Acla Anaesthesia1 .%and 1994: 38: 775-776 Binfinud tn Denmark ~ all rights resmed

Copykht Q Acla Anaeshiol &and 1994

Acta Anaesthesiologica Scandinavica ISSN 0001-5172

Nitrous oxide in pre-hospital care PETER J. F. BASKETT Consultant Anaesthetist, Frenchay Hospital, Bristol (UK)

Ky wards: Nitrous oxide; pre-hospital care.

Nitrous oxide was first introduced into pre-hospital care in the United Kingdom in 1970 (1, 2) to fill the analgesic gap in the management of patients suffering from trauma, myocardial infarction and other painful con- ditions. Hitherto, because of the strongest legal constric- tions applied to administration of the opiates, members of the ambulance service had been unable to provide any form of analgesia at all. As a consequence, pain re- lief was delayed until the patient reached the care of a physician - usually in hospital.

Nitrous oxide analgesia was able to be introduced be- cause of the development of a relatively stable mixture of nitrous oxide and oxygen in equal proportions in a single cylinder - known as Entonox. This gas mixture had been introduced initially in 1961 for the relief of pain in childbirth by Tunstall (3). In collaboration with scientists at the British Oxygen Company, he had found that a 50/50 mixture of nitrous oxide and oxygen com- pressed to 1890 lbs per square inch (135 Kg/sq in) would remain stable at temperatures above -5°C with- out any tendency towards liquidification of the nitrous oxide fraction. A few years later Parbrook & Kennedy (4) demonstrated the value of premixed nitrous oxide and oxygen in postoperative pain relief and Parbrook et al. (5) , also showed that the analgesic power of the mix- ture was comparable to 15 mg morphine sulphate given intramuscularly.

Alongside the development of the gas mixture came the design and manufacture of a suitable device for ad- ministration. The original Entonox apparatus designed by BOC worked on the demand principle and was de- signed for patient self-administration. A pin index system was featured as a safety mechanism so that the device could only be used with an Entonox gas mixture cylin- der. Since the introduction of the original inhalational unit, improvements in design have been made by BOC and by Pneupac Ltd., but the basic principles of the sys- tem are retained. The self-administration demand sys- tem is ideal for use in pre-hospital care with supervision by ambulance personnel. Should a patient with a par-

ticular sensitivity to the gas mixture become drowsy then hidher grip on the mask relaxes, the airtight seal at the mask/patient interface is lost and the gas flow ceases.

The features of Entonox self-administered with a de- mand apparatus may be listed in comparison with the ideal analgesic for pre-hospital care:

Effectiveness - The gas mixture is equivalent to at least 15 mg of morphine given intramuscularly (5). There is a rapid onset (2 minutes) and recovery occurs in a similar time after withdrawal. S d e p - There is no significant depression of respir- ation or the protective reflexes. There is little, if any, depression of the circulation when used for analgesia in the pre-hospital setting. There is no effect on the pupillary reaactions. There is no incompatibility with other drugs which might be included in the patients’ previous medication or used in pre-hospital care. Comfort - Neither nausea nor vomiting is a feature of Entonox used in pre-hospital care. Ease of administration - The self-administration tech- nique can be safely supervised by basic Emergency medical technicians and trained first aiders.

Nevertheless, there are some limitations to the safe use of nitrous oxide/oxygen mixtures in pre-hospital care. The contraindications include:

Patients with head injuries and reduced level of con-

Sedated patients with reduced levels of consciousness. Maxillofacial injuries preventing a mask/face airtight seal. (A mouthpiece may overcome this difficulty in some cases). Problems with gas-filled body spaces eg., pneumo- thorax, dysbarism and gastric or intestinal distension. These conditions may be aggrevated by the adminis- tration of nitrous oxide which rapidly enlarges the gas space in an attempt to find equilibrium between the partial pressures of the gas in the space and in the blood. Lack of patient co-operation - principally occurring in the very young and very old.

sciousness.

0 Acta AnaesthesioLo@ca Scandinauica 38 (1994)

776 PETER J. F. BASKETT

Ambient temperature ~ at temperatures below -5°C the nitrous oxide fraction in the pre-mixed gas begins to liquify and falls to the bottom of the cylinder (6). Continued inhalation will, initially, contain a higher and higher proportion of oxygen until that element is exhausted. After that increasingly pure nitrous oxide will be delivered, with the obvious risk of asphyxi- ation.

There have been some concerns expressed as to other possible deleterious effects of nitrous oxide administra- tion. These include the possibility of bone marrow de- pression with prolonged inhalation. However, this has not been reported as a clinical problem in inhalation duration of under 6 hours and, therefore, does not apply in virtually any case of pre-hospital care. Some have raised the depressant effect of nitrous oxide on cardiac output but this has not been reported in patients with pain in the pre-hospital phase and indeed nitrous oxide has been advocated in patients with myocardial infarc- tion for many years (7, 8, 9).

There remains a concern of risk to attendants working in an atmosphere polluted by nitrous oxide. In pre-hos- pital care practice this is generally not a problem but levels might potentially build up in the patient compart- ment of a poorly ventilated ambulance. If there is a con- sidered risk then the patient’s exhaled air can be diverted to the outside atmosphere using a simple scavenging sys- tem comparable to those installed in operating rooms.

Entonox has not been accepted in all countries of the world. It is primarily used in Britain and Australia and to some degree in South Africa and countries hitherto particularly related to Britain. In certain European Con- tinental countries strict laws categorically prevent non- physicians administering gas mixtures. In the United States there has been limited use of nitrous oxide in the pre-hospital sphere. The FDA decided at the outset that the mixture of two of the oldest gases in medical practice was to be declared a new agent and would require full proof of justification for clinical use.

As a result, in the mid 1970s the Fraser Harlake Com- pany in Buffalo, New York, developed the Nitronox ap- paratus for delivery of a 5O0/o nitrous oxide/oxygen mix- ture. The equipment consisted of two cylinders, one of pure nitrous oxide and one of pure oxygen with a fixed, fail safe, mixing valve located between the two cylinders. The apparatus also worked on a self-administration de- mand principle. Although over twice the weight and

bulk of the Entonox apparatus, the system carried the advantage that no perceived new agent was being intro- duced and perhaps, most importantly, there were no limitations by ambient temperatures to safe use. The Ni- tronox system has been used in pre-hospital care in sev- eral centres in the USA and favourable results have been reported (10, 11) similar to those emanating from the United Kingdom and Australia.

Entonox continues to be the mainstay of analgesia provided by the ambulance service in the United King- dom and it has been used without adverse report in tens of thousands of cases since its introduction into pre-hos- pital care 25 years’ ago. Its continued use seems assured at least until a better agent can be found which will offer greater efficacy with no reduction in safety or ease of administration and with no increase in side effects. Such a replacement is not visible at this time.

REFERENCES I . Baskett P J F, Withrell A. The use of entonox in the ambulance

2. Baskett P J F. The use of entonox in the ambulance service. Proc R

3. Tunstall M E. Obsteric analgesia. Lancet 1961: 2: 964. 4. Parbrook G D, Kennedy B R. Values of pre-mixed nitrous oxide

and oxygen mixtures in relief of post-operative pain. Br Med 3 1964: 2: 1303.

5. Parbrook G D, Rees G A D, Robertson G S. Relief of post-operat- ive pain ~ comparison of 25% nitrous oxide and oxygen mixture with morphine. Br Med3 1964: 2: 480.

6. Bracken A, Broughton G B, Hill D V. Safety precautions to be observed with cooled pre-mixed gases. Br MedJ 1968: 3: 715.

7. Petrovsky B V, Yefuni S N. Therapeutic inhalational anaesthesia. BrJAnaesth 1965: 37: 42.

8. Kerr F, Ewing D J, Irving J B, Kirby B J. Nitrous oxide analgesia in myocardial infarction. Lancet 1972: 8: 63.

9. Thompson P L, Lawn B. Nitrous oxide as an analgesic in acute myocardial infarction. 3 M 1976: 235: 924-927.

10. Flomenbaum N, Gallagher EJ, Eagen K, Jacobsen S. Self-adminis- tered nitrous oxide: An adjunct analgesic. 3 Am Coll Emergency Phys- i c ian~ 1979: 8: 3: 95-97.

I 1 . Stewart R D. Nitrous oxide sedatiodanalgesia in emergency medi- cine. Ann Emerg Med 1985: 14: 139-148.

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Soc Med 1972: 65: 7: 1.

Address: PeterJ. R Baskett, B.A., M.B., B.Ch., B.A.O., F.R.C.A., M.R.C.P. Consultant Anaesthetist Frenchay Hospital Bristol United Kingdom