Outline of tracheal intubation in pets

Tracheal intubation is the placement of a tube that extends from the mouth to the trachea. Its purpose includes the delivery of inhalational anaesthetic drugs, ensuring an open airway in an unconscious animal, and the administration of oxygen for ventilatory support.

The aim of this article is to provide beginners with a standardised procedure for the correct performance of tracheal intubation, focusing on the purpose of tracheal intubation, how to intubate (relevant anatomy and physiology, tools and equipment, and procedures), and the possible risks and complications. In addition, a case of tracheal intubation in a clinically exotic pet (New Zealand rabbit) is shared.

Advantages and disadvantages of tracheal intubation:

(1) Tracheal intubation is one of the many ways to ensure the provision of oxygen to the respiratory system, but is more often used primarily to ‘protect the airway’, i.e., to prevent foreign bodies (e.g., oral secretions and gastric contents) from entering the trachea and the lower airway (which is prevented by normal physiology through the swallowing and pharyngeal reflexes);

② The second function of tracheal intubation during general anaesthesia: i.e. to prevent the escape of anaesthetic gases (WAG, Wastes Anesthetic Gases) into the surrounding environment, to protect healthcare workers from inhaling anaesthetics in a higher volume and to improve the utilisation of anaesthetics;

(iii) The third function of tracheal intubation is to promote positive pressure ventilation.

(iv) The fourth function of tracheal intubation is that it allows for the performance of closer suction monitoring. A carbon dioxide monitor, spirometer or gas analyser can be connected to the distal end of the tracheal tube and can accurately measure inhaled and exhaled carbon dioxide and oxygen as well as inhaled anaesthetic medication, and can also measure airway pressure and compliance.

The ETT can also be used to measure inhaled and exhaled carbon dioxide, as well as inhaled anaesthetics.5 Potential complications of tracheal intubation include injury or tearing of the tracheal mucosa, which can lead to subcutaneous emphysema or pneumothorax.A prolonged ETT can lead to inadequate ventilation or bronchial intubation. However, these complications are uncommon when the healthcare provider performs a standardised procedure and can usually be avoided by choosing an appropriate ETT and carefully inserting and managing the tube.

Translated with www.DeepL.com/Translator (free version)

This diagram mainly shows the importance of measuring the endotracheal tube (ET). This 3 mm tube was used in a variety of guinea pigs and was the appropriate length from nostril to scapula. In this guinea pig, this ET tube needed to be further shortened manually to ensure that unilateral intubation did not occur (see Section III below for details of the exact length required for intubation).

Image source: https://www.theveterinarynurse.com/content/clinical/endotracheal-intubation-of-small-exotic-mammals

I. Anatomy and Physiology Overview
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In order to master the basic clinical skills of endotracheal intubation, pet healthcare professionals should be aware of the relevant anatomy and physiology.

Image source: https://www.theveterinarynurse.com/content/clinical/endotracheal-intubation-of-small-exotic-mammals

(1)The mouth is separated from the nasal cavity by the hard palate. In the mouth, the tongue is a larger muscle that moves food to the back of the throat in a co-ordinated and controlled manner. Therefore, diseases affecting the tongue can also damage the patient's airway.

(2) The hard palate is caudal to the soft palate, which separates the oropharynx from the nasopharynx, and the soft palate is caudal to the pharynx (or common pharyngeal area). The tonsils are located at the pharyngeal junction, and they may become enlarged, thereby reducing visibility during intubation.

(3) The epiglottis is located at the base of the tongue. This triangular cartilaginous structure is part of the larynx. During swallowing, the epiglottis moves dorsal-caudally to cover the vocal folds and prevent swallowed material from entering the trachea.

(4) The soft palate generally terminates at the epiglottis, and the anatomy of the soft palate does differ in dogs and cats from other animals. An excessively long soft palate can lead to some breathing and swallowing problems, especially in specific breeds such as the short-headed dog. An overly long soft palate can catch in the epiglottis, interfering with normal swallowing and respiratory function, and may lead to choking, vomiting, or abnormal breathing sounds.

(5) The entrance to the oesophagus is located on the dorsal side of the larynx, and its proximity is the cause of accidental oesophageal intubation.

(6) The larynx is a group of cartilaginous structures. There are four types of five laryngeal cartilages, namely the unpaired epiglottic cartilages, the thyroid cartilages, the cricoid cartilages, and the paired spatulate cartilages, which are connected to each other by joints or ligaments.
The laryngeal cartilages move laterally with each breath to increase airflow into the trachea. Paralysis of the nerves that power these structures reduces airflow, leading to respiratory distress.

In dogs and cats, the trachea consists of an incomplete C-shaped ring of cartilage around which the oesophagus can dilate as food passes. The trachea is lined with ciliated mucosa, which can easily cause bruising, necrosis or rupture during tracheal intubation.

Expanded knowledge: median section of the canine head

1. frontal and intershield muscles 2. roof of the skull (parietal) 3. frontal sinuses 4. nasal bones 5. dorsal turbinate 6. ventral turbinate 7. third internal turbinate 8. nasal septal cartilage 9. upper lip 10. incisive teeth 11. lower lip 12. hard palate 13. soft palate 14. tip of the tongue 15. body of the tongue 16. root of the tongue 17. nucleus accumbens 18. nucleus hyoidus 19. mandibular hyoidus muscle 20. mandible 21. rectus sinus 22. cerebral hemispheres 23. corpus callosum Vaults and septum of the body terminale 24. interoptic thalamic adhesions (middle block) 25. cerebellum 26. cerebral bridge 27. medulla oblongata 28. cricoid vertebrae 29. spinal cord 30. spinal dura mater 31. cardinal vertebrae 32. cephaloventral rectus 33. cervicalis longus 34. cephalic dorsal rectus 35. collateral ligament 36. optic saccule crossings 37. pituitary gland 38. metanephros 39. external laryngeal ventricle 40. vocal cords 41. cricoid cartilage 42. oesophagus 43. trachea　44. sternohyoid muscle, thyroid cartilage 45. hyoid body, hyoid arch 46. pharyngeal cavity 47. eustachian tube orifice 48. nasal turbinates.

Image credit: Wu Liping, ‘Anatomy and Construction of the Respiratory System,’ https://www.cloudvet.org/Mapi/article/416#95;

1. Basic equipment for safe tracheal intubation: (some tools and equipment may vary)
Includes an endotracheal tube (ETT), some suitable material for securing the tube (often referred to as an ‘ETT tie’), a laryngoscope, sterile water-based lubrication and local anaesthetic spray:

(1)A laryngoscope with a Miller laryngoscope blade;

(2)Adhesive tape (or other supplies for securing the endotracheal tube);

(3)0.25 mL of 2% lidocaine (appropriate anaesthetic depending on the clinical situation);

(4)Polypropylene guide catheter, cut to size as needed. (The non-rounded end is marked with a permanent marker to clearly indicate which end should not be inserted into the trachea (arrow);

(5) Airflow detection device;

(6) Apply sterile lubricant to the distal end of the tube;

(7) Tracheal intubation without a sleeve.

Image source：Endotracheal Intubation of Rabbits Using a Polypropylene Guide Catheter (Article) | JoVE，https://app.jove.com/t/56369/endotracheal-intubation-rabbits-using-polypropylene-guide；

2. Endotracheal Tube
Today, endotracheal tubes (ETTs) are primarily made of polyvinyl chloride (PVC) and are single-use. They have many typical design features, as shown below. Many variations of these designs are present in endotracheal tubes used for specific applications, e.g., re-inforced tubes, RAE tubes, laser tubes, etc.

Expanded information: detailed information can be read
①《Endotracheal tubes》 | Anesthesia Airway Management (AAM) (ucsf.edu) https://aam.ucsf.edu/endotracheal-tubes
②《The endotracheal tube in detail》 | Deranged Physiology https://derangedphysiology.com/main/required-reading/equipment-and-procedures/Chapter%202812/endotracheal-tube-detail

In this article, we will only describe the ‘standard’, most widely used version of the endotracheal tube.

(1) Standard 15 mm airway connector: The connectors are 15 and 22 mm (inner diameter) and comply with the international standard ISO 5356-1. The reason for this standard is self-evident: all airway devices and accessories should be connected to all other airway devices.

(2) Radiopaque blue line: This is just a convenient way of identifying the position of the tube tip on X-rays. The radiopaque blue line is usually made of only slightly ‘impure’ PVC. Many ET tubes include a radiopaque line that extends all the way to the tip. This is helpful when you want to confirm the fit of the tube on a chest X-ray, as the rest of the tube is not visible.

(3) Magill curve: Most ETTs have a pre-formed curve, called a Magill curve, which makes it easier to insert the tube because the curve follows the anatomy of the upper airway. The ‘Magill curve’ was a pleasant accident: when Sir Ivan Magill first used single-lumen rubber tubes for nasotracheal intubation in the 1920s, he cut them from a loop of tubing (so they were curved) and found the curve useful in inserting these tubes.

(4) Markings on tubes: depending on the type and manufacturer, ET tubes have several markings on the outside. The ones that almost all ET tubes have are inner and outer diameter dimensions (inner and outer diameter measurements in millimetres) and length markings (in centimetres from the tip).

(5). Tracheal intubation sleeve: Once the sleeve is filled with air, it seals the lungs and prevents fluid secretions from sloshing around in the upper airway. Furthermore, it ensures that the environment below the cannula can be pressurised and ventilated by a carefully controlled gas mixture. Generally, there are two types of tracheal intubation cannulae used: high volume - low pressure cannulae, and low volume - high pressure cannulae.

Different trocar designs on ‘standard’ ETT (left) and Bivona TTS® tubes (right); image credit: Endotracheal tubes | Anesthesia Airway Management (AAM) (ucsf.edu)

(6) Valve and indicating balloon: The balloon is inflated through a spring-loaded valve with a luer lock connector. Attached to the valve is an indicator balloon, which allows for (gross) tactile and visual confirmation of balloon inflation after intubation or before extubation.
Numerous clinical cases have demonstrated that the ‘feel’ indicator balloon does not allow an accurate estimation of balloon inflation, especially when the balloon is overinflated, which may lead to tracheal mucosal ischaemia and subsequent scarring, especially in chronically intubated affected pet individuals. It is therefore important to avoid high balloon pressures and to check inflation status using a balloon manometer.

(7) Tube tip design and Murphy's eye: The tip of the endotracheal tube (ETT) usually has a left-facing bevelled design. This is a way to ensure that the tip never becomes blocked by direct juxtaposition with the tracheal wall. However, even a bevelled tip can be obstructed by the tracheal wall; it is simply a matter of positioning it in the correct way.

Image credit: Endotracheal tubes | Anesthesia Airway Management (AAM) (ucsf.edu) infringed.

In fact, the left-facing bevel provides a roughly contoured view of the eye looking down the throat. A perfectly cylindrical ETT (no bevelled design) will block most of the vocal folds as it passes through them, thus interfering with the operator's ability to see if it has entered the vocal folds correctly (blind insertion).
In short, the left-facing bevelled design improves visibility during intubation and has the additional collateral benefit of being more difficult to block with phlegm or other mucus.
The main significance of Murphy's eye is to act as an additional ventilator. Even when the tip of the ETT is blocked by secretions in the airway, some gas may still enter through the Murphy's eye.

Image source: https://www.theveterinarynurse.com/content/clinical/endotracheal-intubation-of-small-exotic-mammals

3, Laryngoscope:

(1) Laryngoscopes are battery-powered, hand-held devices used to examine the oropharynx, larynx, and surrounding structures. They consist of two parts: a handle and a blade. The handle contains batteries and is available in standard or narrow widths.

(2) The light source for the laryngoscope is a small bulb, which may be located in the handle or the blade. The bulb in the handle is often a brighter LED.
There are several styles of laryngoscope blades available, but the Miller and Macintosh are the most common in veterinary medicine, with the former usually being more suitable for dogs and cats.

(3)The laryngoscope aids intubation by depressing the tongue and providing enough light to place the ETT correctly. They should be considered essential, not complementary, during routine tracheal intubation.
The ETT should be firmly secured after completion of tracheal intubation to prevent accidental extubation or further tube movement.

1、Preparation for tracheal intubation:

(1) Select the appropriate catheter (material, length, diameter, etc.) After selecting the catheter, pressure check the sleeve to ensure that there is no leakage and ensure that the tube remains clean.
The correct tube length can be estimated as the distance from the incisura to the thoracic inlet.
A. An excessively long catheter may result in increased instrument dead space, which may increase the burden on respiratory function and lead to hypoventilation.
B. Intubating the bronchus with a long tube may lead to contralateral lung atelectasis.

Extended Information:
I. Accidental insertion of a long tube into a bronchus may result in contralateral lung atelectasis (pulmonary atrophy). This may occur when, during intubation, the tube enters a branch of the trachea, which in turn enters one side of the bronchus, while simultaneously obstructing airflow to the opposite lung. This obstruction can lead to inadequate ventilation of the lung in question, which in turn can lead to atelectasis.
II. The thoracic inlet is the uppermost opening of the chest, which is the main access route to and from the respiratory tract and the heart. It is located between the neck and the chest and is formed by the cervical vertebrae, the first thoracic vertebrae, and the first ribs, and the main inlets include the larynx and trachea.
C. Placement of a short tube may result in the balloon not being placed in the trachea.
Comparison of weight and endotracheal tube type in dogs and cats

Image data source: https://www.cvma.org.cn/upload/shouyi/upload/2022/04-02/09-17-390624125506825.pdf
A. Wider diameter catheters offer less resistance to flow, which is beneficial in reducing work of breathing and reducing the risk of airway obstruction (e.g. mucus). However ETTs that are too wide in diameter can damage upper airway tissue or in rare cases become stuck.
B. Narrow tubes may not produce an airway seal with airbag inflation levels. The ideal tube size is the widest, easy to place, and has no resistance during intubation.

2. canine and feline restraint and intubation positioning:
Tracheal intubation is performed on: animals that are unconscious due to trauma, disease or sedation/anaesthetics. For any anaesthetised patient pet, it is always necessary to establish intravenous access (via an intravenous catheter). Intubation can be performed with the animal in dorsal or lateral recumbency; however, tracheal intubation is easiest to perform when the assistant places the animal in sternal recumbency.

At the same time, the assistant must reposition the animal so that the head and neck are not twisted to the sides. The neck must be extended and the animal's mouth opened wide, with one hand holding the palate. Strips of gauze or plastic tubing placed behind the canine teeth are particularly useful for opening the mouth, and additionally allow the assistant's hand to not block the view and therefore allow for better observation.

(1) Restraint:
When the affected pet is in sternal recumbency, the assistant should use a tracheal intubation gauze bandage to elevate the patient's maxilla. This prevents accidental injury to the assistant's fingers if the affected pet suddenly clamps its jaws.
In large dogs, the patient's back and neck may need to be elevated. The head and neck should be straight and aligned with the spine.

When the dog is in sternal recumbency, the assistant holds the dog, traction neck extended and mouth open. The operator grasps the tongue with a piece of gauze and pulls it out of the mouth to visualise the vocal folds with a laryngoscope.
Image source: https://www.nature.com/articles/laban0207-23/figures/2
(2) Positioning of the intubation: clear visualisation of the vocal cords and arytenoid cartilage
To position the larynx with the laryngoscope, place the tip of the laryngoscope on the root of the tongue, ventral to the epiglottis. Push down on the tip of the laryngoscope (i.e., gently press on the root of the tongue) and the epiglottis automatically moves ventrally and makes the vocal folds visible.
Do not press on the epiglottis as it is a delicate structure.

The vocal folds of (a) a cat and (b) a dog are clearly seen with the help of a laryngoscope.
Image source: https://www.theveterinarynurse.com/content/clinical/endotracheal-intubation-of-small-exotic-mammals,

3, Tracheal intubation:
(1) Next, hold the laryngoscope in the non-dominant hand. Use the tip of the blade to push the tongue out of the mouth (again to prevent accidents to the fingers). Simultaneously hold the tongue with the index and forefinger of the non-dominant hand, and hold and position the laryngoscope with the index and thumb.
(2) Apply a small amount of sterile lubricant to the end/edge of the catheter to make it easier to slide in (taking care to avoid the tip or Murphy's eye);
Note: Cats have a small airway and are at greater risk of laryngospasm and therefore require a topical laryngeal spray of lidocaine prior to intubation.
In cats, spray lidocaine once into the larynx to avoid laryngospasm (or or apply lidocaine gel to the endotracheal tube tip ahead of time). Consider the appropriate dose for smaller cats (an insulin syringe may be necessary). Wait 30-60 seconds before continuing.
EXPANDED INFORMATION: Airway complications associated with anaesthesia are almost twice as common in cats (9.4%) as in dogs (4.9%)! Cats are particularly susceptible to tracheal injury or laceration! (Source: A practical guide to endotracheal tube selection in small animal practice - Veterinary Practice (veterinary-practice.com))
(3) Insert the catheter into the tongue and epiglottis, towards the vocal folds. Use the bevelled tip to pass through the vocal folds (you will need to rotate the catheter slightly); ease it into the trachea until the tip is midway between the larynx and the thoracic inlet (you have pre-measured this before induction so you should know how far to insert it).
You must visualise the tube entering the trachea; do not blindly attempt intubation as this can easily lead to serious airway complications!

Position for lateral intubation; image credit: https://www.theveterinarynurse.com/content/clinical/endotracheal-intubation-of-small-exotic-mammals,;
(4) Secure the tethered gauze around the tracheal tube and tie it to the patient (behind the ear or on top of the muzzle, depending on your procedure).
(5) Inflate the sleeve and then auscultate the breath sounds on both sides of the chest. Have the affected pet lie on his or her side. Be careful to fully inflate the sleeve and not over-inflate it (cats are particularly susceptible to tracheal injury from over-inflation of the cuff).
EXPANDED INFORMATION: Proper inflation of the cuff is essential. Over-inflation can lead to high pressure, which can impede blood flow in the mucosa of the trachea, leading to tracheal necrosis or even tracheal tears; under-inflation can lead to an incomplete seal.

The cuff pressure, which should be 20 to 30 cm H2O, can be inflated and measured using a manometer or a digital cuff inflation syringe (AG Cuffill, above) (Hung et al., 2020). Different types of cuff inflation syringes (Tru-Cuff) contain a colour-coded pressure indicator;
Image credit: A practical guide to endotracheal tube selection in small animal practice - Veterinary Practice (veterinary-practice.com).
(6) Connect the tube to the anaesthetic machine. After connecting the tube, I usually take a few assisted breaths. Listen carefully for any air leaks as you breathe. If you hear air, make sure the cuff is properly inflated. The tube should always fit well and should not be too loose in the trachea so that air should not escape once the cuff is inflated.

(7) Before you start connecting the rest of the monitoring equipment, you can connect the pulse oximeter meter. This is the fastest way to determine oxygen levels and heart rate when you begin.
Don't forget to monitor catheter/cannula status throughout the procedure. If you move the patient during the procedure (turning sideways, etc.), disconnect the catheter from the machine and then reconnect it after the move. Take special care to ensure that the cannula remains inflated and that the tube does not kink.

4. Catheter removal
(1)After completing the initial procedure (which requires anaesthesia and intubation), turn off the flow of inhalation anaesthetic but continue to administer oxygen until the animal is ready for extubation. Loosening the anchoring tether on the tube allows for quick removal of the tube when the animal begins to swallow.

(2) When the swallowing reflex returns, the trocar must be deflated before removing the tube. Removing the tube with the sleeve inflated can cause trauma to the trachea and larynx, and healthcare professionals should avoid this infraction.
If there is a large accumulation of fluid around the catheter that cannot be effectively aspirated, the trocar can be partially deflated and gently removed to avoid aspiration of fluid proximal to the trocar.

(3) In the semi-conscious state after anaesthesia, the animal may die due to airway obstruction, so the condition of airway obstruction must be carefully monitored until the animal has fully recovered.
Adjust the position of the animal so that the head, neck and tongue are extended and monitor it until fully conscious. If problems occur, immediate reintubation may be necessary.
Conventional dogs: recovery of jaw tone, eyelid reflex, swallowing reflex and tongue retraction reflex/ (conventional cats: recovery of eyelid reflex and partial jaw tone, no significant tongue retraction reflex), the tube can be removed.

IV.Analysis and Solution of Common Abnormal Situations

1. Poor visualisation: In order to improve the visualisation, try the following methods:
(1) Adjust the position - make sure the head is straight in the midline and the neck is not abnormally curved;
(2) Consider whether the epiglottis is stuck on the soft palate - if so, use the ETT to gently remove it;
(3) Check and adjust the position of the laryngoscope and tongue - ensure that the tongue is fully pulled out and that the tip of the laryngoscope is pushing down on the base of the tongue;
(4) Ask the assistant to provide some ventral traction on the neck;
(5) If there is additional soft tissue, a second laryngoscope or some tongue depressors held by a second assistant may be required;
(6) A swab or suction may be used to remove excess saliva or liquid material from the mouth.

2. Inability to push the catheter into the glottis: If the glottis can be visualised but the ETT cannot be pushed in, the following can be tried:
(1) Use a smaller size ETT;
(2) aim at the ventral side of the vocal folds using the bevelled side of the tube
(3) Assist intubation by guiding the catheter (mainly used to guide other medical devices, such as guidewire, catheter, etc.) and use it to intubate first, then push the ETT into the trachea.

3、Abnormal intubation problems persist:
(1)If intubation is still unsuccessful, a more experienced physician or medical assistant should take over the intubation operation.
(2)If intubation remains unsuccessful, but the patient is breathing normally and maintains good oxygen saturation, they should be allowed to recover from anaesthesia under close monitoring and a revised intubation plan should be developed.
A situation in which intubation proves difficult and the patient becomes hypoxic (or rapidly becomes hypoxic) is known as a ‘Cannot Intubate, Cannot Ventilate’ (‘CICV’) situation and is an emergency. Emergency tracheotomy should be considered and the patient's vital signs should be closely monitored as they may rapidly progress to cardiopulmonary arrest.

4. Short head obstructive airway syndrome
In the specific case of healthcare professionals dealing with short-headed patients with short-headed obstructive airway syndrome (BOAS), initial visualisation may be obstructed by the soft palate or surrounding pharyngeal tissues, and access to the vocal folds may be more difficult than usual to the extent that their reduced size and lack of space prevents visualisation.
In such cases, the affected pet should have been pre-oxygenated and prepared with additional supplies, including a smaller than expected ETT.If the soft palate continues to obstruct visualisation after detaching from the epiglottis, it can be secured dorsally using a wooden tongue depressor. A tube-centre needle can then be fed through the ETT, and this smaller diameter tube can be used to guide the patient's intubation before the ETT is fed into the trachea.
EXPANDED INFORMATION: Short Head Obstructive Airway Syndrome (BOAS) refers to a syndrome of airway obstruction symptoms commonly seen in certain dog breeds (e.g., Bulldogs, Pugs, etc.) due to craniofacial structural abnormalities. This condition usually presents with symptoms such as difficulty breathing, snoring, coughing, and poor exercise tolerance. Some of the features of short-headed dogs, such as their flattened facial structure and narrow airways, make them susceptible to obstruction when breathing. Appropriate treatment may include weight management, surgery, or an improved living environment.

Case Source: Endotracheal Intubation of Rabbits Using a Polypropylene Guide Catheter (Article) | JoVE; https://app.jove.com/t/56369/endotracheal- intubation-rabbits-using-polypropylene-guide ;

1, Anaesthesia:
(1) Anaesthetics (e.g., 35 mg/kg ketamine and 3.5 mg/kg xylazine hydrochloride) are injected intramuscularly into the quadriceps muscle of the hind leg of an approximately 4-month-old, 3.0-3.5-kg New Zealand White rabbit using a 3 mL syringe with a 5-gauge needle.
Note: This anaesthetic regimen is a commonly used combination of injections in young, healthy, pathogen-free study rabbits, but different regimens may be more appropriate depending on the clinical situation and/or experimental goals.
(2) After the rabbit is anaesthetised, a sterile ocular lubricant is applied to the eyes to protect the corneal tissue.

2. Tracheal intubation
(1) Measure the tracheal tube from the incisors to the thoracic inlet to determine the appropriate length for insertion of the tracheal tube. Place the end of the tube at the level of the thoracic inlet, making sure that the opening of the tube crosses the larynx but is located cranially at the bifurcation of the trachea.
(2) Place the rabbit on the preparation table with the head slightly protruding from the edge of the table. The rabbit should be squarely positioned with the head and spine aligned in a straight line.

Sternum positioning for tracheal intubation

The rabbit is placed in a chest-down position with the forelimbs extended to the edge of the table. The hind limbs are extended backwards and the rabbit's head is aligned with the spine. The left hand is used to lift the head for extension in preparation for insertion of the laryngoscope.
(3) Lift the head and use a gauze pad to pull the rabbit's tongue to the side of the rabbit's right lower incisor (in the interdental space), taking care to avoid trauma from the incisor.
(4) Grasp the laryngoscope and blade with the right hand and place it into the mouth behind the incisors, entering from the left side of the rabbit.
(5) Run the tip of the blade along the roof of the mouth in a caudal direction until the soft palate can be seen.
(6) Tilt the laryngoscope forward with the right hand (bend the wrist and hold the laryngoscope downward) while extending the rabbit's head back and stretching the neck forward to keep the airway open and view the larynx. Use this method to gently press the base of the tongue to move the epiglottis and expose the vocal folds. To prevent oxygen desaturation, keep the neck extended as described to ensure that the airway remains open during intubation.

Insert the laryngoscope:
A. Insert the laryngoscope into the left interspace with the right hand and follow the curve of the palate with the tip of the blade until the soft palate is visible. Once the soft palate is visible, tilt the laryngoscope forward while tilting the rabbit's head back and extending the neck forward to view the larynx.

B. Pharyngeal visualisation.
(7) Release the tongue and switch the laryngoscope blade to the left hand to free the right hand for placement of the guiding catheter.
(8) While holding the laryngoscope and maintaining a view of the vocal folds, pass the rounded tip of the guiding catheter through the opening in the larynx. The guide catheter will stop naturally at the tracheal bifurcation.

Guide the advancement of the catheter:

(A). After switching the laryngoscope blade to the left hand, the right hand advances the rounded tip of the catheter into the laryngeal opening.

(B). The catheter was passed through the vocal cords under direct visualisation. The rabbit is placed in a chest-down position with the forelimbs extended to the edge of the table. The hind limbs were extended backwards and the rabbit's head was aligned with the spine. The left hand is used to lift the head for extension in preparation for insertion of the laryngoscope.
(9) While maintaining control of the guiding catheter, the tracheal tube is grasped and threaded onto the catheter, taking care not to apply force to the catheter by jamming the catheter connector. Advance the tube until there is resistance to the vocal cords.