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Pre‐procedural considerations

Equipment

Cylinders

Entonox cylinders are available from the BOC in a variety of sizes. All cylinders have blue and white markings on their shoulder (Figure 22.12). The lightweight smaller cylinders have the following advantages:

They are easier to carry.
They have a live contents gauge.
Changing an empty cylinder is simple because it is not necessary to fit a regulator or use a cylinder key.

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Figure 22.12 Entonox cylinder and hose. Source: Dougherty and Lister ([55]).

Demand apparatus

There are a number of different companies that supply the demand apparatus for self‐administered Entonox use. Examples include the Ease demand valve (Sabre Medical) and the Carnet single patient use demand valve (Figure 22.13).

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Figure 22.13 Patient using Entonox demand valve. Source: The Royal Marsden.

Bacterial filters and mouthpieces/facemasks

Because Entonox equipment is a potential source of cross‐infection, bacterial filters (single use only) should be fitted between the facemask or mouthpiece and demand valve if the demand valve is not for single use (Chilvers and Weisz [38]). Facemasks and mouthpieces must also be single use only and disposed of once therapy ceases. Local policies must also be followed for the cleaning/sterilization of non‐disposable equipment between patients.

Assessment

The patient's ability to administer Entonox safely and effectively (particularly the very young or old) must be assessed prior to use. Patients should be able to:

understand the instructions for Entonox use
hold the demand valve to self‐administer the gas
inhale the gas through the mask or mouthpiece while breathing normally (patients who have impaired lung function may not be able to inhale the gas sufficiently to provide adequate analgesia).

Pharmacological support

Entonox is designed for self‐administration by the patient. This method of administration makes use of a demand unit which safeguards the patient from excessive inhalation of Entonox. The demand unit ensures that gas can be obtained only by the patient inhaling from the mouthpiece or mask and producing a negative pressure. The gas flow stops when the patient stops inhaling and removes the mouthpiece or mask from their face. Thus patients are able to self‐regulate the dose of Entonox (a method of patient‐controlled analgesia). Therefore, the patient must hold the mask firmly over the face or mouthpiece to the lips to produce an airtight fit and breathe in before the gas will flow. Expired gases escape by the expiratory valve on the handpiece. It is essential to adhere to this method of self‐administration as it is then impossible for patients to overdose themselves because, if they become drowsy, they will relax their grip on the handset and the gas flow will cease when no negative pressure is applied. However, should inhalation continue, light anaesthesia supervenes and the mask drops away as the patient relaxes. Entonox may be self‐regulated, but additionally may be administered by attendant medical personnel trained in its use, for example within obstetric/accident and emergency units, and accident ambulances.

Entonox has an oxygen content 2.5 times that of air and is therefore a good way of giving extra oxygen as well as providing analgesia.

Duration and frequency of administration

The duration and frequency of Entonox administration should always be tailored to individual patient needs. Because prolonged exposure to Entonox causes inactivation of vitamin B₁₂, impaired folate metabolism and pernicious anaemia (BOC [19]), it is recommended that:

Entonox is used on a short‐term rather than a long‐term basis.
It should not be used for more than a total of 24 hours, or more frequently than once every 4 days (BOC [19]).
If daily use is required for more than 4 days, this should be accompanied by close supervision and haematological monitoring (blood tests) to check for changes in red and white blood cells (BOC [19]). Consideration should also be given to the administration of B₁₂ and folate supplements.

Procedure guideline 22.5

Entonox administration

Table 22.9 Prevention and resolution (Procedure guideline 22.5)

Problem	Cause	Prevention	Suggested action
Patient not experiencing adequate analgesic effect.	Entonox cylinder empty. Apparatus not properly connected.	Check before procedure commences.	Change to a full cylinder.
	Patient not inhaling deeply enough (BOC [17]).	Education of patient prior to starting procedure.	Encourage the patient to breathe in until a hissing noise can be heard from the cylinder. Reassess suitability of patient for Entonox use. The patient may not be strong enough to inhale deeply or may have reduced lung capacity.
	Patient inhaling pure oxygen, that is, cylinder has been stored below –6°C and nitrous oxide has liquefied and settled at the bottom of the cylinder.	All cylinders should be stored horizontally at a temperature of 10°C or above for 24 hours before use (BOC [16]).	Initially safe, but later the patient may inhale pure nitrous oxide and be asphyxiated. Discontinue the procedure. Ensure adequate warming of the cylinder and inversion of the cylinder to remix the gases adequately.
	Not enough time has been allowed for nitrous oxide to exert its analgesic effect.	Allow at least 2 minutes of Entonox use before commencing the procedure.	Stop procedure. Allow 2 minutes of Entonox use. Restart procedure.
Patient experiences generalized muscle rigidity.	Hyperventilation during inhalation (BOC [17]).	Education of patient prior to starting procedure.	Discontinue Entonox and allow the patient to recover. Explain the procedure again, stressing deep and regular inspiration. Use a mouthpiece in place of a mask.
Patient unable to tolerate a mask.	Smell of rubber, feeling of claustrophobia.	Assess patient preference to use either mask or mouthpiece before Entonox use.	Use a mouthpiece in place of a mask.
Patient feels nauseated, drowsy or giddy.	Effect of nitrous oxide accumulation (BOC [17]).	None.	Discontinue Entonox administration: the effect will then rapidly disappear. Restart Entonox use; if the same effect occurs, stop using Entonox and use alternative analgesia.
Patient afraid to use Entonox.	Associates gases with previous hospital procedures, for example anaesthesia before surgery (BOC [17]).	Assess suitability of patient for Entonox use. Address fear and anxiety issues before starting Entonox use.	Reassure patient and reiterate instructions for use and short‐term effects.