Australasian Anaesthesia 2011 - Australian and New Zealand ...

62 Australasian Anaesthesia 2011Reappraisal of Adult Airway Management 63COMBINING THE TWO-CURVE THEORY AND THREE COLUMN MODELFigure 4 shows how both the Anterior and Posterior Columns of the Three Column Model for Direct Laryngoscopyinfluence the airway passage (Middle Column) configuration, that is the Two-Curves. Changes in the PosteriorColumn have a direct effect on the Primary and Secondary Curves – extension: flattening Primary Curve, head lift:flattening Secondary Curve.In contrast, the effect of the Anterior Column on the Primary and Secondary Curves is the result of both pathologyof the column and the impact different airway devices have on it. For instance, variations in the Anterior Columnthat occur in retrognathia and macroglossia will affect the shape of the airway curves. In addition, the effect ofairway devices on the curves may vary from little or none in the case of fibreoptic bronchoscopy and Glidescopevideolaryngoscope TM to marked flattening of the Primary Curve with the C-Mac video laryngoscope TM .Finally, intrinsic lesions of the airway passage (or Middle Column), such as foreign bodies and airway tumours,may distort the Primary and Secondary Curves.Figure 4. Both the anterior (triangle) and posterior (black line) columns of the Model for DirectLaryngoscopy influence the shape of the airway passage configuration (black dotted line) as describedin the Two-Curve Theory.EXAMPLES OF AIRWAY MANAGEMENT BASED ON THE TWO-CURVES THEORY AND THE THREECOLUMN MODELAnterior Column ProblemsAnterior Column problems are a diverse group of pathological conditions which may be divided into (i) reducedvolume of the submandibular space (retrognathia/micrognathia), (ii) reduced compliance of the submandibulartissues (including Ludwig’s angina, post-radiotherapy to the submandibular space and tumour of the tongue base)and (iii) restriction of temporo-mandibular joint function.Optimising head and neck position is essential to ensure flattening of the Secondary Curve before focusing onthe Primary Curve with Anterior Column problems. With reduced volume or reduced compliance of the submandibularspace, the Primary Curve is the major focus of management. There are two potential management plans for dealingwith Primary Curve problems.1) Paraglossal or retromolar insertion of a straight laryngoscope:Magill described a technique later called by Bonfil 38 “homolateral retromolar intubation” which allows the operatorto bypass the Primary Curve and enter the supraglottic space (rostral part of Secondary Curve). Henderson 39,40re-visited this concept, using a low profile straight blade with a paraglossal approach rather than the midline for apatient with a hypoplastic mandible 40,41 and limited forward movement of the hyoid. 402) Following the Primary Curve without displacing the submandibular tissues:Methods that manoeuvre around the Primary Curve without causing its displacement include flexible fibreopticbronchoscopy, rigid indirect laryngoscopy devices and blind nasal intubation. Successful use of the GlideScopevideolaryngoscope TM has been described for tracheal intubation of patients with mandibular hypoplasia. 42,43 Blindintubation techniques including blind nasal intubation 44 and lightwands such as the Trachlight TM have been usedfor mandibular hypoplasia 45-47 and patients with limited mouth opening. 47,48The choice should be based on the experience of the operator and the suitability of the available device ortechnique.POSTERIOR COLUMN PROBLEMSElective patients with normal Anterior Column requiring manual in-line neck stabilisation (MILNS):The levered laryngoscope when activated produces a fulcrum at the base of the tongue, which increases pressureon the hyo-epiglottic ligament. This elevates the epiglottis in patients with normal Anterior Columns and flattensboth the Primary and importantly, the Secondary Curve. The levered blade has been used successfully in at leastthree studies examining its role in patients requiring manual in-line stabilisation of the neck or wearing cervicalcollars49-51 where the Anterior Column is normal.CLINICAL CONDITIONS WITH COMBINED ANTERIOR, MIDDLE AND POSTERIOR COLUMN PROBLEMSAcromegaly:Features of acromegaly that may cause difficult laryngoscopy and intubation 52 include macroglossia, prognathism,cervical spine osteophyte formation with decreased range of movement, thickening of the pharyngo-laryngeal softtissues and recurrent laryngeal nerve palsy. The problems are three-fold. Firstly, there is inability to compress theenlarged swollen tongue (Anterior Column) during direct laryngoscopy despite the increased submandibular volumecreated by the prognathism. Secondly, there is encroachment into the airway passage by swollen upper airwaymucosa (Middle Column). Lastly, restricted neck mobility may interfere with positioning in the sniffing position.Though all patients may not exhibit all of these aspects, they do have a potential for problems with all three columns.If there is no osteophyte formation and the difficulty is mainly due to the Anterior Column, then a straightlaryngoscope blade used in conjunction with a curved-tip bougie may be effective in displacing the enlarged tongue.An airway passage restricted by thickened or redundant mucosa may, however, make the narrower straight bladeless effective. It seems that an indirect laryngoscopy device, intubating laryngeal mask or fibreoptic bronchoscopyshould be more reliable, but mixed results have been found with indirect laryngoscopy devices. The intubatinglaryngeal mask has a 47.4% failure rate. 53 This appears to be related to the difficulty in matching the Primary Curveof acromegalics to the fixed curvature of a device that has an integral tracheal tube conduit. Preliminary reports 52indicate that use of a flexible bronchoscope through a Classic LMA TM may provide flexibility to match the patient’sPrimary Curvature. The GlideScope videolaryngoscope TM separates the device from the tube/stylet and allows foreasier manoeuvrability in the airway with high success. 52Awake fibreoptic intubation 54-56 provides a useful alternative to indirect laryngoscopy methods, especially if allthree aspects of the model are involved.CONCLUSIONIn Eugene Wigner’s “The Unreasonable Effectiveness of Mathematics in the Natural Sciences”, he states that themathematical structure of physics often leads to further advances in theory and even empirical predictions. This isa significant statement. They are interwoven and, despite their complexities, predictable. We need a similarmathematical model for airway management that will explain what we know and will predict what we should cometo understand.The Two-Curve Theory and the Three Column Model of Direct Laryngoscopy provide a basis for teaching andongoing research in this area. The foundation stone of this approach is assessment of the airway. This may beeither embarking on airway management or as an urgent reappraisal when an unexpected difficult airway isencountered. The diagnosis will assist the operator to select an airway manoeuvre that is likely to be successfuland to avoid those that are not.These two theories are based partly on experimental work and partly on “scholastic reasoning”. With time theymay remain unchanged, be modified or need replacing.“Imagination is more important than knowledge. Knowledge is limited; imagination encircles the world.”~ Albert Einstein