These are labelled ENTONOX. They are colour coded blue with white quadrants on the shoulder. A pin index system obviates the risk of inadvertent connection of the delivery unit to the wrong cylinder. A pressure gauge mounted on the inhalation unit provides an indication of cylinder content. A 500 litre cylinder is available for domiciliary use and 2000 litre and 4250 litre cylinders for hospital use.
The proportions of the mixture are unaffected by variations in cylinder pressure and flow. The functional characteristics of delivery and resistance to gas flow remain constant despite variations in cylinder pressure.
Delivery system
The apparatus incorporates a simple demand valve. It is capable of delivering a peak inspiratory flow in excess of 275 litres/minute.
The demand valve ensures that the gas does not flow unless a negative pressure is achieved. This requires an airtight seal between mask and face.
Administration
If the patient has not previously used entonox, he should be instructed in its use and reassured quietly. "This gas will help your pain. It is cool and pleasant and has a sweetish taste." Speak softly and clearly. Offer the mask to the patient advising that it must form an effective seal on the face before he breathes in - and that an air leak will cause the inhalation to be ineffective. Tell him to: "Keep the mask in position while you breathe out - and breathe normally while you keep breathing with the mask held firmly in position".
Faster, deeper inhalations will speed up the onset of the initial pain relief which should commence within 20-40 seconds and be maximum within two minutes. It takes only 15 seconds for nitrous oxide to go from the lungs to the brain. There is a distinct flowing sound with each inhalation and the demand flow takes place only with inhalation.
When the correct level of pain relief is achieved, the patient should still be able to obey commands, e.g. "lift the mask", "bend your knee".
When entonox is self administered from a demand apparatus under trained supervision, loss of consciousness is unlikely. If it does occur, it will be momentary as the mask falls aside, the flow of gas ceases and the patient wakes up and pain will return. The patient should be assisted to reapply the mask. The attendant should not hold the mask.
A mouth piece may be used instead of a mask.
Indications in motorsport
Entonox is indicated to provide pain relief:
In the ambulance service for splinting of fractures, extrication from vehicles and during transport of the injured.
Contraindications
Entonox should not be used:
where the patient's conscious state is impaired.
where the patient:
is overtly drunk.
has facial or jaw injuries.
has broken ribs (pneumothorax will be increased).
Cautions
Use a clear plastic mask to reveal blood or vomitus.
Nitrous oxide is a drug of dependence - do not leave the delivery unit accessible to the patient.
Inhalation by staff "as a joke" should not be permitted.
Cylinders should be stored in a secure area.
Prolonged use of nitrous oxide (72 hours) results in pancytopaenia - always time the duration of inhalation and record it on the chart.
If the patient is hypovolaemic, closer observation is necessary as the patient may rapidly lose consciousness.
If there is any risk of vomiting, the patient should be in the lateral position.
Entonox is flammable. Caution is necessary if metal cutting equipment is used at an accident site.
The relative proportions of gases issuing from cylinders are affected by temperatures below -7oC when the contents separate.
The cylinders should always be stored at a temperature above 5oC. In cold climates they must not be stored in vehicles or outdoors.
If exposed to temperatures below 0oC or if frost (not dew) forms on the lower half of the cylinder during use, the cylinder must be warmed and then turned end over end three time to mix before use. Warming involves immersing in warm water not exceeding 37oC for not less than 5 minutes.