(2013). Mechanical ventilation and respiratory mechanics during equine anesthesia. Vet. Clin. North Am. Equine. Pract. 29: 51–67.
4 Van Loon, J.P., DeGrauw, J.C., and Oostrom, H. (2018). Comparison of different methods to calculate venous admixture in anesthetized horses. Vet. Anaesth. Analg. 45: 640–647.
Evaluation of the Respiratory System
Tom Doherty
A Patient history
Should include information on the following:Nasal discharge.Coughing.Abnormal lung sounds.Increased respiratory rate and effort.
B Physical examination
The rate, rhythm and character of respiration should be determined.
Observe the horse from all sides to assess bilateral symmetry and the thoracic and abdominal components of respiration.
An increase in the abdominal component of respiration may signify recurrent airway obstruction (heaves).
A reduced thoracic movement is a feature of acute pleuritis.
Assess airflow through each nostril to check the patency of the nasal passages.Closing each nostril in turn and assessing airflow can verify an obstruction.Airflow can be assessed by placing one's ear close to the nostril.
Abnormal odors usually signify anaerobic infections (e.g. sinus, dental, lung abscess).
The pharynx and larynx should be palpated externally for gross abnormalities that may affect airflow or intubation.
Auscultation
In adult horses, it is often difficult to hear lung sounds.
It may be necessary to “amplify” the sounds by making the horse “re‐breathe” prior to auscultation.
Lungs sounds are generally audible in the foal.
Airway Management
Tom Doherty
Airway management includes:Maintaining airway patency.Protecting from aspiration.Providing adequate oxygenation and ventilation.
Intubation of the trachea, either via the oral or nasal passages, requires knowledge of airway anatomy.
It must be remembered that the horse is an obligate nasal breather.
Although it is generally relatively easy to intubate the trachea of the horse, it is important to recognize situations when it may be difficult.
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