Definition and indications for the use of Entonox
Entonox is an inhaled analgesic gas comprising of 50% nitrous oxide and 50% oxygen that is indicated in the management of moderate pain and also for labour pain.1 Entonox is rapidly absorbed into the blood stream of the pulmonary vasculature, but does not combine with haemoglobin or with any other body tissues. Its low solubility in blood produces the rapid onset and offset of effect.2 Nitrous oxide (the anaesthetic agent within Entonox) has very few side-effects and no incompatibility with other drugs has ever been demonstrated.2 It has been noted that the nitrous oxide constituent of Entonox inactivates vitamin B12.3 The mixture of nitrous oxide and oxygen remains stable at temperatures above –6oC.
There are different theories as to the exact mechanism of action of Entonox. It has been suggested that Entonox interacts with the endogenous opioid system,4 and it has been shown to act preferentially on areas of the brain and spinal cord that are rich in morphine sensitive cells.5 BOC suggest that it also has an effect on gamma aminobutyric acid (GABA), which increases inhibition of nerves cells, causing drowsiness and sleep.6
Advantages of Entonox
Entonox offers the practitioner and patient several advantages:
- Analgesic effect is rapid and side-effects are minimal.
- It confers no cardiac or respiratory depression.
- It can be self-administered.
- It has a very short half-life so the effects wear off rapidly once inhalation has ceased. This may be useful for short-term relief when carrying out painful procedures such as wound dressings or applying traction to fractures. In addition, Entonox may be administered as a temporary analgesic agent whilst preparing the patient for the administration of intravenous analgesia.
- The oxygen content may confer benefit to patients with certain medical or trauma conditions.
Contraindications to the use of Entonox
According to the Joint Royal Colleges Ambulance Liaison Committee (JRCALC), Entonox is contraindicated in the following circumstances:1
- Severe head injuries with impaired consciousness.
- Decompression sickness (anybody who has been diving in the previous 24 hours should be considered at risk).
- Violently disturbed psychiatric patients.
Caution should be applied when administering Entonox to a chest injured patient when a pneumothorax is suspected as it may expand the pneumothorax.
There appears to be some conflict between JRCALC and the contraindications listed by the BOC datasheet, which includes any patient:
- Where air is entrapped within a body and where its expansion might be dangerous.
- Artificial, traumatic or spontaneous pneumothorax.
- Air embolism.
- Gross abdominal distension.
- Intoxication.
- Maxillofacial injuries.
Essentially N2O is contraindicated where there is trapped air, which covers the first four in the BOC list. Dr Tom Clarke of JRCALC states that intoxication is not defined by BOC but would appear to indicate a state where the ‘balance of the mind is disturbed’ due to alcohol, drugs or psychosis.7 Maxillofacial injuries are contraindicated due to the possible access of Entonox to the sinuses and also due to the difficulty of administration.
It has been suggested that Entonox should not be given during the first trimester of pregnancy due to the risk of skeletal teratogenic complications; however no increased incidence of foetal malformation has been discovered in 8 epidemiological studies and case reports in human beings. There is no published material showing that nitrous oxide is toxic to the human foetus, therefore, there is no absolute contra-indication to its use in the first 16 weeks of pregnancy.3
Recommendations
- Entonox should be contraindicated to patients where air is entrapped within a body and where its expansion might be dangerous, those with severe head injuries with impaired consciousness, decompression sickness, and intoxication or psychiatric patients where the balance of the mind is disturbed.
Equipment
- Entonox cylinder
- Regulator
- Face mask or mouthpiece.
The neck on an Entonox cylinder is blue and white segmented, which is a UK norm; colours may vary in other Countries. The newer CD (440 L) and HX (2200 L) cylinders feature a valve with built in regulator, an Entonox Schrader outlet connection, and a permanently live contents gauge. The cylinders are 25% lighter than conventional steel, which makes them ideal for prehospital use. The traditional 500 L, D sized cylinder is still in use and requires the application of a pin-index pressure regulator.
Use of cylinders and general safety for both Entonox and Oxygen are covered at the end of this chapter.
Principles of administration of Entonox
Entonox is self-administered by the use of a face mask or mouthpiece connected through a demand valve to the Entonox cylinder. Gas is drawn from the cylinder by the negative pressure created during the patient’s inhalation; as soon as the face mask or mouthpiece is removed, delivery stops. For this reason, the face mask must be held firmly over the face or the mouthpiece to the lips to produce an airtight seal.
Doses are regulated by the patient and the risk of overdose is low. Since pain is usually relieved by a concentration of 25% nitrous oxide, continued inhalation does not occur. However, should inhalation continue, light anaesthesia supervenes and the mask drops away as the patient relaxes.3
Mouthpiece or face mask?
Searches revealed no studies investigating the relative effectiveness of delivering Entonox via a mouthpiece or mask. One study evaluated patient preferences and the efficacy of pre-oxygenation using face mask or mouthpiece with or without nose clip, which may provide some insight into the best method of administering Entonox.8 Most conscious patients tolerated a mask well but a significant minority (12%) refused it. The mouthpiece on its own was insufficient to attain full preoxygenation owing to nose breathing of room air, which may explain why anecdotally, Entonox does not always provide analgesia. The mouthpiece with nose clip produced comparable results to the face mask but was less popular amongst patients.
Recommendation
Entonox should preferentially be delivered via face mask or mouthpiece with nose clip to maximise the effect of the drug.
Complications
Inappropriate, unwitting or deliberate inhalation of Entonox will ultimately result in unconsciousness, passing through stages of increasing light-headedness and intoxication. The treatment is removal to fresh air and resuscitation as appropriate.3
Procedure for administering Entonox
| Procedure | Rationale/further information |
| 1. Ensure that the patient is in a comfortable position prior to administration. | |
| 2. Check the patient has no contraindications for the administration of Entonox. | Avoids exacerbating underlying conditions. |
| 3. Explain the procedure and effects of the medication to the patient and gain informed consent. | Aids patient compliance and is a legal requirement. |
| 4. Ensure that the cylinder is turned on; check the gauge to determine how much Entonox is in the cylinder. | Avoids risk of running out of medication. |
| 5. Demonstrate how to obtain a seal with the face mask or mouthpiece. Explain that when the patient is breathing in and out regularly and deeply a hissing sound will be heard. | Maximises inhalation of the gas and indicates that the gas is being inhaled. |
| 6. Encourage the patient to breathe deeply for at least two minutes before attempting any painful procedures. | Allows time for the analgesic effect to take place. |
| 7. Coach the patient’s respirations as necessary. | To ensure maximum inhalation of the gas. |
| 8. Evaluate the effectiveness of Entonox by using verbal or visual pain scoring tools. | Identifies if further measures are required. |
| 9. Evaluate the effect during and after painful procedures, and throughout the duration of use. | |
| 10. Turn off Entonox supply on completion. | Avoids inadvertent leakage.. |
| 11. Dispose of mask/mouthpiece as per Trust guidelines, or clean in accordance with guidelines if using reusable items. | |
| 12. Record the administration, effects and side effects of Entonox on the case report form. Any adverse effects should also be recorded. | A legal requirement and provides information for those caring for the patient afterwards |
Troubleshooting
Procedure
| Problem | Possible explanation | Suggestion |
| 1. Patient not achieving adequate analgesia. | Cylinder empty. | Check prior to administration, change if necessary. |
| Patient not inhaling deeply enough. | Coach respirations. Reassess suitability of Entonox for the patient and find alternative analgesia if necessary. | |
| 2. Patient refuses or can’t tolerate a mask. | Feeling of claustrophobia or dislike of the smell of the mask. | Use mouthpiece with nose clip as an alternative. |
| 3. Feelings of nausea, drowsiness or giddiness. | Effect of nitrous oxide accumulation. | Discontinue therapy and effects will wear of rapidly. |
| 4. Patient afraid to use Entonox. | Possible associations with previous experiences. | Reassure patient about the benefits of receiving Entonox and the short-term effects. Reiterate the procedure for administration. |
Definition of oxygen therapy
Oxygen (O2) is a colourless, odourless gas that supports combustion and is essential to human life. It is provided in compressed form in a cylinder that is typically colour coded white. Modern cylinders are lightweight and feature a valve with built in regulator, an Oxygen Schrader outlet connection, and a permanently live contents gauge. The sizes most commonly used in prehospital care are the CD size (460 L) and the HX size (2300 L).
Oxygen is the most frequently used drug in prehospital emergency care and may be lifesaving when given correctly. However, it is often administered without careful consideration of its potential benefits and side-effects.9
Indications for use
JRCALC revised the oxygen guidelines in April 2009 to reflect concerns surrounding inappropriate administration of oxygen by practitioners in prehospital care.1 The new guideline challenges many of the traditional practices, such as the routine administration of high concentration supplemental oxygen to all patients suffering a suspected or confirmed acute myocardial infarction. It also advocates the more widespread use of pulse oximetry to guide supplemental oxygen therapy and the use of alternative methods of delivery, such as nasal cannulae.
It is imperative that the practitioner is aware of the updated guidelines and also understands the physiology of respiration and tissue oxygenation. It is recommended that this be reviewed in a suitable physiology textbook.
