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High laryngeal mask airway pressures resulting from nitrous oxide do not increase pharyngeal mucosal injury in dogs

[Les pressions élevées du masque laryngé, causées par le protoxyde d'azote, n'augmentent pas les lésions muqueuses chez les chiens]

Tânia Mara Vilela Abud, MD^*, José Reinaldo Cerqueira Braz, MD PhD^*, Regina Helena Garcia Martins, MD PhD, Elisa Aparecida Gregório, PhD and João Carlos Saldanha, MD

^* From the Department of Anesthesiology, The Department Of Otorhinolaryngology, Ophthalmology and Head and Neck Surgery,
the Department Of Morphology Of The Institute Of Bioscience
of The School Of Medicine Of The University Of São Paulo (unesp), Bottucatu, São Paulo; and The Department Of Pathology,
School of Medicine of the University Federal of Triângulo Mineiro, Uberaba, Minas Gerais, Brazil.

Address correspondence to: Dr. Tânia Mara Vilela Abud, Departamento de Anestesiologia, Faculdade de Medicina, UNESP, Distrito de Rubião Junior, Botucatu (SP), Brazil, ZIP CODE 18.618-970. Phone: 55 14 6802-6222; Fax 55 14 6822-4026; E-mail: anestesi{at}fmb.unesp.br

Purpose: During general anesthesia, nitrous oxide (N₂O) diffuses rapidly into the air-filled laryngeal mask airway (LMA) cuff, increasing intracuff pressure. There is no clear correlation between LMA intracuff pressure and pressure on the pharynx. We have studied the effects of high LMA intracuff pressures secondary to N₂O on the pharyngeal mucosa of dogs.

Methods: Sixteen mongrel dogs were randomly allocated to two groups: G1 (intracuff volume, 30 mL; n=8) breathed a mixture of O₂ (l L•min^–1) and air (l L•min^–1) and G2 (intracuff volume, 30 mL; n=8) a mixture of O₂ (l L•min^–1) and N₂O (l L•min^–1). Anesthesia was induced and maintained with pentobarbitone. LMA cuff pressure was measured at zero (control), 30, 60, 90 and 120 min after #4 LMA insertion. The dogs were sacrificed, and biopsy specimens from seven predetermined areas of the pharynx in contact with the LMA cuff were collected for light (LM) and scanning electron microscopic (SEM) examination by a blinded observer.

Results: LMA intracuff pressure decreased with time in G1 (P <0.001) and increased in G2 (P <0.001). There was a significant difference between the groups (P <0.001). In both groups, the LM study showed a normal epithelium covering the pharyngeal mucosa and mild congestion in the subepithelial layer. There were no differences between the groups (P >0.10) or among the areas sampled (P >0.05). In both groups, the SEM study showed a normal pharyngeal mucosa with mild superficial desquamation. Few specimens in G1 and G2 showed more intense epithelial desquamation.

Conclusion: High LMA intracuff pressures produced by N₂O do not increase pharyngeal mucosal injury in dogs.

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