Abstract
Direct laryngoscopy should be preceded by airway assessment and discussing rescue strategies with assisting staff. The initial plan and rescue techniques should be based on an understanding of normal airway anatomy and its variants. This chapter uses the two curve theory and the three column model to describe and functionally classify both the normal and difficult airways. This classification then provides the operator with an understanding of the various causes of difficult airways and how they are related to each other and airway morphology (two curve theory). These two concepts form the basis for the decision process to performing direct laryngoscopy and choosing the right laryngoscope blade for the job. Endobronchial intubation is discussed in detail.
‘If you know the enemy and know yourself, you need not fear the result of a hundred battles…’
Successful direct laryngoscopy and tracheal intubation is based on a clear understanding of upper airway anatomy and how it can be manipulated to achieve successful airway management and patient safety. Understanding the mechanisms of normal and difficult direct laryngoscopy can be illustrated by two concepts: the two curve theory and three column model. This enables the difficult airway to be managed predictively rather than reactively.
Airway assessment and preparation before performing airway manoeuvres have been highlighted in a number of airway management audits, including NAP4. These topics are covered in detail in Chapters 5–7. The reader should be familiar with these subjects before reading this chapter.
The Key Elements of Successful Direct Laryngoscopy
These are
1. Correct positioning of the head and neck
2. Correct insertion and manipulation of the laryngoscope blade within the upper airway
The two curve theory is an alternative to the three axes alignment theory and allows an understanding of airway configuration in different head and neck positions (Figure 14.1). This theory divides the airway passage into two curves:
1. Primary or oropharyngeal curve
2. Secondary or laryngotracheal curve
Figure 14.1 Airway passage (solid curved line) superimposed over MRI scan showing primary (solid green line) and secondary (solid red line) curves. (A) ‘Neutral position’ (no pillow): small radii of curvature of both curves. (B) ‘Sniffing position’: head lift (flexion of lower cervical spine) and head extension (extension of upper cervical spine) causes flattening of both curves.
How to Flatten the Primary Curve through Patient Positioning
When the adult is in the neutral position (i.e. no pillow and the patient has a vertical gaze with no head extension, such as the position commonly found during manual in-line neck stabilisation) each curve has a small radius of curvature making airway manoeuvres difficult.
The optimal airway position is that which enables easy airway management. This occurs when the external auditory meatus and the sternal notch lie on the same horizontal plane. This occurs in a small child with a small head-ring under the head, in a large child and adult with a single pillow and in the obese patient with ‘ramping’ using several pillows or wedges under the upper torso and head (Figure 14.2).
Figure 14.2 Head and neck positioning during airway management. (A) Obese patient with standard head lift: external auditory meatus and sternal notch are not in the same horizontal plane (white dotted line) leading to poor flattening of secondary curve (solid red line) and difficult direct laryngoscopy. (B) Obese patient in ramped position with head elevation and shoulder support: external auditory meatus and sternal notch are in the same horizontal plane (white dotted line) leading to flattening of secondary curve (solid red line) and easy direct laryngoscopy. (C) Obese patient with head-up positioning: similar effect to ramped position with external auditory meatus and sternal notch in the same horizontal plane (white dotted line) leading to flattening of secondary curve (solid red line) and easy direct laryngoscopy. (D) Non-obese patient with standard pillow: external auditory meatus and sternal notch are in the same horizontal plane (white dotted line) leading to flattening of secondary curve (solid red line). (E) Child with small pillow (i.e. relatively large head in relation to small antero-posterior diameter of chest): external auditory meatus and sternal notch are in the same horizontal plane (white dotted line) leading to flattening of secondary curve (solid red line).
In the ‘sniffing position’ there is a combination of lower cervical flexion and upper cervical extension. This flattens both curves (Figure 14.1B) and makes airway manipulation easier, thereby improving the chance of successful direct laryngoscopy.
An alternative to the sniffing position is to tilt the upper body 25° ‘head-up’. This rotates the oroglottotracheal curve and aligns it with the operator’s viewing angle during direct laryngoscopy. This position may be of use for emergency airway management when ramping the patient is logistically difficult (e.g. an unconscious obese patient).
The Function of a Standard Macintosh Laryngoscope Blade
The laryngoscope blade displaces the tongue to the left and completes the flattening of the primary curve by elevating the mandible and compressing the tissues in the submandibular space. The operator pushes the laryngoscope blade anterodistally (not levering on maxillary teeth) to (i) lift the epiglottis anteriorly by pressure on the hyoepiglottic ligament, (ii) displace the submandibular tissues anteriorly and (iii) push the tongue laterally. The result is to provide sufficient space for the operator to view the vocal cords and insert the tracheal tube into the trachea.
Correct Technique When Using a Macintosh Laryngoscope Blade
The blade is optimally placed along the floor of the mouth to the right of the tongue and used to displace the tongue to the left (Figure 14.3A). This contrasts to placing the blade over the tongue (Figure 14.3B) where greater force is required as it is necessary to try to compress both the tongue and submandibular tissues.
Figure 14.3 (A) Laryngoscope blade in paraglossal position causing lateral displacement of tongue and lifting submandibular tissues, providing wide field of view (yellow shaded area); straight line of sight over patient’s head. (B) Laryngoscope blade placed over the tongue reducing field of view due to: (1) less displacement as the tongue is anterior to the blade and (2) tongue bulging over spatula into field of view; angled line of sight to view around bulging tongue.
An alternative technique is the retromolar approach when the patient’s head is turned to the left and the blade is inserted over or behind the right molar teeth. This approach reduces the volume of submandibular tissue to be displaced and has been recommended for patients with a large tongue or at-risk upper teeth.
Anatomical Causes of Difficult Direct Laryngoscopy
An understanding of difficult direct laryngoscopy may be based on (i) why the two curves cannot be flattened and (ii) factors preventing correct insertion and manipulation of the laryngoscopy blade. These factors may be summarised in the three column model of difficult airways.
The Three Column Model of the Airway
In this model the airway is divided into three columns (Figure 14.4):
Anterior column – formed by the mandible and submandibular tissues
Middle column – formed by the airspace
Posterior column – formed by the cervical spine
Figure 14.4 Three Columns: Anterior Column – mandible and submandibular tissues (blue triangle), Middle Column – airway passage (green line) and Posterior Column – cervical spine (red line).
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