Philip Hopkins, MB BS MD FRCA, Professor of Anaesthesia, University of Leeds, UK
Abstract
Early attempts to use nitrous oxide as a sole anaesthetic foundered because of its low potency. It has been used successfully as an adjunct to more potent anaesthetics, however, since 1868. By enabling reduced doses of more potent anaesthetics, nitrous oxide reduces the cost of anaesthesia and limits cardiorespiratory side-effects Nitrous oxide does increase the incidence of postoperative nausea and vomiting in cases where risk of this side-effect is increased, but it seems likely that use of antiemetics prophylactically will negate this factor.
Perhaps the greatest argument for the continued use of nitrous oxide is that it reduces the incidence of recall of intra-operative awareness. Reduced pharmacokinetic variability compared with other anaesthetics, especially intravenous agents, is likely to be a most important reason for this, although evidence is emerging that nitrous oxide also has pharmacodynamic advantages.
There are specific situations in which nitrous oxide should not be used, but in the absence of these, its use can be favourably recommended.
Key words
Inhalational anaesthetics, nitrous oxide; Nitrous oxide, side-effects; general anaesthesia, complications, awareness; Nitrous oxide, pharmacokinetics, pharmacodynamics.
Clinical Practice points
- Nitrous oxide reduces dose requirements of potent anaesthetic agents, limiting cardiorespiratory side-effects of the latter.
- Nitrous oxide reduces the likelihood of awareness
- Nitrous oxide increases the risk of postoperative nausea and vomiting only in patients at high risk: this added risk of nitrous oxide can probably be eliminated by the use of antiemetics prophylactically.
- Nitrous oxide should not be used in situations where expansion of air-filled spaces may be problematic
- Nitrous oxide toxicity can occur in individuals who are vitamin B12 deficient and those exposed to nitrous oxide for long periods, or repeatedly within a short period of time.
Research agenda
- Further exploration of effects of different anaesthetics on memory processes and the interaction with noxious stimulation.
- Effectiveness of prophylactic antiemetics in preventing nitrous oxide induced nausea and vomiting.
Nitrous oxide has an important place in the history of anaesthesia. Its analgesic properties were recognised in the first decade of the nineteenth century by Humphrey Davy. Despite the lack of success of Horace Wells in his attempted demonstration of anaesthesia using nitrous oxide in 1844, the drug became popular with dentists in the United States and later in Europe.
The pure gas (produced on the premises by heating ammonium nitrate) was breathed, unconsciousness ensued rapidly and teeth could be extracted painlessly. It was quite reasonable, considering this technique, that a prevailing view emerged that the effects of nitrous oxide were a result of asphyxia. This view was challenged, however, by Bert, a French physiologist who demonstrated the anaesthetic properties of 50% nitrous oxide in oxygen administered in a high-pressure chamber.
In 1868 nitrous oxide became available compressed to a liquid in metal cylinders and this practical availability soon led to its widespread adoption as an adjunct to ether anaesthesia. The prior administration of nitrous oxide was found to smooth the inhalational induction of ether anaesthesia, to reduce the requirements of ether for maintenance of anaesthesia and subsequently to produce a more rapid recovery.
In the ensuing years several generations of anaesthetists have been trained into the habit of using nitrous oxide as a consistent “back-drop” to general anaesthesia. It is timely, in this era of balanced anaesthesia with designer anaesthetic drugs, to consider whether nitrous oxide still has a role.
Of course, we need to be clear about precisely what question is being addressed. We do not need to consider if anaesthetic practice in the developed world would collapse if nitrous oxide became unavailable. This is clearly nonsense, just as it would be nonsense if the drug under question was propofol, sevoflurane, remifentanil or even a combination of these three popular newer drugs. It is important, however, that we consider:
- Does nitrous oxide have any important clinical advantages that cannot be matched by another drug, or combination of drugs, in a cost-effective way?
- Do any clinical advantages of nitrous oxide outweigh its unwanted effects in a sufficient number of clinical situations?
If the answer to both of these questions is “yes” then I would argue that nitrous oxide is indeed important for anaesthesia.
Benefits of the use of nitrous oxide
With a MAC50 value of approximately 104%, nitrous oxide is not suitable for use as a sole anaesthetic agent. In lower concentrations it is an effective analgesic and is used as such, for example, for trauma and obstetric analgesia. In anaesthesia, per se, however, it is used principally in order to reduce the amount needed of more potent anaesthetics. When a potent inhalation anaesthetic drug is used for maintenance of anaesthesia it is generally accepted that the reduction in required end-tidal concentration of potent agent is equal (as a proportion of its MAC value) to the MAC of nitrous oxide delivered 1.
The relationship is not so well defined for the propofol-sparing effect of nitrous oxide, but administration of 65% nitrous oxide has been estimated to reduce propofol requirements by 30-50%. The potential advantages in using nitrous oxide to reduce amounts of more potent anaesthetics used include less cardiovascular and respiratory depression, less likelihood of problems from drug interactions involving the more potent drugs, and cost savings.
Indeed, two large trials 2,3 demonstrated substantial drug cost savings without additional costs from increased side-effects when anaesthetic techniques incorporating nitrous oxide were compared to nitrous oxide-free techniques.
Reduced cardiovascular and respiratory depression
All of the commonly used potent maintenance anaesthetics cause cardiovascular depression. This becomes especially important clinically for the potent inhalational anaesthetics when they are administered in concentrations exceeding one MAC. Propofol induces hypotension throughout the concentration range used for maintenance anaesthesia. Combination of 65% nitrous oxide with isoflurane or sevoflurane produces less hypotension than equi-MAC concentrations of either potent agent alone 4. Similarly, 70% nitrous oxide added to increasing target concentrations of propofol had no effect on blood pressure until the target concentration of propofol exceeded 5 mg/mL 5.
The haemodynamic benefits of reducing the amounts of propofol or potent inhalation agent used are likely to be even more significant in patients with reduced cardiovascular reserve because of age, pathology or medication.
For example, the hypotensive effects of propofol and the potent inhalation anaesthetics are compounded in patients taking calcium channel blocking drugs because of an interaction with myocardial and peripheral vessel smooth muscle calcium channels. Nitrous oxide does not affect these calcium channels and has been shown not to interact with calcium channel blocking drugs 6.
The effect on respiratory function of adding nitrous oxide to the more potent anaesthetics has not been as well studied as the effect on cardiovascular function but it seems likely that any reduction in the required concentration of a more potent drug by the addition of nitrous oxide will lead to a reduction in the respiratory depression caused by the former. Indeed, decreased respiratory depression during emergence from anaesthesia was observed with a nitrous oxide/sevoflurane combination compared to sevoflurane alone 7.
Awareness
Intuitively, one would envisage that as long as you give enough anaesthetic, be it as a combination of drugs or a single potent agent, the choice of drug(s) should have no impact on the incidence of awareness. This assumes, however, that the pharmacokinetic variability dictating the accuracy of the estimate of blood concentration of the different drugs and their pharmacodynamic profiles are similar. The fact that nitrous oxide may have such pharmacokinetic and/or pharmacodynamic benefits is supported by the results of a meta-analysis of awareness from randomised controlled trials 8.
The number-needed-to-treat (NNT) with nitrous oxide to prevent one case of awareness was 46, a staggeringly low number. To put this into context, consider the results of the recently reported large randomised trial of BISTM monitoring to prevent awareness 9. This study found an approximately 5-fold reduction in the incidence of awareness when BISTM monitoring was used. However, assuming the overall incidence of awareness in unselected patients is 0.1%, the NNT with BISTM to prevent one case of awareness is 1,250.
If we assume the consumable and depreciation costs of BISTM monitoring is €20 per patient 10, the cost of BISTM monitoring to prevent awareness is €25,000 per case prevented. The use of nitrous oxide, in contrast, shows greater efficacy and actually reduces the cost of anaesthesia!
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Figure 1
Pharmacokinetic variability of nitrous oxide compared with isoflurane and target-controlled propofol. The ordinate is the blood concentration at equilibrium expressed as a fraction of the delivered (nitrous oxide and isoflurane) or target (propofol) concentration. Nitrous oxide produces the most predictable blood concentration. |
Pharmacokinetic advantages underlying reduced awareness
If kinetic benefits contribute to this advantage of nitrous oxide, it is a combination of the accuracy and precision of the estimate of blood concentration that is important. For nitrous oxide, as with the potent inhalation anaesthetics, our best estimate of the blood concentration is obtained from the end-expired concentration. When we use a propofol infusion for maintenance, the blood concentration can be estimated if a target-controlled device is used.
After a suitable equilibration period, using a DiprifusorTM system for example, the actual blood concentration of propofol will be ±20% of the target concentration in 95% of patients. For isoflurane, after 15 minutes of stable end-expired concentration, the arterial concentration may be 35 % less than the end-expired 11. After nine minutes of breathing nitrous oxide, the blood concentration is under 30% less than inspired concentration and reaches 90% of inspired concentration after 15 minutes 12.
The major advantage, however, with inhalational techniques over propofol target-controlled maintenance is that the divergence of the blood concentration of the former from the end-expired concentration is so much more predictable than the divergence of the blood propofol concentration from the target concentration. This is because the factors responsible for the difference between blood and end-expired concentrations (shunt, deadspace, failure of equilibration between alveolar gas and pulmonary end-capillary blood) are uni-directional and show less inter-individual variability compared with the pharmacokinetic parameters used in estimates of blood propofol concentration.
For example, if a target concentration of 4 mg/mL of propofol is set, the 95% CI for the blood concentration will be 3.2 – 4.8 mg/mL. Compare this to the administration of isoflurane at end-tidal concentration of 1.5%: in this case the 95% CI for the blood concentration will be 0.93 – 1.05% 10, a much narrower range. For nitrous oxide the advantage is even greater: for example, with an inspired concentration of 67% the 95% CI for the blood concentration is 60 – 63% 11. I believe this consistency between individuals (illustrated in Figure 1) is a major factor in the reduced likelihood of awareness when nitrous oxide is used and, albeit to a lesser extent, why awareness is more likely when propofol is used for maintenance compared with potent inhalational anaesthetics.
Pharmacodynamic advantages underlying reduced awareness
The pharmacodynamic benefits of nitrous oxide contributing to reduced recall of awareness are less tangible. Like the potent inhalational agents and propofol, nitrous oxide is more potent as a suppressor of memory than as an anaesthetic. This makes it extremely curious as to why any patient should recall being aware as it implies a large margin of safety. To a large extent, however, the comparative effects on memory of the different anaesthetics have been studied in either sedated volunteers, anaesthetised patients prior to surgery or patients emerging from anaesthesia after relatively minor surgical procedures.
These contexts ignore the potentially vital interaction between noxious stimuli, awareness and recall. It seems entirely plausible that the amnesic effects of a drug are less likely to be affected by noxious stimuli if the drug also has analgesic efficacy with a similar dose-response profile to the amnesic effect. This would certainly give nitrous oxide an advantage over propofol and possibly also over the potent inhalational agents.
Our understanding of the potential mechanisms of awareness may also be confused by a failure of many studies to distinguish between effects on learning and effects on memory. A recently published animal study by Alkire and Gorski 13 demonstrates that inhalational anaesthetics do indeed have differential dose-dependent effects on learning and memory. Interestingly, nitrous oxide was found to have a negligible effect on learning at sub-anaesthetic doses, but it had the most potent (on a relative MAC basis) effect on memory of a noxious stimulus compared with isoflurane, sevoflurane, desflurane and halothane.
Indeed the ED50 inspired concentration for the amnesic effect of nitrous oxide was 14% 13. Figure 2 illustrates the dose-response curves for the five inhalation anaesthetics.
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Figure 2
Dose–response curves for amnesic potential plotted on a common logarithmic scale of relative minimum alveolar concentration (MAC) values. The data show that memory retention is most potently inhibited by nitrous oxide and least potently inhibited by halothane. Desflurane was significantly more potent only than halothane. The slopes of the curve fits for nitrous oxide and desflurane were significantly lower than those for sevoflurane, isoflurane, and halothane, but the nitrous oxide and desflurane slopes did not differ from each other. The slopes also did not differ among sevoflurane, isoflurane, and halothane. From: Alkire: Anesthesiology, Volume 101(2).August 2004.417-429 |
Interpretation of Alkire and Gorski’s results must be tempered by the usual cautions when considering the clinical relevance of animal studies. They do, however, offer a potential explanation for why human studies (e.g 14) in which amnesia is tested as a composite of learning and memory (in the absence of noxious stimuli), have failed to demonstrate nitrous oxide to be a superior amnesic to other anaesthetics. Furthermore, Alkire and Gorski have provided evidential continuity between the systematic review evidence 8 that nitrous oxide reduces awareness and the demonstrated importance of glutamatergic processes to memory 15.
Potential adverse effects of nitrous oxide
Expansion of air-filled spaces
Nitrous oxide has the potential to expand or increase the pressure within any air-filled space in the body. It should not be used, therefore, in patients undergoing middle ear surgery, in patients where air may be present under abnormal conditions (pathological or iatrogenic), for example in the eye, within the skull, in a pneumothorax or pneumopericardium or as an air embolus. The ability of nitrous oxide to cause expansion within the gut depends on the proportion of bowel gas that is air. Some workers have reported that nitrous oxide delays bowel function after colonic surgery 16 but larger studies have found no such effect 17,18.
Postoperative nausea and vomiting
A meta-analysis of the influence of nitrous oxide on postoperative nausea and vomiting (PONV) reported an overall small but significant reduction in the severity of PONV when nitrous oxide was not used 8. This overall result reflected a greater effect (NNT = 5) in patients at high risk of PONV but no significant effect of omitting nitrous oxide in patients not at high risk of PONV. However, it is now common practice for prophylactic antiemetics to be used in patients at high risk of PONV. For this purpose either dexamethasone 19 or ondansetron 20 is more effective than nitrous oxide while the combination of dexamethasone and 5-HT3 receptor anatagonist is better still 19.
As omitting nitrous oxide does not improve PONV outcomes when the baseline incidence is low 8, it would seem unlikely that omitting nitrous oxide will have any additional effect when effective prophylactic drugs are to be used.
Toxicity
Nitrous oxide inactivates vitamin B12, thereby inhibiting the conversion of homocysteine to methionine by methionine synthetase and thereby inhibiting folate metabolism: potential toxic effects are megaloblastic anaemia, agranulocytosis and acute demyelination polyneuropathy. In healthy individuals, significant depression of methionine synthesis requires prolonged exposure (~24 hr) to clinically used concentrations. Shorter periods of exposure may be harmful in patients with vitamin B12 deficiency and this should be excluded positively, for example, if prolonged nitrous oxide administration is contemplated in elderly patients.
There is also a potential interaction with methotrexate, a folate antagonist immunosuppressant that is commonly used in the treatment of rheumatoid arthritis: it is interesting, however, that nitrous oxide has no added effect on methionine synthetase activity in the presence of methotrexate 21.
More recent work has focussed on increased plasma homocysteine concentrations after nitrous oxide use. These have been correlated with postoperative myocardial ischaemia in high-risk individuals 22, but can be prevented by vitamin B12 supplements prior to surgery 23. Interestingly, other workers who have sought carefully to achieve equivalent depths of anaesthesia when comparing nitrous oxide-free with nitrous oxide-inclusive techniques have found a trend favouring fewer myocardial ischaemic events when nitrous oxide is used 24.
Environmental considerations
Nitrous oxide is an important environmental pollutant. In the troposphere it acts as a greenhouse gas while it destroys ozone in the stratosphere. However, all medical uses of nitrous oxide combined constitute less than 2% of nitrous oxide pollution, the majority arising from agriculture and the burning of fossil fuels 25. Indeed, the effects of all inhalational anaesthetics contribute only 0.0005% of ozone destruction.
Are there better alternative supplements to general anaesthesia
Benzodiazepines, opioids and a2 adrenoceptor agonists are all used as supplements to general anaesthesia and each might be considered likely to reduce the incidence of intraoperative awareness. From the above discussion, a combination of amnesic and analgesic properties seems to be necessary to the efficacy of nitrous oxide. Benzodiazepines are good amnesic drugs but not analgesic. Opioids are potent analgesics but relatively poor amnesics.
The a2 adrenoceptor agonists are relatively weak amnesics and analgesics. A combination of benzodiazepine and opioid, eg, midazolam and remifentanil, might be considered a suitable alternative to nitrous oxide. However, the cardiovascular depression caused by remifentanil is more marked than that of nitrous oxide. All of these alternatives, of course, are administered intravenously, and subject to the pharmacokinetic variability which seems to underlie, at least in part, the potential for awareness with propofol anaesthesia.
Xenon, on the other hand, is an inhalational anaesthetic with profound analgesic properties and is even more inert on the cardiovascular system than nitrous oxide 26. Its effect on memory has not been as well characterised but it is probably the only realistic potential replacement for nitrous oxide. Its utility is severely diminished, however, by its cost – current estimates are that, even with an appropriate delivery system, xenon would add €100-350 to the drug cost of each case. At these rates it is unlikely to be marketed for general use.
Conclusion
Nitrous oxide is an unique drug. Its pharamcodynamic profile makes it the best available supplement to minimise the use of more potent anaesthetics, thereby reducing the cost and side-effects of general anaesthesia.
This combines with its predictable pharmacokinetic characteristics and possible advantages as an amnesic to noxious stimuli, to make it a most important factor in prevention of intraoperative awareness, a potentially devastating complication of anaesthesia. As with any drug, there are specific contraindications to the use of nitrous oxide but in the absence of these it is difficult to justify not using it.
References
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- Arellano RJ, Pole ML, Rafuse SE, Fletcher M, Saad YG, Friedlander M, Norris A, Chung FF. Omission of nitrous oxide from a propofol- based anesthetic does not affect the recovery of women undergoing outpatient gynaecologic surgery. Anesthesiology. 2000 Aug;93(2):332-9.
- Visser K, Hassink E~A, Bonsel GJ, Moen J, Kalkman CJ. Randomized controlled trial of total intravenous anesthesia with propofol versus inhalation anesthesia with isoflurane-nitrous oxide: postoperative nausea with vomiting and economic analysis. Anesthesiology. 2001 Sep;95(3):616-26.
- Inada T, Inada K, Kawachi S, Takubo K, Tai M, Yasugi H. Haemodynamic comparison of sevoflurane and isoflurane anaesthesia in surgical patients. Can J Anaesth. 1997 Feb;44(2): 140-5.
- Shiga T, Wajima Z, Inoue T, Ogawa R. Nitrous oxide produces minimal hemodynamic changes in patients receiving a propofol- based anesthetic: an esophageal Doppler ultrasound study. Can J Anaesth. 2003 Aug-Sep;50(7):649-52.
- Ramsay JG, Arvieux CC, Diedericks J, Foex P. Verapamil and nitrous oxide exert only minor effects on the normal myocardium in the presence of fentanyl. Eur J Anaesthesiol. 1991 May;8(3):203-12.
- Einarsson S, Bengtsson A, Stenqvist O, Bengtson JP. Decreased respiratory depression during emergence from anesthesia with sevoflurane/N20 than with sevoflurane alone. Can J Anaesth. 1999 Apr;46(4):335-41.
- Tramer M, Moore A, McQuay H. Omitting nitrous oxide in general anaesthesia: meta-analysis of intraoperative awareness and postoperative emesis in randomized controlled trials. Br J Anaesth. 1996 Feb;76(2):186-93.
- Myles PS, Leslie K, Forbes A, Chan M, McNeil J. A Large Randomized Trial of BIS® Monitoring To Prevent Awareness in High Risk Patients: The B-Aware Trial. ASA Annual Meeting Abstracts. Anesthesiology 2003;99:A320.
- Abenstein JP, Narr BJ, Rose SH. Is Reduction of Intraoperative Awareness with BIS® Monitoring Cost-Effective? ASA Annual Meeting Abstracts. Anesthesiology 2002;96:A548.
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- Alkire MT, Gorski LA. Relative amnesic potency of five inhalational anesthetics follows the Meyer-Overton rule. Anesthesiology 2004; 101:417–29
- Dwyer R, Bennett HL, Eger EI II, Heilbron D: Effects of isoflurane and nitrous oxide in subanesthetic concentrations on memory and responsiveness in volunteers. Anesthesiology 1992; 77:888–98
- Riedel G, Platt B, Micheau J: Glutamate receptor function in learning and memory. Behav Brain Res 2003; 140:1–47
- Scheinin B, Lindgren L, Scheinin TM. Perioperative nitrous oxide delays bowel function after colonic surgery. Br J Anaesth. 1990 Feb;64(2):154-8.
- Krogh B, Jensen JP, Henneberg SW, Hole P, Kronborg O. Nitrous oxide does not influence operating conditions or postoperative course in colonic surgery. Br J Anaesth. 1994 Jan;72(1):55-7.
- Jensen AG, Kalman SH, Nystrom PO, Eintrei C. Anaesthetic technique does not influence postoperative bowel function: a comparison of propofol, nitrous oxide and isoflurane. Can J Anaesth. 1992 Nov;39(9):938-43.
- Henzi I, Walder B, Tramer MR. Dexamethasone for the prevention of postoperative nausea and vomiting: a quantitative systematic review. Anesth Analg. 2000 Jan;90(1):186-94.
- Tramer MR, Reynolds DJ, Moore RA, McQuay HJ. Efficacy, dose- response, and safety of ondansetron in prevention of postoperative nausea and vomiting: a quantitative systematic review of randomized placebo-controlled trials. Anesthesiology. 1997 Decf;87(6):1277-89.
- Fiskerstrand T, Ueland PM, Refsum H. Folate depletion induce by methotrexate affects methionine synthase activity and its susceptibility to inactivation by nitrous oxide. J Pharmacol Exp Ther. 1997 Sep;282(3):1305-11.
- Badner NH, Beattie WS, Freeman D, Spence JD. Nitrous oxide- induced increased homocysteine concentrations are associated with increased postoperative myocardial ischemia in patients undergoing carotid endarterectomy. Anesth Analg. 2000 Nov;91(5):1073-9.
- Badner NH, Freeman D, Spence JD. Preoperative oral B vitamins prevent nitrous oxide-induced postoperative plasma homocysteine increases. Anesth Analg. 2001 Dec;93(6):1507-10.
- Kozmary SV, Lampe GH, Benefiel D, Cahalan MK, Wauk LZ, Whitendale P, Schiller NB, Eger EI 2nd. No finding of increased myocardial ischemia during or after carotid endarterectomy under anesthesia with nitrous oxide. Anesth Analg. 1990 Dec;71(6):591-6.
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Address for correspondence:
Prof PM Hopkins
Academic Unit of Anaesthesia
St James’s University Hospital
LEEDS
LS9 7TF
United Kingdom
Tel: +44 (0) 113 2065274
Fax +44 (0) 113 2064140
Email: p.m.hopkins@leeds.ac.uk
Reprinted from Best Practice & Research: Clinical Anaesthesiology, Vol 19, Hopkins PM, Nitrous oxide: a unique drug of continuing importance for anaesthesia, pg 381-9, ©2005, with permission from Elsevier.
Issue: June 2008
The increasing cost of the series of volatile anaesthetic agents developed during and since the 1950s led to the development of techniques to allow re-breathing of exhaled gas in a controlled fashion, within anaesthetic breathing systems, and consequently, to conserve volatile agent. This change was facilitated by the introduction of routine monitoring of carbon dioxide and oxygen concentrations. The resurgence in the use of breathing systems incorporating ‘soda-lime’, consisting mainly of the alkali calcium hydroxide for use as a carbon dioxide absorbent, has the added benefit of increasing the humidity of the gas within the breathing system, via its chemical reaction with carbon dioxide. However, a warm, moist environment offers the potential for the proliferation of pathogenic organisms. This contrasts with the relatively arid environment within a semi-closed breathing system, combined with the allegedly bacteriostatic effect of black rubber.
The recent development of non-caustic absorbents removes a further potential barrier to the growth of pathogens. Recognition of this risk led firstly, to the introduction of filters to reduce the potential for entry of pathogens into the anaesthetic breathing system, and secondly, to the introduction of systems to ensure the timely replacement of contaminated breathing system components. Reports of the possible transmission of hepatitis via breathing system components led to a systematic review of practices and an appraisal of the optimal use of all parts of the anaesthetic breathing system, including associated devices, such as laryngoscopes1. This coincided with concerns about possible contamination with ‘prion’ agent, which was found to be very resistant to removal by conventional decontamination procedures. A more consistent risk-based approach was developed, with those components intended to come into direct contact with patient fluids or mucous membranes either undergoing a formal decontamination process or being treated as ‘single-patient’ use. The re-evaluation of the potential of silver as a bacteriocide has led to the development and incorporation of silver-containing compounds into a range of materials, including anaesthetic breathing systems and underwear, promoted in part by the political and epidemiological rise of MRSA.
The concept of protecting the breathing system by means of a filter was questioned both on economic and safety grounds and new, standardised test methods of filtration in this environment were developed. Breathing system filters have a difficult task to perform, since not only do they need to filter air flowing in both directions but also they must be able to cope with water droplets (and sometimes larger amounts of fluid) contacting both sides of the filter: The potential for these two sources of liquid to come into contact led to the adoption of hydrophobic materials in the construction of filtration medium by some manufacturers.
Breathing system filters fall into two categories: the membrane type, which have a screen membrane, with a defined pore size, of material often with hydrophobic properties, which is usually pleated to allow an increased surface to be exposed to the gas, to lower resistance to flow, whilst restricting the filter ‘dead-space’. The second type is variously called electrostatic, depth or impaction filter, and consists of a compacted bundle of fibres with varying sizes of intertices. They work as filters by attracting particles onto fibres with an electrostatic charge and also by causing particles to impact onto, or be trapped between adjacent fibres. This type of filter is generally more efficient at heat and moisture exchange, but cope less-well with a large volume fluid challenge. However, since the membrane type can become a complete barrier to the passage of fluid, there is a possibility of barotrauma to a patient’s lungs, if a large amount of fluid secretions flood a hydrophobic membrane filter, preventing release of gas pressure. Various hybrids have been developed to combine the advantages of both types of filter.
The potential massive rise in cost to UK hospitals faced with a full-scale adoption of single-patient use breathing systems led to the development of a compromise position. Judicious use of filters to protect the breathing system, with the filter changed between patients, and backed up with regular checks of the breathing system integrity, was deemed to be a safe way of prolonging the usable life of the breathing system to as long as one week. However, the reduction in price of breathing system filters to below the cost of the breathing system has compromised the economic argument for re-use. However, the replacement of a filter between cases generates far less bulk in terms of contaminated waste, than changing the breathing tubing and still remains a practical option.
Since there is now a requirement for continuous gas monitoring in breathing systems used in anaesthetics, manufacturers are being encouraged to incorporate gas-sampling lines either coaxially within, or attached to the breathing system tubing. The principal advantage of such a move would be to remove the need for separate tubing connection to be made, and reduce the dis/misconnection risks. It would also ensure that these lines were replaced regularly, reducing blocking and infection risks.
A recent report of a misconnection of a luer connection on a sampling port3 on a capnograph to and intravenous catheter has highlighted the work being undertaken by an international technical task force, hosted by CEN, the European Standards organisation, to completely re-think the design of small-bore connectors used for conveying liquids and gases in healthcare applications. This work was instigated because of a number of misconnection errors involving a wide range of devices fitted with ‘luer’ connectors4. Luer and luer-compatible ports are commonly used for ‘accessory’ tubing connections in breathing systems and have the potential to cause problems5. There is a move to make the body of breathing system connectors from rigid, non-elastomeric materials, to prevent the force fitting of incompatible components.
Inadvertent disconnection of breathing system components is still a problem for the user, despite the improvements in the sureness of fit brought about by the introduction of standardisation of the breathing system tapers6. There are at least two ‘locking’ variants of 22mm breathing system connector but unfortunately, neither are currently available in a single-use type format, and thus are not in widespread use in the UK.
The increased use of disposable medical devices has increased the bulk of medical waste significantly: disposal charges for contaminated waste have risen rapidly, and are set to rise further as fuel costs increase. It is to be hoped that this will speed the drive for the development and introduction of materials offering less-costly disposal methods.
References:
