What is Gas and Air?
Gas and air is the common term for a medical gas mixture trademarked as Entonox® [1.2.1, 1.2.2]. This colorless, odorless gas consists of a 50/50 mix of nitrous oxide (N₂O) and oxygen (O₂) [1.2.2, 1.8.1]. It is classified as a prescription-only medication and is listed on the World Health Organization's List of Essential Medicines [1.2.2]. Its primary function is to provide analgesia (pain relief) and anxiolysis (anxiety reduction) for short-term, painful procedures [1.2.4].
Its use became popular in the 1950s and it is now a staple in settings like maternity wards, emergency departments, and ambulance services [1.2.4, 1.2.5]. The administration is unique in that it is self-administered by the patient through a demand valve with a mouthpiece or mask. The gas only flows when the patient inhales, giving them complete control over their dosage. This inherent safety feature prevents overdose, as the patient will be unable to hold the mask if they become too drowsy [1.8.1, 1.8.4].
Pharmacological Profile: How It Works
From a pharmacological standpoint, gas and air is unequivocally a medication. The active component, nitrous oxide, is classified as an analgesic, a general anesthetic, and an NMDA receptor antagonist [1.2.2].
The precise mechanism of action is not fully understood, but it is believed that nitrous oxide induces the release of endogenous opioid peptides in the brain stem [1.4.1, 1.6.3]. These are the body's own natural pain-relieving chemicals, similar in effect to morphine. Studies have reported that a 30% concentration of nitrous oxide can be equivalent to 10-15mg of morphine in analgesic effect [1.2.4].
Key pharmacokinetic properties include:
- Rapid Onset: The effects begin within 20 to 30 seconds of inhalation, with peak effect after 3 to 5 minutes [1.2.2, 1.2.4].
- Short Duration: Once inhalation stops, the effects wear off very quickly, typically within a minute, as the gas is rapidly exhaled and is not metabolized by the body [1.2.2, 1.8.1].
- High Solubility: Nitrous oxide is highly soluble in the blood, allowing it to be quickly diffused across cellular membranes and eliminated unchanged via the lungs [1.2.4].
These properties make it an ideal choice for the sharp, intermittent pain of labor contractions and other short medical procedures [1.2.4, 1.8.5].
Clinical Applications and Efficacy
While famously associated with childbirth, Entonox is effectively used across many medical specialties [1.2.4].
- Obstetrics: It is the most common labor analgesic in many countries, used by over 50% of birthing people in the UK, Australia, and Finland [1.6.3]. It does not eliminate pain entirely but helps to 'take the edge off' and allows the patient to remain in control [1.5.1, 1.9.5].
- Emergency Medicine: It is used for acute pain from trauma, such as fractures, and for painful procedures in the emergency department [1.2.4, 1.7.2].
- Dentistry and Urology: It's an effective anxiolytic and analgesic for procedures like flexible cystoscopy, prostate biopsies, and pediatric dental work [1.2.4].
- Endoscopy: It is offered for bowel tests like colonoscopy to help with pain and anxiety, with patients recovering psychomotor function more rapidly than with intravenous sedation [1.2.4, 1.2.5].
Side Effects and Contraindications
Though generally safe, gas and air does have potential side effects, which are typically transient and resolve quickly after use is stopped [1.2.5]. The most common are dizziness, light-headedness, nausea, and a tingling sensation [1.5.2, 1.8.1]. Prolonged use (over 24 hours) is not recommended as it can inactivate vitamin B12, potentially leading to bone marrow depression [1.2.4, 1.5.1].
There are specific situations where gas and air is contraindicated due to the risk of nitrous oxide expanding within air-filled body cavities [1.2.4, 1.8.1]. These include:
- Pneumothorax (collapsed lung)
- Bowel obstruction
- Recent eye or ear surgery involving gas injections
- Head injuries with elevated intracranial pressure
- Decompression sickness (the bends) [1.8.1].
It should also be used with caution in patients with severe vitamin B12 deficiency [1.5.1].
Comparison with Other Analgesics
In the context of labor pain, gas and air offers a middle ground between non-medical options and more invasive methods like an epidural.
| Feature | Gas and Air (Entonox) | Opioids (e.g., Pethidine) | Epidural Block |
|---|---|---|---|
| Mechanism | Inhaled gas, NMDA receptor antagonist [1.2.2] | Injected opioid analgesic [1.8.5] | Local anesthetic injected near the spinal cord [1.9.1] |
| Pain Relief | Moderate; reduces awareness of pain [1.6.1, 1.7.5] | Moderate relief [1.8.5] | Excellent; blocks most pain from the waist down [1.6.1] |
| Onset | Very fast (15-30 seconds) [1.2.2, 1.5.1] | ~20 minutes [1.8.5] | 10-20 minutes [1.9.1] |
| Patient Control | High; self-administered [1.5.1] | Low; administered by clinician [1.8.5] | Low; administered by anesthesiologist [1.9.1] |
| Mobility | Full mobility maintained [1.6.2] | May cause drowsiness [1.8.5] | Confined to bed; loss of sensation in legs [1.6.2, 1.9.1] |
| Effect on Baby | No known significant side effects [1.5.1, 1.5.3] | Can cause drowsiness and breathing issues in the baby if given close to birth [1.9.2] | Minimal drug transfer to baby [1.9.2] |
| Recovery | Almost immediate (<1 minute) [1.2.2] | Lasts 2-4 hours [1.8.5] | Can take hours to wear off [1.6.5] |
Conclusion
Gas and air is definitively classified as a medication. It is an inhaled analgesic drug with a specific pharmacological profile centered on the effects of its active ingredient, nitrous oxide [1.2.2, 1.4.1]. Its rapid onset, fast recovery, and patient-controlled administration make it a valuable, safe, and effective tool for managing pain in a variety of clinical settings, from childbirth to emergency procedures [1.2.4]. While it doesn't eliminate pain to the extent of an epidural, its favorable safety profile and the sense of control it provides to patients ensure its continued importance in modern medicine [1.5.1, 1.6.2].
For more information, you can consult the NHS page on pain relief in labour.
