylinder colouring always being blue with blue and white shoulders. Portable cylinders and administration equipment are available. Care should be taken to store the cylinder horizontally above 10 °C; the gases will separate at −6 °C and this may pose a problem to community midwives in the winter until the cylinder is brought back to room temperature.

Entonox® is self-administered by the woman using a mouthpiece or mask to which an expiratory valve is attached. The mask is held over the nose and mouth with an airtight seal or the mouthpiece is placed in the mouth. As the woman breathes, the Entonox® is heard to be released; the apparatus should remain in place during expiration. In this way the woman may breathe through the contraction at a rate that suits her.

Only the woman should hold the apparatus to prevent overdosing; if the woman becomes drowsy she is unable to hold the apparatus to her face. Other side effects may include poor memory of labour, hyperventilation, tingling in hands, nausea and vomiting, dry mouth.

Taken correctly, Entonox® is fully effective within 40 seconds to 1 minute; the effect begins after five deep breaths (approximately 20 seconds; Street 2000). It is excreted from the body within 2–5 minutes.

The skill for the midwife is to support and assist the woman to gain maximum effectiveness from its use. In the first stage of labour, Entonox® needs to be breathed at the onset of the contraction if it is to be effective at the height of the contraction when the pain is at its peak. The midwife needs to palpate the contractions abdominally (see Chapter 1

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