European Journal of Anaesthesiology:
First attempt at characterising ‘Mac-alike’ functionality in a graded model of difficulty in Laerdal SimMan
Darshane, Seema; Charters, Peter
From the University Hospital Aintree, Liverpool, UK
Correspondence to Seema Darshane, FFA, University Hospital Aintree, Liverpool, UK Tel: +44 0 1515295152 5153; fax: +44 0 1515295155; e-mail email@example.com
Published online 4 October 2011
We are grateful to have the opportunity to reply to Xue et al. for their detailed comments1 on our study.2
We have published a couple of studies in recent years on our ideas concerning ‘residual tongue size and space available into which this can be displaced’ during rigid blade laryngoscopy.3,4 However, from their description, we are not certain that we see things in exactly the same way. In our view, most of the issues we and others have described in respect of the tongue displacement are problems specific to the Macintosh blade. In the last few years, we have started to use the term ‘Mac-alike’ for blades that share these limitations. It is our view that the Airtraq blade is not ‘Mac-alike’ because tongue compression and displacement into the submandibular space is not essential in normal usage. Differentiating ‘functional’ differences between rigid blades in the past has not been straightforward because there have been no simple models available for this, though we have made contributions to this field by way of limited mathematical and computer models.5,6 Physical mannequins may provide consistent test conditions but cannot hope to simulate the variation we see in clinical practice. Equally, clinical trials cannot produce reproducible conditions in which blades can be tested in detail for consistent comparisons between blades. Although mannequin studies have tended to become less favoured in recent times, we believe they can be valuable for testing various hypotheses about functional differences between blades and this was the reason for our study.
As far as our choice and use of insert was concerned, this was, as we stated, an attempt to produce an intermediate state and this we justified in our study. As regards the actual displacement that this achieved, we realised that some would consider this non-anatomical. Interestingly, none of the participants, who were blinded as to the actual mode of difficulty, reported anything unusual enough to suggest an entirely inappropriate mechanism for the difficulty encountered. We agree that this is an entirely novel approach to modelling progressive difficulty, but as far as the authors are concerned, the effect was equivalent to results that we have seen when we have previously attempted to use the AirSim mannequin with the tongue in various stages of inflation with air.7 In that sense, we consider that whatever the precise mechanism here, the effect was equivalent to an intermediate stage of worsening tongue size/mandibular space reduction. It is our view that whatever the mechanism, we were achieving this critical limitation for Macintosh and ‘Mac-alike’ blades.
We have previously used mannequins like AirSim (Trucorp Ltd., Belfast, Northern Ireland, UK) with varied tongue inflation to produce progressive imbalance between residual tongue size and space availability but found this model insufficiently robust for repeated laryngoscopies.7 With SimMan, the model is robust enough, but tongue inflation has only two options (resting and maximal inflation). Equally the laryngeal structures are fairly immobile and so BURP manoeuvres are not feasible. This answers their second point as to why we did not allow such manoeuvres. However, Xue et al. are right to correct us for not mentioning head and neck positioning. We were remiss in our methodology not to state that a standard intubating pillow was used in all cases to provide a consistent ‘sniffing position’.
On the final point about the C-Mac blade, we agree that it would have been interesting to include this blade in our study. Unfortunately, this instrument was not available when the study was at the stage of planning.
1. Xue FS, Wang Q, Liao X, et al. Can model of graded difficulty in Laerdal SimMan exactly compare performances of direct and indirect laryngoscopes? Eur J Anaesthesiol 2012; 29:53–54.
2. Darshane S, Ali M, Dhandapani S, Charters P. Validation of a model of graded difficulty in Laerdal SimMan: functional comparisons between Macintosh, Truview EVO2, Glidescope Video Laryngoscope and Airtraq. Eur J Anaesthesiol 2011; 28:175–180.
3. Horton WA, Fahy L, Charters P. Factor analysis in difficult tracheal intubation: laryngoscopy-induced airway obstruction. Br J Anaesth 1990; 65:801–805.
4. Charters P. What future is there for predicting difficult intubation? Br J Anaesth 1996; 77:309–311.
5. Charters P. Analysis for mathematical model for osseous factors in difficult intubation. Can J Anaesth 1994; 41:594–602.
6. Then SK, Gillies D, Vennila R, Charters P. Biomechanical modelling of the upper human airway. Br J Anaesth 2010; 104:521.
7. Darshane S, Ali M, Charters P. Tongue pressure in AirSim ‘airway management trainer’ during Macintosh laryngoscopy: effect of increasing head flexion and tongue volume. Br J Anaesth 2007; 99:761.
© 2012 European Society of Anaesthesiology