Supraglottic airway devices: Placement and pharyngeal seal matters!

Abstract

Supraglottic airway devices (SADs) are now an indispensable instrument in the operating room as well as in non-operative room anaesthesia practice. They have evolved greatly from Archie Brain's laryngeal mask airway (LMA).[1] Use of SADs has expanded over the time from short surgical procedures initially, to all types of surgeries including laparoscopic/robotic and obstetrics. SADs have also been used in oral, cervical tracheal, prone position surgeries[23] and during cardiopulmonary resuscitation.[4] From the first-generation LMA classic to the most advanced third-generation Baska mask®, there is much progress in the field of SADs. Despite their improved utility, SADs do have disadvantages like not providing 100% protection from pulmonary aspiration of gastric material like a cuffed tracheal tube (TT).[5] Although the SAD is considered a minimally invasive device, complications and problems have been associated with its use. The 4th National Audit Project (NAP4) of the United Kingdom reported that the incidence of death/brain damage is 9.1, 6.5 and 5.0 per million with TT, face mask and SAD, respectively. NAP4 has found that aspiration is the most frequent cause of death accounting for 50% (8/16) of deaths and two cases resulted in brain damage during airway management.[6] Factors that may prevent these complications include selecting patients appropriately, choosing an SAD with better sealing pressure, the correct size of SADs, and using a recommended or improved insertion technique. There are various types of SADs available and among all these, perilaryngeal sealers (e.g., ProSeal LMA, Supreme LMA, I-gel™, LMA Protector™ and Baska mask®) are most commonly used. Perilaryngeal sealers form a seal around the larynx by fitting into the shape of the hypopharynx and tongue. Head movement and a change in the position of the patient can compromise this seal and lead to air leakage and tidal volume loss during controlled ventilation. A perfect and appropriate seating of the SAD is very important to prevent displacement. A tug-test can be performed after placement of the SAD. Tug-test assures that the cuff of the SAD is beyond the base of the tongue and does not slip out into the mouth. Tug-test helps in placing the tip of the SAD above the upper oesophageal sphincter and also helps in correcting accidental epiglottis down folding.[2] For adequate tidal volume delivery and ventilation, a proper oro-pharyngeal seal is required; this is determined by the oro-pharyngeal leak pressure (OPLP). Oro-pharyngeal leak pressure is measured by closing the expiratory valve of the anaesthetic circle system at a fixed gas flow rate (usually 3 L/min) and noting the equilibrium airway pressure. Second generation and third generation SADs are known to give better OPLP than first-generation SADs. Oro-pharyngeal leak pressure is the gold standard in SADs but a proper oro-pharyngeal seal cannot be attained without proper placement of SAD over the laryngeal inlet. Proper placement of LMA was studied by Brimacomb and Berry[7] and they proposed a grading system. A fibreoptic scope is passed through the ventilation tube of the LMA up to a position just proximal to the aperture bar of the LMA and the view is graded as follows: Grade 4 – only vocal cords seen; grade 3: vocal cords plus posterior epiglottis seen; Grade 2 – vocal cords plus anterior epiglottis seen; Grade 1 – vocal cords not seen, but LMA function adequate; Grade 0 – failure to function and vocal cords not seen. They mentioned that LMA function is not relevant for Grades 4, 3 and 2 because the vocal cords are clearly visible and functional failure is not due to improper positioning and warrant ruling out other causes. Ultrasonography can be used for confirmation of misplacement/malrotation of SAD and the cuff of the LMA can be visualised with USG if inflated with saline. Nevertheless, further research is needed in the use of USG for confirmation of LMA placement.[89] Supraglottic airway devices