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

Purpose: During general anesthesia, nitrous oxide (N2O) diffuses rapidly in to the air-filled laryngeal mask airway (LMA) cuff, increasing intracuff pr essure. There is no clear correlation between LMA intracuff pressure and pr essure on the pharynx. We have studied the effects of high LMA intracuff pr essures secondary to N2O on the pharyngeal mucosa of dogs. Methods: Sixteen mongrel dogs were randomly allocated to two groups: G1 (in tracuff volume, 30 mL; n = 8) breathed a mixture of O-2 (1 L.min(-1)) and a ir (1 L.min(-1)) and G2 (intracuff volume, 30 mL; n=8) a mixture of O-2 (1 L.min(-1)) and N2O (1 L.min(-1)). Anesthesia was induced and maintained wit h pentobarbitone. LMA cuff pressure was measured at zero (control), 30, 60, 90 and 120 min after #4 LMA insertion. The dogs were sacrificed, and biops y specimens from seven predetermined areas of the pharynx in contact with t he 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 i ncreased 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 la yer There were no differences between the groups (P > 0.10) or among the ar eas sampled (P > 0.05). In both groups, the SEM study showed a normal phary ngeal 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 ph aryngeal mucosal injury in dogs.