A comparison of supraglottic airway devices versus endotracheal intubation for use in rabbit anaesthesia

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Appraisal, application and reflection

Facemasks are commonly used in first opinion practice for rabbit patients due to the difficulty of endotracheal intubation by veterinarians with limited experience with rabbit patients. Current evidence suggests that face masks are inadequate for airway maintenance as it is difficult to achieve an effective seal to provide adequate oxygen leading to hypoxaemia, hypercapnia and an inability to achieve positive pressure ventilation in the event of apnoea (Wenger et al., 2017). Endotracheal tubes have been used successfully in animal practice for many years and when using a technique allowing visualisation of the trachea, are an excellent choice for maintaining a patent airway in rabbit patients. The use of the blind technique can result in mucosal damage and multiple intubation attempts. For those practitioners who are untrained in endotracheal intubation in rabbits, supraglottic devices may be an alternative. There are a range of supraglottic devices available including the rabbit specific v-gel®. Both endotracheal tubes and supraglottic airway devices have evidence demonstrating their effective use in maintaining anaesthesia with no significant differences in anaesthetic parameters.

All studies used subjects that were healthy and young and with low American Society of Anesthesiologists (ASA) scores and only one study assessed the use of both devices whilst the subjects were undergoing a procedure (Comolli et al., 2020). Only one study assessed the recovery of patients (Crotaz, 2013), and the subjects of two studies were euthanised whilst still under anaesthesia (Engbers et al., 2017; and Comolli et al., 2020). Considering this, it is difficult to extrapolate the findings to rabbits seen in general practice who would be put under anaesthesia for surgical procedures where recovery is an important part of an anaesthetic protocol. Rabbit patients may also present with higher ASA scores due to age or disease and selection of premedication and induction agents may be limited by clinic protocol, availability of different agents and patient requirements. Additionally, no studies looked at dwarf breeds and whether application of a supraglottic device or endotracheal tube was easier or possible given the smaller oral cavity size. Majority of studies used New Zealand White rabbits as subjects (Enbers et al., 2017, Comolli et al., 2020, Tman et al., 2015; and Wenger et al., 2017). This breed is commonly used for research purposes and as such the results of these studies may be more readily applicable in a research setting.

Crotaz et al. (2010) concluded that the v-gel® supraglottic airway device was simpler to use which is further supported by other studies which demonstrated that the time to place v-gel® was faster, more consistent and required fewer attempts when compared to endotracheal intubation (Crotaz, 2010; Cruz et al., 2000; Engbers et al., 2017; and Comolli et al., 2020). As induced patients are at risk of apnoea, the length of time it takes to achieve a patent airway is critical to prevent hypoxaemia (Navarrete-Calvo et al., 2014). This is even more clinically significant when considering patients in respiratory arrest where fast airway access is imperative. Cruz et al. (2000) demonstrated that a significantly higher dose of induction agent, in this case thiopentone, was required to achieve endotracheal intubation than supraglottic device insertion¹⁵. However, the studies premedicated and induced their patients with a variety of different agents, including induction chamber or facemask with isoflurane, intravenous administration of thiopentone, propofol, ketamine and xylazine or alfaxalone. The use of different agents could impact the ease or difficulty of the application of different airway devices as would impact sedation quality at induction and the stress levels of each subject. In patients where using less anaesthetic agents would be ideal such as geriatric patients or patients with organ dysfunction, supraglottic devices may be a the better choice if available premedication and inductions drugs at higher dosages increases risk for these patients.

Power calculations were only undertaken in one study (Wenger et al., 2017). As all studies had small sample sizes, it is possible that differences between the groups were only insignificant because there were not enough subjects in each group, resulting in a type 2 error. When comparing mucosal damage to the trachea, Engber et al. (2017) found histological evidence of damage to the tracheal mucosa was significantly higher when using an endotracheal tube, whereas Comolli et al. (2020) found no significant difference between either device (Engbers et al., 2017; and Comolli et al., 2020). Whilst this could be a type 2 error due to small sample size, it could also be explained by the fact that the Engber et al. (2017) study used the blind technique, resulting in multiple intubation attempts and leading to more mucosal damage. There could also be bias in histological scoring. Comolli et al. (2020) in contrast visualised the trachea during endotracheal intubation, thus reducing the attempts required to achieve intubation. The Wenger et al. (2017) study showed that more attempts were required to place the endotracheal tube than there were with the supraglottic device, which could explain why there was further damage to the oral cavity and tracheal mucosa. Use of the blind technique could also increase the risk of oesophageal perforation (Ranchère et al., 1992) and this must also be considered as it may have long term effects on patients. Long-term recovery was not assessed in these studies; thus, it is not possible to assess whether mucosal damage to the trachea has significance to a patient’s recovery and future health or is clinically significant. Other studies have shown that tracheal damage can lead to tracheal strictures which can increase patient morbidity and mortality (Grint et al., 2006).

Bateman et al. (2005) provided good evidence that supraglottic devices could be used successfully in rabbit patients but resulted in gastric tympanism in some cases when positive pressure ventilation was provided. Further investigation is warranted to see if gastric tympany has any long-term effects of clinical significance, especially when considering that gut stasis is a common complication of recovery. Cruz et al. (2000) by use of barium sulphate as a radiographic contrast directly into the stomach of the subjects concluded that there was no regurgitation present on radiographs following use of endotracheal tubes or supraglottic devices. However, one should note that gastric oesophageal reflux is highly unlikely in rabbits due to their strong cardia preventing backflow of gastric contents. Crotaz et al. (2010) and Bateman et al. (2005) noted that there was no evidence that the supraglottic v-gel® device would protect against aspiration of fluids and was displaced easily during patient movement, increasing the likelihood of aspiration (Bateman et al., 2005; Crotaz, 2013; and Comolli et al., 2020). Dental procedures are common in rabbit patients, and it may be risky to use supraglottic devices without knowing if the seal is adequate to prevent aspiration considering the fluid produced and introduced into the patient’s oral cavity via dental instruments.

Engbers et al. (2017) used computed tomography (CT) scans to measure the height and width of the larynx, showing that it was significantly narrower in subjects using the v-gel® supraglottic device when compared to the laryngeal mask airway device or facemask. The Wenger et al. (2017) study used CT scans to assess the correct placement of the v-gel® supraglottic device where half of the subjects required readjustment with three subjects showing laryngeal compression with increased mucous accumulation. This is an important consideration when using supraglottic devices as narrowing of the larynx due to the device size or mucous accumulation can lead to laryngeal compression which may result in an increased breathing effort as a result of increased airway resistance. Considering this, monitoring of respiratory effort when using supraglottic devices is warranted, although the clinical significance of increased breathing effort was not discussed.

Whilst a device may be effective for spontaneous ventilation, positive pressure ventilation or controlled mechanical ventilation may be required in the event of apnoea. Bateman et al. (2005) showed that in four of the six subjects using supraglottic devices, gastric tympany was noted and there was leakage of anaesthetic gas. This suggests whilst controlled mechanical ventilation can be achieved with supraglottic devices, there is leakage into the environment and the alimentary tract, which can make it more difficult to provide adequate ventilation in apnoeic patients. This would also potentiate environmental contamination with the gaseous anaesthetic agent which is an important consideration for the safety of staff and other patients. Bateman et al. (2005) suggested that keeping the pressure below 14 cmH₂O could mitigate some leakage however, this would likely also decrease alveolar ventilation and lead to hypercapnia. Comolli et al. (2020) demonstrated that endotracheal intubation is the most effective device to decrease dead space as supraglottic devices showed higher levels of PaCO2. However, in the event of unsuccessful endotracheal intubation, supraglottic devices are still superior to facemasks as controlled mechanical ventilation has been shown to lead to much more significant leakage, increased dead space and PaCO₂ in facemasks compared to supraglottic devices (Bateman et al., 2005; and Wenger et al., 2017).

More research is warranted in clinic specific settings to determine the suitability of supraglottic airway devices in rabbit patients. The easier loss of airway seal during movement when using a supraglottic device is not ideal in procedures that require repositioning. They also take up more space in the oral cavity, making them difficult to use in dental procedures, and potentially more difficult to use in smaller breeds. v-gels® do have the benefit of being easily sterilised via autoclave which can reduce bacterial growth and contamination which can be important in patients where respiratory defences are compromised. They may also be easier to apply in rabbit patients, providing an alternative for the practitioner lacking the experience or equipment to perform endotracheal intubation safely. Endotracheal intubation is more effective than supraglottic devices in achieving positive pressure ventilation in the case of apnoea and may be a better option for dental procedures as less space is taken up in the oropharynx and the better seal reduces the risk of aspiration. It should be noted however, that the blind technique has a higher risk for mucosal damage and increased likelihood of repeated intubation attempts.

The current evidence demonstrates that both devices can be used effectively to maintain anaesthesia in rabbit patients and that the final choice of airway maintenance device should be based on availability of equipment, the training of the practitioner and the procedure to be undertaken.

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