Last updated: Oct 2003
Keywords: Entonox; gas and air; procedural pain relief; nitrous oxide and oxygen; inhaled analgesia; wound dressing changes.
Nitrous oxide and oxygen gas mixture (also known as 'gas and air' or Entonox) has been available for many years and is an excellent method of providing short-term pain relief and relaxation during a number of painful procedures.
Entonox is safe and simple to use, provided certain cautions are observed.
Nurses can become involved in developing training and operational protocols for use, and should actively drive the development of these protocols to meet the need of patients in their care.
There are a number of procedures that patients undergo without appropriate analgesia, even though the means to provide adequate pain relief has been available for many years by the administration of a mixture of nitrous oxide and oxygen. For some patients, even apparently trivial procedures such as having an intravenous cannula sited, or sutures removed can cause a great deal of distress and discomfort. However, the profession has been slow at taking the lead in developing the organisational structure to support the more widespread use of 'gas and air'.
Nitrous oxide (N2O) gas has been known to have analgesic and sedative properties for over two hundred years. The gas was discovered by a Yorkshire chemist named Joseph Priestly and in 1799 the scientist Humphry Davy inhaled the gas and found it gave him rapid pain relief from an infected tooth; on one occasion he reported momentarily losing consciousness, waking up laughing about the pleasurable feelings he had experienced : hence the term 'laughing gas'. As a medicinal gas, it is available as a mixture containing equal parts of N2O and oxygen (O2). In many countries this is commercially available as Entonox, although it is also available as Kalinox in France or Medimix in Sweden. In the UK it is often known to patients and staff as 'gas and air' and since the 1960s has most frequently been associated with childbirth and use by ambulance crews.
Entonox is ideal in situations where pain is predictable and of a short duration . It can provide relief of pain during minor procedures such as wound dressing changes, debridement, the removal of drains or sutures and even turning a patient with a fracture or a pressure ulcer. In some circumstances the use of Entonox may aid the patient's ability to comply with lengthy or uncomfortable procedures, such as flexible sigmoidoscopy examination . All are situations where the nurse can instigate and supervise the use of this rapid action analgesic gas mixture.
The gas is administered using a face mask or mouthpiece; gas flow is controlled by a sensitive demand-valve activated by the patient's inspired breath. This allows pressurised gas from the cylinder to flow through a pressure regulator into the lungs at a steady rate. Longer and deeper breaths allow greater volumes of gas to be taken into the lungs if necessary.
The gas is rapidly absorbed on inhalation, providing analgesia within minutes. It is excreted, largely unchanged, by the lungs and its rapid elimination from the body on cessation of inhalation makes it ideal for controlling pain during short procedures. The patient safely controls the dosage and, under normal conditions, there is no risk of overdose as the patient's level of consciousness governs his/her ability to maintain the flow of gas .
With mobile equipment, Entonox can be used anywhere. This may be by a hospital bed or treatment room, or in the patient's own home. In each case the patient must be made comfortable, be assured of privacy, and be guaranteed a short period of rest following the procedure.
Entonox is safe for any age group as long as the patient is able to comprehend the activity and be physically capable of operating the system. The same cautions and care in use must therefore apply for all ages.
Entonox is an excellent analgesic for children, providing pain relief, distraction and relaxation but preparation and training may need to be more sensitive and involve parents or carers. However, while parents or carers may support the child during the procedure they must not take over the positioning of the face mask. Younger children who are either unable to understand or handle the equipment may require assisted administration, but this is outside of the scope of the current paper and is generally carried out by specially trained paediatric nurses.
There are no major incompatibilities with other drugs and its use with other analgesics is appropriate when given in a controlled manner. For example, oral analgesia can be given at least an hour prior to a procedure so that this has started to take effect by the time the Entonox gas has worn off. Any additional sedative action by other analgesics, such as oral morphine elixir, will be safely accommodated as the patient's active participation is needed to operate the Entonox system.
Entonox will cause an enclosed air-pocket in the body to expand rapidly in volume as the gas mixture is absorbed from the blood into the space, resulting in a build up of pressure . It must therefore never be used if the patient has any conditions where air is trapped in the body and expansion would be dangerous ; for example, it will exacerbate the onset and development of a pneumothorax (air inside the chest cavity but outside of the lung) and can increase the pressure of intracranial air following head trauma. Entonox can also be drawn into other cavities, such as the sinuses, middle ear and gut, causing expansion and pain. In such cases the patient should stop using the gas.
It is recommended that Entonox gas should not be used in the following situations:
Following a head injury where consciousness is impaired. However, for patients who require sutures to a head wound, the use of Entonox may be appropriate providing the patient is alert and able to follow simple commands . Such judgements regarding Entonox use should only be made by an experienced practitioner
Where there is artificial, spontaneous or traumatic pneumothorax
If there is an air embolism
In cases of decompression sickness
Where there is abdominal distension or suspected bowel obstruction
In maxillofacial injuries.
Repeated exposure to Entonox may result in megaloblastic anaemia owing to interference with the action of vitamin B12. If a procedure is to be carried out more frequently than every four days, the patient should be monitored and a routine blood cell count carried out . As the gas mixture is designed for short-term use only it should not be used in place of general continuing analgesia; for example in the case of burns or fractures, where an appropriate analgesic assessment is necessary for long-term management. It has been suggested that using Entonox for a short-duration but with repeated daily exposure in burns patients may increase the risk of developing megaloblastic anaemia and leukopenia in a group that invariably have blood abnormalities as a result of the tissue trauma and serus fluid loss following the burn damage .
Entonox should be administered according to a locally agreed protocol following a period of instruction or training. The protocol should cover a number of key areas as follows:
Assessment: Consider the need for Entonox during the planned procedure and whether other analgesics may be required. It is important to remember that oral analgesics can take some time to work and should be given in advance if they are to have any benefit in the immediate post-procedure phase when the gas has worn off. Entonox is not as effective as intravenous sedation and analgesia , for example midazolam plus an opioid such as fentanyl, but it is simpler to administer, wears off more quickly and is potentially safer. Entonox should not be used as a substitute for general anaesthesia or more profound supervised sedation where this is in the patient's best interests. Expert advice should be sought where there is any doubt before commencing what could be a major procedure.
Patient involvement: For patients who have not previously used Entonox, explain what the gas is for and what is required of them. Give them an opportunity to practice a few breaths to check their technique prior to commencing the procedure. Time spent at this stage will ensure the patient is relaxed and gains full benefit from the system. Select a mask or mouthpiece that is appropriate for the patient and instruct him/her on how to hold the mask over the nose and mouth, or in the case of the mouthpiece, between the teeth sealing around it with the lips, and to then breath normally.
Documentation: Consent is not normally required as the patient is self-administering the gas, but nursing documentation should confirm that proper patient instruction took place, and record the time period in which the gas was used. Generally, record keeping should also document an assessment of the pain experienced during and after the procedure and what steps have been taken to provide on-going comfort.
Practical considerations: Check the amount of gas in the cylinder prior to commencing therapy as it is important not to run out part way through a painful procedure. Check that the cylinder valve is turned on fully (at least two full turns) and use the demand valve test-button to demonstrate to the patient that the gas is flowing and familiarise him/her with the noise made on inspiration.
Procedure for administration: Encourage the patient to breathe the gas for around two minutes before commencing the procedure. Never hold the mask on the patient's face, as his/her active involvement is crucial to the safety of the Entonox system. Continually assess the patient's progress during the procedure. At all times the patient should be able to obey commands, but if a momentary loss of consciousness does occur, the seal around the mask or mouthpiece will be lost as it falls away and the demand valve will fail to operate causing the flow of gas to stop. On breathing normal air these effects will quickly wear off, at which point the patient may choose to continue using the gas. During dressing changes or debridement, the amount of pain will vary throughout the procedure. Sometimes painful events happen intermittently throughout the procedure and the patient must be given adequate warning to top-up as required in advance. If only the clinician is involved in treatment, it is important to be especially vigilant and maintain frequent verbal contact. Elderly patients may require a higher level of support and can experience difficulties maintaining a seal where dentures have been removed. If an assistant is helping they should support and supervise the patient, but it is the responsibility of the clinician to ensure that the assistant is aware not to hold the mask onto the patient's face. Patients using Entonox for the first time will require observation after treatment until they have fully regained their normal level of alertness and balance.
In the UK, Entonox is supplied in blue cylinders with a white and blue band around the top. A unique pin-index valve port on the cylinder prevents the wrong type of gas cylinder from being connected to the Entonox regulator valve and tubing. Local policies should cover access to medical gas cylinders and ensure staff training in assembling the necessary components. Where necessary, this should also cover transportation of the systems by nurses in their cars. It is advisable to keep the cylinder stored in the boot, away from the driver area, in case of a leak. The tubing should be disconnected and the cylinder turned off prior to starting a journey. It may also be worth checking with an employer about whether any additional insurance cover is required to carry Entonox.
The gas is stored in cylinders at a pressure of 137 bar, where it remains gaseous at temperatures above 6oC. If the gas has been stored in the boot of a car, or outdoors in cold weather, it is important to check that the cylinder has no ice on it. It should be allowed to return to room temperature (above 10oC for at least 2 hours or placed in water at body-temperature for 5 minutes) and the cylinder inverted three times to ensure an even mixture .
Occasional patient supervision by staff should not subject them to any undue risks of prolonged exposure to Entonox, but frequent use in a treatment room should prompt management to provide a gas scavenging system or increased ventilation.
Although Entonox is rapidly eliminated from the body, the British Oxygen Company (BOC) advises that patients should not drive or operate machinery for at least 12 hours . For patients having repeated procedures, where there has been time for individual assessment as to the duration of effect, a less strict regime may be appropriate. Experienced clinicians may also develop confidence with the safety of the system through seeing its predictable effects on a number of patients undergoing similar procedures. For example, Martin et al  conducted a controlled study in patients undergoing flexible sigmoidoscopy as a day-case procedure. A total of 40 men and 40 women were enrolled in the study group, and matched controls who were not given Entonox were sourced from the same procedure list. A computerised 'tracking test' was employed to measure the subjects' complex motor skills before and after the procedure. The study found no difference between the group receiving Entonox and the control group. The authors concluded that their study supported the view that Entonox does not impair driving ability, and in the context of flexible sigmoidoscopy, its use is expected to lead to an increase in compliance with the procedure. The assessment of individuals is most important, and local policy should allow for an element of professional judgement, lest the benefits be denied unnecessarily.
NOTE: Peate and Lancaster  have discussed the practical tips on the safe use of medical gases in the clinical setting in more detail. For further Health and Safety information consult the manufacturers' data sheet .
One perhaps overlooked aspect of delivering Entonox gas is the need to ensure patient safety from infectious agents contaminating the apparatus. The Blood Borne Viruses Advisory Panel of the Association of Anaesthetists of Great Britain (AAGBI) produced an advisory report in 1996  that included consideration of the possible transmission of infections via anaesthetic breathing systems. Following cases of cross infection with hepatitis C (HCV) its recommendations highlighted the need for an appropriate filter to be placed between the breathing system and the patient, and for the filter to be changed between patients. Although Entonox was not specifically mentioned in this document, the same risks of cross infection can be assumed to apply . Patients should therefore be protected from possible contamination, not simply by providing a clean mask or new mouthpiece, but also by providing a physical barrier between the Entonox equipment and the patient; the mouthpiece or facemask, tubing and control valves would otherwise have to be discarded after each use .
Some researchers have examined practices relating to infection control around the UK and found an almost complete lack of consistent guidance for staff. Chapman and Clarke , in 1999, contacted all training managers in the UK ambulance service and obtained a 90% response rate (n=36) to a factual questionnaire. This study found that simple soap and water was the most common method of cleaning equipment after use, and that none of the ambulance services used filters. The authors cite staff education as the cornerstone of any infection control programme, adding that relying on the use of disposable equipment and sterilising agents alone to counteract poor general hygiene is "both inadequate and unsafe."
Two studies of Entonox looking at cross infection in obstetric units have been published recently. The first  surveyed the use of anti-infective filters being used in Entonox systems following the guidance from the Association of Anaesthetists. Of 100 delivery-suite units in the UK that provided information, only seven used filters. One quarter of respondents washed the mouthpieces after each case, but this, it was suggested, would probably be ineffective in preventing viral transmission. The second study  was conducted in obstetric units within the Anglia and Oxford regions of the UK. It too found that there was no consensus on the cleaning of Entonox equipment after use, or the use of filters with the apparatus. Interestingly, 75% of respondents (15/20) reported that cleaning procedures were taken more seriously in known 'high risk' cases, but, as the authors pointed out, all patients should be treated as potentially infected, and all patients should be protected from potential cross infection. In the majority of units (18/20) the expiratory valve was not cleaned between patients. Both studies confirm the need for proper decontamination guidelines, including methods of initial cleaning and sterilisation, and the use of a microbiological filter between the mouthpiece or facemask and the control valves and tubing of the system.
Nitrous oxide and oxygen (Entonox) is an excellent method of providing short-term pain relief and relaxation during a number of painful procedures. There are, however, a number of reasons why such a valuable asset has not been made more readily available to nurses and their patients. These may include a lack of awareness and confidence on the part of the nurse about the pharmacological effects of Entonox, misconceptions about the 'extended role' system for nursing task authorisation, and local policy regarding its prescription by a doctor , . In the UK, Entonox is not classed as a Prescription Only Medicine (POM) but is designated as a Pharmacy (P) product. This means that it does not require a doctor's prescription, but must be issued by a pharmacist, who may be unwilling to do so without confirmation from a doctor. Local policies should, therefore, be designed to meet patient needs and provide a pathway for trained staff to have appropriate access to both the gas cylinders and the demand-valve breathing apparatus. This can be developed around training and operational protocols involving medical, pharmacy and nursing staff and other professionals such as physiotherapists who encounter many situations in which the gas would be of benefit.
Entonox gas is an established and safe short-term analgesic. It is ideal for patients undergoing wound dressing procedures where an element of both analgesia and distraction may be of benefit. Its historic use by midwives and paramedics and use in investigative procedures does not preclude its adoption by wound care nurses. The motivation to develop Entonox services must come from the nurses themselves, as it has, perhaps, rarely been seen as a priority by the healthcare management. This lack of access, coupled with a lack of organised training to support its development, has resulted in the under-utilisation of this valuable resource. There need be no great mystique about Entonox but it must be introduced alongside clear department policies regarding its use and training requirements of staff. Box 1 summarises the key elements of a typical operational protocol for a wound dressing change.
1. Fairley P. The Conquest of Pain. London: Michael Joseph, 1978 (Chapter 7).
2. Spencer EM. Inhaled sedative agents. Current Opinion in Critical Care 1999; 5(4): 257-62.
3. Harding TA, Gibson JA. The use of inhaled nitrous oxide for flexible sigmoidoscopy: a placebo controlled trial. Endoscopy 2000; 32(6): 457-60.
4. British Oxygen Company. Data Sheet April 1992 (Revision 2). Manchester, UK: BOC Gases, 1992.
5. Eger EI, Saidman LJ. Hazards of nitrous oxide anesthesia in bowel obstruction and pneumothorax. Anesthesiology 1965; 26(1): 61-66.
6. British Oxygen Company. Entonox Protocol Template Available from: BOC Customer Service Centre, Manchester, UK, .
7. Pal SK, Cortiella J, Herndon D. Adjunctive methods of pain control in burns. Burns 1997; 23(5): 404-12.
8. Forbes GM, Collins BJ, Foster N. Nitrous oxide for colonoscopy: a randomised controlled study. J Gastroenterol Hepatol 1998; 13(Supplement A178).
9. Martin JP, Sexton BF, Saunders BP, Atkin WS. Inhaled patient-administered nitrous oxide/oxygen mixture doesnot impair driving ability when used as analgesia during screening flexible sigmoidoscopy. Gastronintestinal Endoscopy 2000; 51(6): 701-03.
10. Peate I, Lancaster J. Safe use of medical gases in the clinical setting: practical tips. Br J Nursing 2000; 9(4): 231-36.
11. Association of Anaesthetists of Great Britain and Ireland. . A report received by council of the Association of Anaesthetists on Blood Bourne Viruses and Anaesthesia. 1996 http://www.aagbi.org/pdf/20.doc.pdf.
12. Bajekal RR, Turner R, Yentis SM. Anti-infective measures and Entonox equipment. A survey. Anaesthesia 2000; 55(2): 153-54.
13. Chapman G, Clarke TNS. Infection control of breathing circuitry in pre-hospital care. Pre-hospital Immediate Care 1999; 3: 84-88.
14. Chilvers RJ, Weisz M. Entonox equipment as a potential source of cross infection. Anaesthesia 2000; 55(2): 176-79.
15. Sealey L. Nurse administration of Entonox to manage pain in ward settings. Nursing Times 2002; 98(46): 28-29.
16. Lawler K. Entonox: too useful to be limited to childbirth? Professional Care of Mother and Child 1995; 5(1): 19-21.