Blind Endotracheal Intubation in Neonatal Rabbits

Summary

We describe a technique of endotracheal intubation in newborn rabbits after esophageal catheterization with a gastric tube.

Abstract

The newborn rabbit is a useful animal model for various pathologies and procedures. Airway management of the rabbit is complex due to its anatomical characteristics, which is further complicated in the case of the newborn. Of the different methods of advanced airway management, endotracheal intubation is less aggressive than tracheostomy, and is more feasible than supraglottic management given the lack of supraglottic devices of such a small size. As direct glottis visualization is very difficult in animals this size, this blind intubation model is presented as an effective alternative, especially for experiments requiring prolonged anesthesia. Using this method, we performed blind intubations with a 90% success rate.

Introduction

The peculiar anatomy of the rabbit's respiratory system makes endotracheal intubation complicated. The oral cavity is deep and narrow with a limited range of mouth opening and a relatively large tongue, making direct visualization of the larynx difficult even with the use of a laryngoscope^1,2. Furthermore, intubation of the newborn rabbit, which is 10-20 times smaller than the adult, is even more of a challenging procedure. In addition to these anatomical characteristics, rabbits are prone to laryngospasm secondary to manipulation of the airway.

Several intubation techniques have been described in the literature, such as laryngoscope tracheal intubation, blind intubation, otoscope intubation, retrograde intubation, endoscope intubation, tracheostomy and supraglottic device ventilation^{3,4,5,6,7,8,9}. All these techniques have been carried out successfully in adult rabbits; however, there are few publications regarding advanced airway management in the newborn rabbit¹⁰. One study describes performing tracheostomies in animals of this age to assess breathing mechanics and lung volumes, though this is an aggressive and irreversible technique¹¹. This method for endotracheal intubation has proven to be an effective and low-cost method of airway management in newborn rabbits.

Protocol

The intubation procedure outlined below was approved by the Barcelona University Institutional Animal Care and Use Committee (CEEA 180/19)

1. Experimental preparation

1. Obtain the following materials: 2 mm diameter polyvinyl chloride (PVC) endotracheal tube, 6 French (Fr) polyurethane feeding tube, flat and wide tip dissection forceps and sterile lubricant (Figure 1).
2. During the intubation and surgery processes, maintain sterile conditions.

2. Anesthesia

1. Anesthetize the animal with 5 mg/kg xylazine and 25 mg/kg ketamine applied together intramuscularly in the vastus lateralis of the femoral quadriceps. Alpha-2 agonists such as dexmedetomidine can also be used instead of xylazine.
2. Allow 3-5 minutes for the medication to reach effect.
3. Readminister the same dose until deep anesthesia is achieved (negative pedal, palpebral and corneal reflexes). Ideally, the rabbit will maintain spontaneous breathing until intubation.

3. Endotracheal intubation

1. Place the rabbit on a thermal blanket at 37 °C. Place the animal in dorsal recumbency with the head in hyperextension. Align the head as straight and midline as possible. A four-centimeter roll made of sterile gauze placed in the dorsal region of the neck may be helpful to achieve at least 45° of extension (Figure 2).
2. Use flat and wide tip dissection forceps to open the oral cavity approximately one centimeter wide. Use forceps with a blunt and wide end to avoid mouth trauma.
3. Measure the distance between mouth and the region of stomach (first rib). Introduce a lubricated 6 French (Fr) polyurethane gastric tube to the pre-measured mark (Figure 3-4).
4. Move the feeding tube to the left and fix with adhesive tape to the surgical field.
5. Introduce the 2 mm diameter PVC endotracheal tube, covered with sterile lubricant to the 3 cm mark.
   1. If there are any difficulties in advancing the tube, move it completely and reattempt until a smooth glide is achieved. The risk of trauma to the larynx and epiglottis is significant with multiple intubation attempts. Fix the tube with adhesive tape to the surgical field (Figure 5-6).
6. Connect the endotracheal tube to the ventilator with following settings: Tidal volume - 10 mL/kg; respiratory rate: 40 breaths/min; FiO₂ 100%, PEEP 6.
7. Confirm the correct position of the tube by the detection of CO₂ in exhaled air (ETCO₂), direct visualization of thoracic expansion and fogging of the tube. Monitoring of the animal by pulse oximetry and lung auscultation can also be helpful (Figure 7).
8. At the conclusion of the experiment, use a standardized euthanasia protocol that consists of intravenous administration of pentobarbital 120-240 mg/kg. Repeat doses until breath cessation, reflex absence and asystole are acheived.
   NOTE: In the surgical experiments, the animal is alive during the ventilation process and finally is euthanized.

Results

In the laboratory, this technique is carried out by medical personnel with experience in experimentation with New Zealand rabbits and expertise in pediatric airway management. The procedure has been performed by one investigator in 10 animals with a global success rate of 90%. Previously, another researcher had practiced optimizing both the position of the animal and the correct tube fixation length. None of the animals were intubated in esophagus. Only one attempt (10%) failed due to the...

Discussion

Rabbits are widely used in animal experimentation due to their availability, high reproduction rate with short intergenerational times, and easy handling². In our center, we use 5-7-day old New Zealand rabbits (Oryctolagus cuniculus) as a neonatal model to study the neurologic repercussions of extracorporeal surgery on the developing brain. As a result, we utilize procedures that require deep anesthesia and must use endotracheal intubation for periods of time up to an hour.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2 mm diameter Polyvinyl Chloride (PVC) endotracheal tube	Vygon	520.20
6 French polyurethane feeding tube	Vygon	310.06
Anesthesia (Ketamine and Xylazine)
Multiparameter monitor Intellivue MP5	Phillips
Plain and wide tip dissection forceps	LensforVision	AK005
Sterile lubricant (Silkospray)	Rush
Ventilator Servo 900c	Maquet