I Larynx
A Function
The primary function of the larynx is to protect the airway by preventing the entry of food and foreign materials.
The cricoarytenoid dorsalis is the only muscle of the larynx which abducts the arytenoids and opens the rima glottidis.
Phonation is the secondary function of the larynx.
B Innervation
Motor innervation to the cricoarytenoid dorsalis is provided by the recurrent laryngeal branch of the vagus nerve.
C Recurrent laryngeal nerve neuropathy
Occurs primarily on the left side.
Is relatively common in larger breed horses.
Results in an inability to fully abduct the arytenoid cartilages.
Cases of recurrent laryngeal neuropathy generally do not exhibit signs of airway embarrassment at rest.
D Iatrogenic laryngeal neuropathy
May result from depositing anesthetic drugs adjacent to the recurrent laryngeal nerve during an attempted jugular vein injection.
Nerve paralysis can occur with perivascular injection of α2 agonists and local anesthetics.
This situation, while temporary, may cause severe airway obstruction necessitating passage of a nasotracheal tube or a tracheostomy.
Horner's syndrome (ptosis, miosis, enophthalmos) may also occur as a result from blocking sympathetic fibers in the vagosympathetic trunk.
E Hyperkalemic periodic paralysis (HYPP)
Can result in spasm or paralysis of the laryngeal and pharyngeal muscles.May be accompanied by upper airway noise.
In foals, milk discharging from the nostrils may be due to HYPP.
II Assessment of airway
A History
If intubation was difficult previously, determine if the reason has been resolved or if it was due to anatomical malformations.
B Physical examination
It is not possible to directly visualize the larynx and pharynx due to the shape of the horse's head and the minimal opening of the mouth. However, these structures can be visualized using an endoscope.
Palpation of the upper trachea and intermandibular space will give an indication of swelling or increased sensitivity.
Pharyngeal swelling (e.g. from abscess formation) may cause obvious signs of airway obstruction.
Guttural pouch tympany in foals may result in distortion of the pharynx.
Dysphagia resulting from swelling of the tongue or pharynx may indicate a difficult intubation.
In cases of mandibular fracture, use of a mouth gag may be contraindicated and nasal intubation or a temporary tracheostomy may be necessary during surgery.If nasal intubation is to be performed, determine if the nasal passages are patent by assessing airflow at the nostrils as described above.
C Situations in which difficulty is to be expected
Recurrent laryngeal neuropathy
To prevent damage to the adducted arytenoid cartilage it is usually necessary to use a slightly smaller sized endotracheal tube.A smaller tube will also facilitate the surgical approach and may obviate removal of the tube, intraoperatively, for surgical assessment.
Pharyngeal abscessation and lymphadenopathy
May cause misalignment of oral, laryngeal, and pharyngeal structures.
III Airway equipment
While it may be considered ideal to intubate the airway at all times in the anesthetized horse, it is not routinely practiced under field conditions for procedures of short duration.
Airway obstruction is uncommon in the non‐intubated horse during short procedures.
A Mouth gag
It is not absolutely necessary to use a gag, but its use facilitates 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
