To topically anaesthetise the mouth and oropharynx, sit the patient up with their mouth open maximally and spray four metered puffs of lidocaine (10 mg per spray) onto the tongue and wait a few minutes for it to take effect. Depress the tongue and spray a further 4 puffs onto the posterior part of the tongue and pharynx. Although the superior laryngeal nerve can be topically anaesthetised by placing a pledget soaked in 2% lidocaine into each pyrifoam fossa with Krase forceps, once the mouth and tongue are blocked, it is more usually blocked discretely as follows: Place the patient supine with the head extended and palpate the hyoid bone just cephalad to the thyroid cartilage. Displace the hyoid slightly towards the side to be blocked and insert a 25 G, 2.5 cm needle just under the inferior border of the hyoid and inject 2-3 ml of 1% lidocaine, moving the needle gently in and out through the thyrohyoid membrane (Figure RA.37). Repeat on the other side. To complete the airway anaesthesia, with the patient in the same position as above, palpate the inferior border of the thyroid cartilage and having anaesthetised the skin first, insert a 21 G, 2.5 cm needle through the cricothyroid membrane into the lumen of the trachea. Ensure that air is freely aspirable and then rapidly inject 3-4 ml of 2% lidocaine. This will precipitate brisk coughing which spreads the local anaesthetic throughout the trachea and up into the larynx and vocal cords (Figure RA.38). Total dose of lidocaine in an average 70 kg adult should not exceed 200 mg.
The nerve supply of the airway comes from three sources. The trigeminal nerve supplies the nasopharnyx, palate (V2), and anterior aspect of tongue (V3). The glossopharyngeal nerve supplies the oropharnyx, posterior aspect of tongue and soft palate. The vagus nerve gives off two nerves—the superior laryngeal and recurrent laryngeal nerves which supply motor and sensory fibres to the airway below the epiglottis. The superior laryngeal nerve emerges beneath the inferior edge of the greater cornu of the hyoid before it divides into the internal and external branches.
Figure RA.37 Superior laryngeal nerve block
Figure RA.38 Transtracheal block
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