2. Surface anatomy of neck

The hyoid bone lies at the level of the third cervical vertebra (C3). Lying just above and behind is the epiglottis The bifurcation of the common carotid artery is at the level of the fourth cervical vertebra (C4), slightly above the notch of the thyroid cartilage. The larynx lies opposite the fourth, fifth and sixth cervical vertebrae (C4, 5, 6). The cricoid cartilage is at the level of the sixth cervical vertebra (C6). The trachea extends from the sixth cervical vertebra (C6) down as far as the fifth or sixth thoracic vertebra (T5, 6) at end-inspiration. The suprasternal notch is located at the level of the second and third thoracic vertebrae (T2, 3).

The trachea comprises 16–20 C-shaped cartilages, which lie anteriorly in the neck covered by skin and the superficial and deep fascial layers. The second, third and fourth rings are covered by the isthmus of the thyroid. The great vessels of the neck lie laterally, and so identification of the midline is crucial. The cricothyroid membrane spans the inferior border of the thyroid cartilage and the superior border of the cricoid cartilage, and immediately overlies the subglottic region of the larynx. It is covered anteriorly by skin and by superficial and deep fascia. Immediately lateral are the sternocleidomastoid muscle, the sternothyroid and the sternohyoid muscles and the carotid sheath.

Its indications are the same as for formal tracheostomy in the critically ill: typically to simplify airway management in a patient who otherwise would face the problems of long-term tracheal intubation, to allow a reduction in sedation and to facilitate weaning from mechanical ventilation.

Guided by the surface anatomy a skin incision is made to allow a needle and guidewire to be placed through the fibroelastic tissue that joins the tracheal rings. is for this reason that many intensivists now prefer a low approach, at the level of the second and third ring. The diameter of the hole is enlarged with progressively larger dilators to the point at which it will accept a definitive tracheostomy tube. An alternative is the use of a single tapered dilator A second anaesthetist should monitor this procedure from within the trachea by using a fibreoptic bronchoscope. The posterior wall of the trachea may be so ragged and friable that it can easily be perforated.

Haemorrhage (immediate or delayed), the creation of false passage, tracheal or oesophageal perforation, barotrauma, subcutaneous emphysema, failure and accidental decannulation.
Subglottic stenosis is a cause of serious morbidity; it is more common after cricothyroidotomy than after percutaneous tracheostomy.

Both techniques bypass the normal translaryngeal route to secure the airway, but the circumstances and urgency of their use differ considerably. Percutaneous tracheostomy is an elective procedure, whereas cricothyroidotomy is an emergency procedure which is usually invoked only when all other attempts to secure a definitive airway have failed and when critical hypoxia is imminent. The cricothyroid membrane is used for emergency access because it is readily identifiable and because it is relatively avascular.

the cricothyroid membrane is identified using the so-called laryngeal handshake. (The whole hand is used to palpate the cartilaginous structures of the hyoid, thyroid and cricoid, in much the same way that some anaesthetists check for accurate seating of a laryngeal mask airway). If this proves ineffective and ultrasound is not rapidly available, an 8–10 cm midline incision should be made (caudal to rostral). With the cricothyroid membrane exposed, it should be incised with a transverse stab after which the scalpel blade (number 10) is rotated through 90 with the sharp edge caudad. Without removing the blade, a bougie is passed into the trachea over which a lubricated 6.0 cuffed tracheal tube can be railroaded. (If the cricothyroid membrane is easily identified, the extended midline incision will not be necessary.)

This procedure is not as straightforward as this description might suggest: the Fourth National Audit Project. (NAP 4) Major complications of airway management in the UK, that was published in 2011 reported that the emergency cricothyrotomy failure rate was 64%. Another review of failed intubation in obstetrics over several decade reported that of 13 women in whom this procedure was attempted only 6 survived unharmed. A further 6 died, and 1 more suffered hypoxic brain injur