Research Paper
Evaluation of laryngeal mask as an alternative to endotracheal intubation in cats anesthetized under spontaneous or controlled ventilation

https://doi.org/10.1111/j.1467-2987.2004.00195.xGet rights and content

Abstract

Objective

To compare the cardiorespiratory effects and incidence of gastroesophageal reflux with the use of a laryngeal mask airway (LMA) or endotracheal tube (ET) in anesthetized cats during spontaneous (SV) or controlled ventilation (CV).

Study design

Prospective randomized experimental trial.

Animals

Thirty‐two adult crossbred cats, weighing 2.7 ± 0.4 kg.

Methods

The cats were sedated with intramuscular (IM) methotrimeprazine (0.5 mg kg−1) and buprenorphine (0.005 mg kg−1), followed 30 minutes later by induction of anesthesia with intravenous (IV) thiopental (12.5–20 mg kg−1). An ET was used in 16 cats and an LMA in the remaining 16 animals. Anesthesia was maintained with 0.5 minimum alveolar concentration (0.6%) of halothane in oxygen using a Mapleson D breathing system. Cats in both groups were further divided into two equal groups (n = 8), undergoing either SV or CV. Neuromuscular blockade with pancuronium (0.06 mg kg−1) was used to facilitate CV. Heart and respiratory rates, direct arterial blood pressure, capnometry (Pe′CO2) and arterial blood gases were measured. Gastric reflux and possible aspiration was investigated by intragastric administration of 5 mL of radiographic contrast immediately after induction of anesthesia. Cervical and thoracic radiographs were taken at the end of anesthesia. Data were analyzed using anova followed by Student–Newman–Keuls, Kruskal–Wallis or Friedman test where appropriate.

Results

Values for PaCO2 and Pe′CO2 were higher in spontaneously breathing cats with the LMA when compared with other groups. Values of PaO2 and hemoglobin oxygen saturation did not differ between groups. Gastroesophageal reflux occurred in four of eight and two of eight cats undergoing CV with ET or LMA, respectively. There was no tracheal or pulmonary aspiration in any cases.

Conclusions and clinical relevance

The use of an LMA may be used as an alternative to endotracheal intubation in anesthetized cats. Although aspiration was not observed, gastric reflux may occur in mechanically ventilated animals.

Introduction

Cats have small, delicate tracheae and the larynx may be easily damaged. The well‐developed protective reflexes make laryngeal spasm a common complication during or after endotracheal intubation. Consequently, intubation should be performed with particular care in this species (Rex 1994; Taylor 1994). Even when proper endotracheal tube (ET) placement is achieved, laryngeal damage during intubation may cause swelling and airway obstruction after the tube has been removed (Taylor 1994).

The laryngeal mask airway (LMA) is a relatively new device for airway management during inhalation anesthesia (Pennant & White 1993; Verghese et al. 1993; Maltby 1994; Ivens et al. 1995). It was developed in 1981 by Archie Brain and has been commercially available since 1988 (Brain 1983; Pennant & White 1993; Maltby 1994). The LMA was designed for use in human beings, but is suitable for dogs (Braz et al. 1999), pigs (Wemyss‐Holden et al. 1999), rabbits (Cruz et al. 2000) and medium and large‐sized cats (Asai et al. 1998). The LMA is a combination of the face mask and ET. It is manufactured in silicone, may be autoclaved and used several times. It is composed of a rigid tube coupled at the distal end to an oval silicone mask which forms a hermetic seal around the larynx (Maltby 1994). It has been suggested that the use of the LMA would be a safe way of providing a gas‐tight seal in the cat's airway without laryngeal damage (Steffey 1994). Asai et al. (1998) observed that the LMA was as effective as the endotracheal tube in maintaining a patent airway, suggesting that the LMA could be useful in cats and possibly in veterinary practice. The use of an LMA, however, is associated with an increased risk of gastroesophageal reflux and possible aspiration in human beings; especially when mechanical ventilation is used (Valentine et al. 1994; Bapat & Verghese 1997).

The hypothesis examined in this study was that the LMA is a practical device for airway maintenance during inhalation anesthesia in cats. Accordingly the aim of this study was to compare the cardiorespiratory effects and incidence of gastroesophageal reflux with the use of LMA or ET in cats anesthetized during either spontaneous (SV) or controlled ventilation (CV).

Section snippets

Materials and methods

The study was approved by the Animal Care Committee (protocol number 22/99). Thirty‐two healthy adult crossbred cats, weighing from 2 to 3.5 kg (2.7 ± 0.4 kg) were used. Health status was assessed by physical examination and blood gas analysis performed prior to the experiment. Before the beginning of the study, the animals were maintained for 15 days in individual cages where they were treated for parasites and vaccinated. The cats were randomly distributed into four groups of eight, after

Results

There were no differences between groups in terms of mass or sex distribution (Table 1). The animals undergoing endotracheal intubation required higher doses of thiopental for laryngeal reflex suppression (17 ± 2 mg kg−1), when compared with animals undergoing LMA insertion (14 ± 1 mg kg−1).

Heart rate (HR) increased during endotracheal intubation or LMA insertion (mean ranging from 180 to 222 beats minute−1) when compared with the anesthesia maintenance phase (mean ranging from 100 to 170 beats

Discussion

A relatively low concentration of halothane (approximately 0.5 MAC) was used because, in the pilot study, a greater concentration of halothane (1 MAC) produced significant cardiopulmonary depression. This would have made it difficult to investigate the use of LMA in spontaneously breathing cats, as most of the animals developed apnea with 1 MAC. This anesthetic depression was probably related to the combination of methotrimeprazine, buprenorphine and thiopental, potentiating halothane

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