Tracheal intubation is simple in most domestic species of animal, so an endotracheal tube is almost invariably used in preference to a mask for connection of the breathing circuit to the patient's airway.

Benefits of endotracheal intubation

• Assists in maintaining a patent airway.
• Prevents aspiration of saliva or regurgitated gastric contents.
• Seals the respiratory system and breathing circuit.

Types of endotracheal tube

Magill tube
Magill tubes are those most commonly used in veterinary anesthesia. They are curved and have a bevelled tip which facilitates passing the tube through the larynx. They are available in a variety of sizes from 3 mm to 40 mm internal diameter (the internal diameter, ID, being the standard method of referring to the size of tube), accommodating animals ranging in size from cats to large horses.

Magill tubes are manufactured either plain or with an inflatable cuff. Plain tubes are used in veterinary anesthesia when the volume of the cuff would hinder insertion of a tube of sufficient internal diameter, as in very small animals (e.g. cats and birds).

Two types of cuff are commonly used. The traditional high pressure, low volume cuff operates by distending a rubber balloon around the tip of the tube, thus sealing the trachea. This carries the risk that the pressure within the cuff can cause trauma to or necrosis of the tracheal wall.

The more modern low pressure, high volume cuff has a much greater volume than the traditional cuff, and so requires a lower inflation pressure to produce a seal. There is, therefore, less risk of trauma to the trachea. However, there is a danger that, if the cuff is over-inflated, rupture of the trachea can occur. It is important that only very low pressures should be used to inflate these cuffs.

Magill tubes are invariably supplied too long and are intended to be cut to the correct length: this is generally taken to be the distance from the tip of the nostrils to the point of the shoulder. If the tube is too long, endobronchial intubation can easily occur (see below). In addition, respiratory dead-space is increased if the endotracheal tube is too long. To minimize dead-space, the connecting piece between the endotracheal tube and the anaesthesia machine should be kept as short as possible.

The Cole tube

This was designed for emergency use in pediatric anesthesia, but has been used in veterinary anesthesia, particularly in cats and horses. The intention of the design is that the shoulder of the tube should impact in the larynx and so provide a gas-tight seal. In practice, it is found that any degree of movement or intermittent positive pressure ventilation tends to dislodge the tube, so it is not a very satisfactory tube for routine use.

Materials

• Red rubber--less used than in the past because of concerns over cross-infection.
• Polyvinyl chloride--disposable, intended for single use only. They usually incorporate a radiopaque strip to aid visualization. Although they are expensive when used only once, they will tolerate some degree of re-use.
• Silicone rubber--designed for repeated use. They have relatively thick walls, so a tube of smaller internal diameter must be used. They are extremely floppy which makes their introduction difficult unless a stilette is used.
• Latex--little used since it is very soft and perishes rapidly. Its main use is in armored tubes, but has generally been superseded by silicone rubber.

Complications of endotracheal intubation

Esophageal intubation may occur if the tube is not seen to pass between the vocal folds. It can usually be recognized by: an unusually small degree of expansion and contraction of the breathing bag in time with the animal's breathing (some change in volume of the bag is to be expected because of changes in esophageal pressure over the respiratory cycle). respiratory sounds audible around the mouth. occasionally, cyanosis due to failure to provide an oxygen-enriched breathing mixture. inability to provide a gas-tight seal when inflating the cuff of the tube, due to the distensibility of the esophagus. failure of the plane of anesthesia to deepen as would be expected if normal uptake of the inhalation agent were taking place. There may sometimes be difficulty in deciding whether the tube has been correctly placed: if there is any doubt, the tube should be withdrawn and re-introduced.
Endobronchial intubation occurs if too long a tube is used and inserted into one of the mainstem bronchi. The un-intubated lung does not contribute to gas exchange and the large volume of blood flowing through this lung results in a substantial right to left shunt. Signs are those of arterial hypoxemia, including cyanosis and labored breathing. In addition, uptake of the inhalation anesthetic agent may be impaired, resulting in an unexpectedly light plane of anesthesia. This problem may be avoided by trimming of the tube to the correct length and securing it firmly around the muzzle or lower jaw of the patient. If too long a tube is used and it is tied around the back of the patient's neck, movement of the tube during surgery may move the tip of the tube into a bronchus.
Impaction of the tip of the tube against the tracheal wall may result in respiratory obstruction, particularly where the trachea contains a sharp bend, such as the thoracic inlet.
The Murphy eye, incorporated into many modern tubes, permits airflow to take place even if this has occurred.
Herniation of the cuff over the lumen of the tube may happen if the cuff of an old, perished tube is over-inflated. This, again, will cause respiratory obstruction.
Compression of the lumen of the tube by the cuff may be caused by over-inflation of the cuff or by gradual diffusion of nitrous oxide onto the cuff during the course of anesthesia.This problem is more common when silicone rubber tubes are used.
Stretching of the tracheal wall may be caused by over-inflation of the cuff. This may lead to tracheitis, pressure necrosis of the tracheal wall, or tracheal rupture.
Fires are a danger associated with the increasing use of lasers for airway and oral surgery. Steps which may be taken to reduce this extremely serious hazard include: Using special laser tubes, which may be made of jointed metal or clear plastic (with no radiopaque strip). Wrapping exposed portions of the tube with aluminum tape. Use of helium-oxygen mixtures which are less supportive of combustion than oxygen alone or oxygen-nitrous oxide mixtures.

Laryngeal masks