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Since the advent of the laryngeal mask airway (LMA), anaesthetists have become used to the advantages of hands-free anaesthesia. Beforehand it was usual, when anaesthetizing spontaneously breathing patients, to maintain direct patient contact via the facemask used to deliver the anaesthetic gases. Trainees were taught the advantages of direct contact for continuous clinical assessment. By contrast, hands-free anaesthesia offered freedom of movement about the operating theatre, increased independence from assistants, and the opportunity to more readily monitor surgical activity. Over the same period, monitoring has become more sophisticated, but the cynic might suggest that the anaesthetist now needs to be hands-free to attend to monitor alarms. For anything other than minor cases, tracheal intubation was the order of the day as it provided the most secure airway and also mitigated against the risk of aspiration. Progressive exploration of the clinical applications of the LMA has gone on to challenge the need for intubation to protect the airway from aspiration.
In the last decade or so there has been a surge of interest in airway devices. While this may be seen to have been precipitated by the novelty of the LMA, this is not necessarily the whole explanation. Brain1 has acknowledged the relevance of the introduction of propofol and its propensity for pharyngeal smooth muscle relaxation in substantially improving ease of use of the LMA. In a similar sense, the properties of the newer inhalation agents may also be important.2 These changes were highlighted in McIntyres review of historical airway devices3 where he not only described the earlier devices but also recalled the context in which they were to be used (e.g. inhalation ether inductions). This early experience may explain why this problem received so little attention for a long period. The Shipway airway might now be seen as a forerunner of the COPA4 in that both are oropharyngeal devices with a pharyngeal cuff and direct connection to the breathing circuit. By comparison, the concept of the laryngeal mask was itself entirely novel and described by the inventor as a face mask on the larynx (i.e. the idea of the conventional face mask is applied directly over the laryngeal inlet).
The cuffed pharyngeal tube (CPT) described in this issue by Tashayod5 may at first sight appear to be simply a foreshortened tracheal tube with a large pharyngeal cuff. The anteriorly eccentric nature of the cuff is clearly different from that of the COPA where the cuff is by comparison largely posterior. The latter feature has been commented on adversely as it tends to push the tip of the COPA into the tongue substance.6 Non-symmetrical cuffs may not be entirely new, but presumably the technology for exploiting this option has tended to become more efficient with time. Another recently introduced device, the modestly named Airway Management Device or AMD, has at the time of writing no clinical trials reporting its use.7 At first sight this resembles the Combitube because it has two cuffs but just the one lumen. The lower cuff is designed to locate in the hypopharynx and assist in the positioning of the aperture opposite the laryngeal inlet and its upper cuff is designed to elevate the tongue and epiglottis. The upper cuff is somewhat oval in shape and this is claimed to centralize the airway and account for hands-free usage. It is said to be a stable airway and easy to insert. Further clinical experience with both the CPT and AMD is eagerly awaited, but at present it is only the latter that has a CE mark.
In view of these recent changes, it is reasonable to again question what we might expect from an ideal airway device. (Expectations have perhaps been increased by clinical experiences over the last decade and should not be expected to remain unchanged over the next.)
Efficient conduit for pulmonary ventilation bypassing the upper airway.
Easy, appropriate insertion even by non-specialists.
Efficacy not drastically impaired by suboptimal placement.
Stable in use once positioned (i.e. capable of hands-free anaesthesia).
Works equally well in abnormal as well as normal airways.
Easily converted to tracheal tube placement (i.e. a dedicated airway).
Good accept/reject profile (see below).
Minimal/no aspiration risk.
Easy sealing of the upper airway to allow intermittent positive pressure ventilation.
Sealing should be with minimal pharyngeal distortion and/or distension.
Cricoid pressure friendly.
Negligible side effects profile.
It is suggested that of the above list at least half of the features noted are unlikely to have been seriously considered more than 10 yr ago. The term dedicated airway describes devices which maintain airway patency while other major airway interventions are anticipated or in progress.6 It refers to devices which allow uninterrupted controlled ventilation while other measures are undertaken, such as converting from use of the device to tracheal intubation. Guedel airways, for example, would not come into this category. Other important and recently introduced devices may also not be dedicated in this sense, but are advances in airway management and include protheses for sleep apnoea patients.8 9 It is surprising that this class of airway advances so far seem to have had little impact on anaesthetic practice.
The accept/reject profile is a notion advanced in Brains writings on the development of the LMA.1 It describes the relative potential for acceptance or rejection of a foreign body by the oropharynx. Rejection phenomena include excess coughing, secretion outpouring, retching and expulsion of the device. Obviously patient-specific factors are not irrelevant,10 11 but equally some device shapes, constituent materials and designs will be better tolerated than others. It seems likely that we may not yet have discovered what makes for the most well-tolerated devices. Cuffs of tubes placed in the upper pharynx under local anaesthesia have been suggested to cause a swallowing reflex if dilation is too great or too fast.6 Another aspect of the same idea might be the ability to retain an airway device under light sedation (or local anaesthesia) which then becomes rejected as the sedation decreases (or local wears off). The LMA is well tolerated during emergence from anaesthesia, but the ideal must be a device that is not only a good airway but also well tolerated in the awake subject, without the need for local anaesthesia. Paediatric anaesthetists might suggest that childrens dummies have been used as airways in infants for many years to do just that.
The best alternative for the oesophagus/oropharynx interface is also far from clear. The options at present aim to minimize passive regurgitation (by being well tolerated) or try to produce a blocking effect with or without a tube acting as a vent. These alternatives may not be mutually exclusive. The application of cricoid pressure with any airway device may have implications which need to be considered.12 13 It is interesting to speculate how, in the longer term, devices which terminate higher in the oropharynx than the LMA will compare as far as aspiration risk is concerned.
Overall, development of airway devices up to the present time can be considered to have been somewhat empirical. The understanding of the physiological dynamics of the oropharynx is limited and variations in pharyngeal anatomy may be greater than has been previously considered. This is clearly an important area for anaesthesia research and airway device design in the future.
P. Charters
Department of Anaesthesia
University Hospital Aintree
Longman Lane
Liverpool L9 7AL
UK
References
1 Brain AIJ. Historical aspects and future directions. Int Anesthesiol Clin 1998; 36: 118
2 Tanaka M, Watanabe S, Nishikawa T. Minimum alveolar sevoflurane concentrations required for insertion of the cuffed oropharyngeal airway and the laryngeal mask airway: a comparative study. Anaesthesia 1999; 54: 115560[ISI][Medline]
3 McIntyre JWR. Oro-pharyngeal and naso-pharyngeal airways 1 (18801995). Can J Anaesth 1996; 43: 62935[Abstract]
4 Greenberg RS, Toung T. The cuffed oro-pharyngeal airway a pilot study. Anesthesiology 1992; 77: A558
5 Tashayod ME. Two cases of difficult intubation managed by a handmade device. Br J Anaesth 2000; 85: K588
6 Charters P, OSullivan E. The dedicated airway: a review of the concept and an update of current practice. Anaesthesia 1999; 54: 77886[ISI][Medline]
7 Airway Management Device, product literature. Nagor Limited, PO Box 21, Douglas IM99 1AX, Isle of Man, British Isles
8 Schonhofer B, Hochban W, Vieregge HJ, Brunig H, Kohler D. Immediate intraoral adaptation of mandibular advancing appliances of thermoplastic material for the treatment of obstructive sleep apnoea. Respiration 2000; 67: 838[ISI][Medline]
9 Johal A, Battagel JM. An investigation into the changes in airway dimension and the efficacy of mandibular advancement appliances in subjects with obstructive sleep apnoea. Br J Orthodontics 1999; 26: 20510
10 Erskine RJ, Murphy PJ, Langton JA, Smith G. Effect of age on the sensitivity of upper airway reflexes. Br J Anaesth 1993; 70: 5745[Abstract]
11 Erskine RJ, Murphy PJ, Langton JA. Sensitivity of upper airway reflexes in cigarette smokers: effect of abstinence. Br J Anaesth 1994; 73: 298302[Abstract]
12 Asai T, Barclay K, Power I, Vaughan RS. Cricoid pressure impedes placement of the laryngeal mask airway and subsequent tracheal intubation through the mask. Br J Anaesth 1994; 72: 4751[Abstract]
13 Dravid RM, Reed P, Stoneham M, Popat MT. Effect of cricoid pressure on insertion of and ventilation through the cuffed oropharyngeal airway. Br J Anaesth 2000; 84: 3636[Abstract]