The hypopharynx forms the final section of the pharynx before it merges with the oesophagus. The distal cuff tip should occupy the entire hypopharynx forming an oesophageal seal to protect the laryngeal inlet from aspiration from below. This oesophageal seal – also known as the secondary seal – typically comprises pressures upwards of 40 cmH2O once the LMA cuff is inflated. Contrary to its principle function, some evidence suggests the distal cuff tip may interfere with lower oesophageal sphincter tone, predisposing to gastroesophageal reflux.^[1] Second generation LMAs include a gastric conduit to mitigate this risk by allowing gastric drainage and deflation as well as passage of instruments (e.g. endoscopes) into the upper gastrointestinal tract. Brain originally claimed a gastric tube could be passed behind the cLMA to achieve the same effect.

If hypertrophied, the palantine tonsils may impede insertion and/or be traumatised.

Mouth opening of >3cm is typically required for LMA insertion. This is estimated by assessing if the interincisor distance is approximately three fingers wide.

The anterior surface of the proximal cuff rests on the pharyngeal portion of the tongue. A large tongue may obstruct insertion and subsequently make manipulation difficult. Following insertion, the tongue may sag posteroinferiorly onto the LMA cuff within the oropharynx and improve primary seal pressures.

The anterior surface of the proximal cuff aligns with and adheres the epiglottis to the posterior surface of the pharyngeal tongue. If enlarged, the epiglottis may interfere with LMA placement. The epiglottis may also twist or fall into the bowl of the mask and obstruct airflow. Intubating LMAs include an epiglottic elevating bar to remove such obstruction. The epiglottis may be traumatised by LMA insertion, particularly if used as an intubation conduit.

Encased by protective cartilages the larynx houses the vestibular fold and vocal (true) cords. The larynx represents the merger point between the upper airway and trachea. Posterior displacement of the larynx (e.g. by trauma or external pressure) may impede LMA insertion. Laryngeal structures may be damaged by passage of instruments through an LMA.

LMA insertion will project prominent neck cartilages forward, providing a gross indication of mask position. Thyroid pressure may improve airway seal, while cricoid pressure may inhibit intubation through the LMA.

LMA insertion follows the line of the palate. Significant resistance will be met during insertion if the palatine-pharyngeal angle is less than 90 degrees. The posterior surface of the airway tube presses into these structures once completely inserted.

The nasopharynx lies superior to the soft palate. The junction between the naso- and oro-pharynx is a common site of resistance during insertion. In extreme cases, the distal cuff tip may curl upwards at this junction, entering the nasopharynx and causing airway trauma.

Degree of cervical spine extension will influence the dimensions of the pharynx and LMA seal pressures. A small amount of cervical spine movement is incurred with LMA insertion. Intubating LMAs have been shown to be beneficial for ETT placement in patients with cervical spine injuries. [ref]Ferson D, Rosenblatt W, Johansen M, Osborn I, Ovassapian A. Use of the Intubating LMA-Fastrach™ in 254 Patients with Difficult-to-manage Airways. Anesthesiology. 2001; 95: 1175—81.[/ref]

The oropharynx is bordered by the soft palate superiorly and base of the tongue inferiorly. It is the site of highest mucosal pressures once the LMA is inserted.

Bound superiorly by the epiglottis and inferiorly by the junction between the glottic inlet and oesophagus, the laryngopharynx typically contains the bulk of the LMA mask. The mask’s bowl aligns with the laryngeal inlet, while the lateral aspects of the cuff abut the pyriform fossa forming an airtight seal. This seal is called the primary seal and typically reaches 20 cmH2O. The primary (laryngeal) seal may also be referred to as the oropharyngeal leak pressure, which is determined by measuring the equilibrium pressure in a closed anaesthetic system at a constant flow rate. Posterior displacement of the larynx (e.g. anatomical variant or cricoid pressure*) may impeded LMA placement. Arytenoid cartilage within the larynx may be damaged by incorrect insertion. Compression of pharyngeal structures may also damage the recurrent laryngeal nerve as it enters the larynx from below.
*Cricoid pressure: External pressure applied over the cricoid cartilage during endotracheal intubation. Posterior projection of the laryngeal cartilage compresses the collapsible oesophagus and may reduce the risk of gastroesophageal reflux and aspiration. Of questionable benefit, this manoeuver is not routinely used, unless there are significant concerns of aspiration.

The hypopharynx forms the final section of the pharynx before it merges with the oesophagus. The distal cuff tip should occupy the entire hypopharynx forming an oesophageal seal to protect the laryngeal inlet from aspiration from below. This oesophageal seal—also known as the secondary seal—typically comprises pressures upwards of 40cm H2O once the LMA cuff is inflated. Contrary to its principle function, some evidence suggests the distal cuff tip may interfere with lower oesophageal sphincter tone, predisposing to gastroesophageal reflux. [ref]Owens T, Robertson P, Twomey C, Doyle M, McDonald N, McShane A. The incidence of gastroesophageal reflux with the laryngeal mask: A comparison with the face mask using esophageal lumen pH electrodes. Anesth Analg. 1995; 80: 980—4.[/ref] Second-generation LMAs include a gastric conduit to mitigate this risk by allowing gastric drainage and deflation as well as passage of instruments (e.g. endoscopes) into the upper gastrointestinal tract. Brain originally claimed a gastric tube could be passed behind the cLMA to achieve the same effect.

The posterior pharyngeal wall abuts the LMA backplate or posterior cuff (if present) at the level of the second to seventh cervical vertebrae. It is the most common site of airway trauma.