Does the use of supraglottic device in rabbits cause less injury than other airway management devices?

[...]

Appraisal, application and reflection

The increasing popularity of pet rabbits and the willingness of owners to pay for veterinary services (Mayer et al., 2017) means that more rabbits are presenting to veterinary practices for care. Veterinary practices need to accommodate the medical and welfare needs of rabbit patients and provide the necessary services, including general anaesthesia for surgical procedures such as ovariohysterectomy. General anaesthesia in rabbits has higher risks and higher mortality rates compared to cats and dogs (Brodbelt et al., 2008). The main factors contributing to this high risk are difficulty establishing a secure airway, underlying respiratory diseases, and cardiorespiratory complications during anaesthesia (Brodbelt et al., 2008; and Eatwell, 2014).

Securing the airway in rabbits is made difficult due to their unique airway anatomy. Rabbits have small and narrow mouths with large incisors, large molars and large tongues relative to their skull size, which makes it difficult to visualise the larynx for intubation (Donnelly & Vella, 2020). The most common methods used to secure the airway in rabbits are endotracheal intubation and placement of a well-fitting face mask. However, both endotracheal tubes and face masks have disadvantages that can impact the risk of anaesthesia (Eatwell, 2014). Face masks allow delivery of oxygen and anaesthetic gases but do not guarantee airway patency, thus can cause clinically significant hypoxaemia and hypercapnia. Endotracheal intubation can cause mucosal damage to the upper airways, leading to swelling and potential narrowing (Bateman et al., 2005; and Hawkins & Pascoe, 2021). Endotracheal intubation also requires significant training and practice. To address these issues, supraglottic airway devices (SGAD), similar to those used in human medicine, have been introduced and trialled as an alternative airway management device for rabbits. The use of a human supraglottic airway device was tested in rabbits and reported to cause lingual cyanosis, possibly due to pressure on the lingual vasculature (Crotaz, 2013; and Kazakos et al., 2007). In recent years, a new rabbit-specific SGAD (v-gel®) has been designed to decrease the risk of complications observed with the use of a human SGAD.

The three studies included in this critical appraisal reported varied outcomes. Significant limitations were identified for all three studies, including small sample size, use of multiple investigators for device placement and the use of rabbit breeds that differ in size from breeds commonly kept as pets.

Comolli et al. (2020) and Engbers et al. (2017) both compare the placement of v-gel® and an endotracheal tube (ETT) in rabbits. However, the findings of the two studies are incongruent. Comolli et al. (2020) concluded that overall, the trauma caused by v-gel® and ETT placement is not significantly different. The authors report that v-gel® caused more injury to the larynx and epiglottis, and ETT caused more injury to the trachea, although the differences between the devices were not statistically significant. In contrast, Engbers et al. (2017) identified that ETT causes more significant injury to tracheal mucosal and submucosal airways compared to v-gel®. The different findings in these two studies could be due to differences in ETT placement method, given that Comolli et al. (2020) used endoscopic guided endotracheal intubation, while Engbers et al. (2017) performed a blind technique for placement of ETT. Another possible explanation could be the experience of investigators placing the devices. Goldmann & Ferson (2005) report that training for airway management by anaesthetists can reduce anaesthesia-related morbidity and mortality. The third study (Wenger et al., 2017) compared two additional airway devices, laryngeal mask, and face mask, in addition to v-gel® and ETT. Wenger et al. (2017) mainly studied the amount of laryngeal compression caused by the different devices and found that v-gel® compressed the larynx more significantly than laryngeal mask and face mask. This study did not look at the direct injury to the upper airways, but it was assumed that compression of the larynx would lead to airway damage, and therefore was relevant to the PICO question.

The methods used to evaluate airway injury were similar between the Comolli et al. (2020) and Engbers et al. (2017) studies. Both papers performed histological evaluation of post-mortem samples following anaesthesia. However, the time of sample collection differed between the studies. In Comolli et al. (2020) study samples were collected 4 days after anaesthesia. In Engbers et al. (2017) study samples were collected 2 hours after anaesthesia. The different time intervals between device placement and sample collection could impact the observed histological changes to the tissues. Collection after 2 hours would show any acute injury changes, such as acute inflammation and damage to tissue integrity, but not long-term damage such as chronic inflammatory infiltrate, fibrosis or stricture formation which could take days or weeks to develop. On the other hand, collection of tissue after 4 days may fail to identify the extent of the acute inflammatory changes immediately following the injury. If the tissue damage is significant but not severe enough to cause chronic change, it may still cause airway complications during the recovery period with resolution prior to sample collection at day 4 (Anderson, 2013).

The level of experience of the investigators performing the placement of each airway device differed between the three studies. In the two blinded studies (Comolli et al., 2020; and Engbers et al., 2017) the pathologist evaluating the tissue histology was blinded to treatment, however, the investigators placing the airway devices could not be blinded due to the nature of the procedure. This could impact the technique used by the investigator due to unconscious bias about which device was superior. Moreover, in the Engbers et al. (2017) study the investigator was more experienced using ETT compared to the v-gel®. In Wenger et al. (2017) the level of experience with device placement was not clear as multiple investigators were involved. One limitation that stood out in the Comolli et al. (2020) study is the use of visual inspection of upper airway. The authors do not describe how the measurements were recorded and analysed for visual inspections of the upper airways. Finally, it should be considered that the studies included only one breed of rabbit, that does not reflect the common pet rabbit breeds.

Considering the findings and limitations of each study, there is currently insufficient evidence to confirm that the use of v-gel® during general anaesthesia in rabbits causes less injury to the upper airways compared to other airway management devices. More research such as randomised blinded clinical trial with larger sample size can be helpful to learn if v-gel® use in rabbits is beneficial to them routinely in the clinical setting, given that less damage to the upper airways, will lead to better recovery from anaesthesia and fewer complications.

[…]