Evaluation of Functional Dimensions of Macintosh Laryngoscope Blade During Direct Laryngoscopy in Patients With Normal Mouth Opening

Upper airway access is one of the essential components of general anaesthesia, and more often than not, it is gained by direct laryngoscopy followed by tracheal intubation. While direct laryngoscopy depends on laryngoscope blade design and the anesthetists' skill set, the ease of tracheal intubation is influenced by the line-of-view to the glottis gained by the laryngoscopy maneuver. In patients with normal mouth opening (≥4.0 cm), the act of conventional airway access is not difficult given that the neck extension and the TMJ joint mobility is within normal limits. Conversely, it becomes difficult in patients who have mouth opening restriction, and hence they are adjudicated as 'anticipated difficult airway' (ADA). The ADA, attracts several issues that, not uncommonly, converges on to time inefficiency, heightened equipment cost, operator anxiety, and unfortunately, patient morbidity. There are many causes of restricted mouth opening, defined technically as reduced inter-incisor distance (IID) including trauma, TMJ movement limitation, intra/extra-oral pathology resulting in pain, or the most common pathology in the Indian subcontinent, oral submucosal fibrosis (SMF). While the pathology itself is a forerunner of oral mitosis, it is the associated restricted mouth opening component that put conventional airway access efforts of the anaesthesiologist in spot, and consequently, a whole, sometimes unnecessary rigmarole has to be activated under the heading of 'advanced approach to ADA'.

Oral submucous fibrosis (SMF) is a well-recognized, potentially malignant progressive condition afflicting the oral cavity that is characterised by a reduced mouth opening. The reduction in mouth opening occurs due a vertical reorientation of collagen fibrils that are normally haphazardly arranged. Of the currently available multiple classifications to identify disease stage/severity, the functional classification based on the inter-incisor distance (IID) seems to be most relevant.

IID forms an important component of airway examination and a value of less than 3-cm is suggestive of difficulty in airway management. However, despite the above, SMF is largely unrecognized as a cause of airway difficulty, and even when recognized, no set protocol is available to ensure (as far as possible) a favourable outcome in terms of least morbidity to the patient, and stress to the anaesthesiology team in question.

Need Assessment for the Study Generally, in a tertiary level centre, with all facilities at hand, cases with a reduced IID are normally tackled using a fiberoptic bronchoscope guided tracheal intubation. However, in India, the burden of this disease lies to a great extent in rural/suburban settings where access to a fiberoptic bronchoscope is unlikely. As a result these cases are referred to 'higher' centres for management. This imposes undue burden (logistics/ financial) onto patients and their families. Hence, there is a great need for anaesthesiologists working at the primary/secondary level healthcare set up to get oriented to this problem and be able to manage it with conventional options (direct laryngoscopy \& intubation) available to them.

Scientific Contention Direct laryngoscopy remains the foremost among the various techniques available to access the upper airway and the Macintosh blade has been the most preferred blade type. In order to improve direct laryngoscopy, many modifications of blade design have been tested/ compared with the Macintosh blade, but apparently, the clarity as to which laryngoscope blade is most suited for a given airway remains low. For the majority of practicing anaesthesiologists, the availability of different laryngoscope designs has only increased the hit-and-trial option for laryngoscopy.

Since the time it was first introduced, the Macintosh blade (size 1, 2, 3, 4, and 5) has weathered many newer laryngoscope blade designs and stood by our side with consistent/optimal performance. Per se, from a technical standpoint, although shaped the same, various sizes of the Macintosh blade are not exactly similar, the discrepancy becomes obvious when comparing sizes 1, 2 with 3 \& 4. Unfortunately, investigators are still unclear as to how design characteristics and the various sizes of the Macintosh blade impact the process of direct laryngoscopy. Generally, the selection of laryngoscope blade (type and sizes) is largely a function of individual experience and preferences. The absence of predefined parameters for the selection of the most appropriate laryngoscope blade for a given airway remains arbitrary. Apparently, an anaesthesiologist will be able to intubate an adult trachea with any of the laryngoscope blade, however, blade type and size selection assumes significance in the presence of a difficult airway.

The three factors which influence successful direct laryngoscopy performance are:

1. The ability to place the laryngoscope blade tip in vallecular recess and engage the hyoid bone. This requires an adequate length of the blade to be introduced orally to reach the vallecula (transition space between base of the tongue and the epiglottis) position. ii) The ability to displace the tongue aside for clear view to the glottic-epiglottic region iii) Achieving a clear line-of-sight to the glottis-opening. To ensure that the tip of the laryngoscope blade engages the hyoid by placement in the vallecula, an adequate blade length must be inserted into the oral cavity. Normally, this is easily achieved. However, when faced with a situation involving reduced mouth opening/inter-incisor distance (e.g.as in case of SMF), the length of blade that can be inserted into the oral cavity (functional length) becomes a function of the width of the blade's flange at the incisor point. Thus the size of the flange, corresponding to the width of the blade that must be accommodated in the oral cavity assumes particular significance in patients with reduced inter-incisor gap. Typically, for the Macintosh blade's curved design confers different length to its flange's glossal (concave) and the palatal (convex) surface, the functional width of its flange increases from the tip towards the heel [Figure]. Based on the degree of mouth opening restriction and the available IID (in cm), during preoperative evaluation, one can predict and plan how long a Macintosh blade length will be required and what size of the blade would suffice to be placed orally through the available IID.

Hypothesis Development Marks et al, utilized an x-ray based method to devise a method to evaluate laryngoscope blade design during active laryngoscopy procedure. Technically, they found that Macintosh sizes 3 and 4 have a more refined blade design that provides greater degree of forward space enhancement (FSE, allows greater mandibular space for the tongue to be moved into) than other conventional blades (Miller, Soper, etc.). This FSE-effect gets further magnified with increase in functional length of the blade (the length of the blade inside the oral cavity during active laryngoscopy).

Their elaborate work addressed the performance of the laryngoscope blade (size, type) selection in a theoretical setting. The information gleaned from their assessment offered valuable suggestions on optimal blade position during direct laryngoscopy and included:

1. Mandibular space 'encroachment/enhancement' with the use of different blade types/sizes, and ii) 'Line-of-vision' (view to the glottis) versus 'mandibular space restriction' (space for tongue displacement during laryngoscopy).

However, still, the clinical feasibility of knowledge thus gained from performance assessment facilitated by x-ray/fluoroscopy is difficult to translate into routine clinical practice.

As the investigator prepares to take the conventional direct laryngoscopy process into difficult airway situations where the primary difficulty is due to reduced mouth opening, there is a need to convert theoretical knowledge into clinical suggestions. Investigators thus contend that unless they get accustomed with greater clarity on the use of routine equipments, and analyse their practical feasibility in difficult situations, the element of randomness and subjectivity will continue to be a setback.