Manual of Equine Anesthesia and Analgesia. Группа авторов

keeping the jaws open to allow passage of an ET tube.However, there is generally no need to use a gag in neonatal foals as they have minimal jaw tone.

       The gag is fitted between the upper and lower incisors and care must be taken to avoid pressure on the hard palate.

       A variety of gags may be purchased from commercial vendors or a gag may be fashioned from a piece of PVC pipe (see Figures 4.6 and 4.7).PVC gags are lightweight and unobtrusive.

      B Laryngoscope

       Laryngoscopes are not used in the horse, as direct visualization of the larynx is not possible with this method. However, endoscopes may be used to assist intubation in some circumstances.

      C Endotracheal tubes

       Most ET tubes are made from non‐toxic plastic or silicone and are numbered according to their internal diameter (mm).Figure 4.6 PVC mouth gag used to hold mouth open and protect endotracheal tube.Figure 4.7 Oral speculums used in equine patients: Weingart mouth gag (a), Gunther mouth gag (b).

       Tube selection is generally based on the body mass of the horse.

       It is important that the tube not be too tight a fit for the airway.Most adult, full‐size horses (400–500 kg) require a tube size of 24–26 mm.Larger horses (≥500 kg) require a tube of 26–30 mm or greater.The airway of the newborn foal (40–50 kg) should accommodate a tube diameter of 10–11 mm.A newborn miniature horse breed (~10 kg) will need a smaller diameter tube (6–7 mm).

       There is generally little danger of bronchial intubation in horses.

D Inflatable cuffs

       Inflation of the cuff creates a seal with the tracheal mucosa.

       This allows the airway to be inflated under positive pressure and protects the lungs from aspiration of foreign material (e.g. gastric contents).

       It is important not to over inflate the cuff.

      IV Complications of airway intubation

      A Tissue damage

       To avoid tissue damage, it is important that a gentle technique be employed.

       Even the most seemingly gentle intubation can cause bruising of the tissues of the pharynx, larynx, and trachea.

       A rough technique may result in impaction of the epiglottis into the rima glottidis.

      B Edema

       May result from persistent attempts to pass the tube.

       This causes narrowing of the glottis and hinders air flow.

      C Over inflation of the cuff

       An increase in cuff pressure will be transferred to the capillaries in the tracheal mucosa and may occlude blood flow.Arteriolar capillary pressure is ~30 mmHg, and during ventilation of adult horses cuff pressures of ~60–70 mmHg are necessary to prevent leaks.

       Ischemia may result in necrosis of the airway mucosa.

       Selection of a suitably sized tube will prevent having to over‐inflate the cuff to create a seal.

       Low‐volume cuffs have an inherently high‐cuff pressure once inflated. A problem with low volume, high‐pressure cuffs on equine ET tubes is that the pressure measured at the pilot balloon only reflects the elastic force of the cuff, and not the pressure at the mucosa. Thus, checking the cuff pressure with a manometer will not prevent over inflation.

      D Lubrication

       Lubrication of the tube with a water‐soluble gel will facilitate its passage.

       It is especially important that a lubricant gel be used liberally for nasotracheal intubation.

      V Intubation of trachea

      A Difficulty

       Horses are relatively easy to intubate due, in part, to the poor reflex responses of the larynx.

       Flushing the mouth with water prior to induction prevents food materials from being pushed into the airway (see Figure 1.1).

       Foals often have straw or shavings from the bedding in their mouths, and these should be removed.

B Position

       Intubation is usually performed with the horse in lateral recumbency.

       However, if there is a likelihood of reflux of gastric contents (e.g. colic case) the horse should be kept sternal until the tube is inserted and the cuff inflated (see Figure 4.8).

      C Technique

       Intubation is accomplished blindly.

       The head and neck should be extended and the tube advanced into the mouth toward the pharynx.Avoid rubbing the dependent eye on the ground if the horse is in lateral recumbency.

       At this location, the tube may touch the underside of the epiglottic cartilage, which must now be dislodged from its position dorsal to the soft palate.

       Withdrawing the tube slightly and rotating it, will usually reposition the epiglottis and allow the tube to advance. If not, repeat the procedure until successful.

       Avoid the temptation to force the tube forward.

      D Confirmation of tracheal tube placement

       The tube will advance easily (unless it is too large) if it is in the trachea. If the trachea is held gently in the free hand, the movement of the tube along the tracheal rings can be perceived.

       Gentle compression on the thoracic wall will force air out through a correctly placed tube. However, this is usually unnecessary.Figure 4.8 Intubation of a horse in sternal recumbency if there is a risk of regurgitation and subsequent aspiration of stomach contents.

       Esophageal placement of the tube is easily detected, as the tube does not advance easily and will recoil slightly if the driving hand is removed.The esophagus is a “potential space” and will not be dilated in the normal state; hence, the slight resistance to advancing the tube.

      E Intubation of the difficult airway

       On rare situations, when a horse has signs of distress due to of airway obstruction it may be necessary to perform a tracheostomy and insert an ET tube prior to sedation.

       In the majority of these cases, the passage of an orotracheal tube can be greatly facilitated by using a narrow‐bore tube (e.g. stomach tube) as a guide.

       A suitably sized stomach tube is inserted into the ET tube and advanced forward