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Respiratory: anesthesia

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Podcast: Respiratory: anesthesia

Indications

Oronasal surgery

Complications
  • The oronasal mucosa has the potential for profuse hemorrhage.
  • Patients with ethmoid hematoma   Ethmoid: hematoma  or guttural pouch empyema   Guttural pouch: empyema  may present with epistaxis or purulent nasal discharge . A cuffed endotracheal tube   Eyelid: tarsorrhaphy - temporary  is essential to prevent aspiration of blood or purulent material. This should be left in place where possible until the animal has recovered consciousness and is standing, in order to ensure a patent airway.
  • Surgery of the oronasal mucosa can be painful, so good analgesia   Anesthesia: analgesia - overview  eg butorphanol   Butorphanol  should be considered during and after the procedure.
  • Hematology prior to anesthesia is advisable in such cases. In cases of more severe blood loss, a preoperative blood transfusion   Blood: transfusion  may be necessary. It is advisable to blood type the patient for the transfusion   Blood: cross-matching  .
  • Because of the potential for profuse hemorrhage, premedication with fenothiazine drugs, eg acepromazine   Cyclosporine  is inadvisable due to their vasodilatory effects.

  • Assessing blood loss
  • Patients which suffer excessive hemorrhage during surgery can become suddenly hypotensive   Anesthesia: monitoring - cardiac output and blood pressure  . Blood pressure should be monitored   Cardiovascular: blood pressure monitoring  on a continual basis.
  • A sudden decrease in diastolic pressure can indicate that the degree of blood loss due to hemorrhage has exceeded the ability of the patient to compensate by vasoconstriction. In this case, immediate infusion of intravenous fluids is indicated.
  • Blood loss may be monitored by measuring the volume of blood in the surgical suction apparatus, weighing swabs and estimating the volume on the drapes and the floor.
  • The accurate estimation of blood loss is important so the appropriate type and volume of replacement may be carried out immediately.

  • Correcting blood loss
  • Intravenous administration of crystalloid solution   Fluid therapy: overview  during anesthesia at a rate of 10 ml/kg/hour for all patients where there is a risk of hemorrhage.
  • Where the packed cell volume (PCV)   Blood: packed cell volume (PCV)  is normal prior to surgery, the first 25% of blood volume loss may be treated with crystalloid fluids (2-3 times estimated blood volume lost). Total blood volume can be estimated at 75-100 mL/kg with the higher values in thoroughbred horses.
  • Monitoring of the PCV and of the total serum protein are important to ensure the serum protein remains above 3.5 g/dL. This will help to prevent pulmonary edema secondary to hypoproteinemia and can develop when large volumes of crystalloid solutions are administered.
  • Cross-matched blood should also be available where possible for emergency transfusion where blood loss becomes excessive (greater than 25% total blood volume), or the serum protein falls below 3.5 g/dL.
  • Soft palate resection can lead to formation of edema, so the administration of anti-inflammatory drugs, eg phenylbutazone   Phenylbutazone  during or after the surgery is advisable.

Laryngeal surgery

  • Most patients presenting for laryngeal surgery are young, fit thoroughbreds, or performance horses.
  • Fit patients carry their own special anesthetic problems   Anesthesia: monitoring - cardiac output and blood pressure  eg postanesthetic myoneuropathy   Muscle: myopathy - postanesthetic  , but on the whole, the surgical risk is less for this group than for elderly or compromised patients.
  • Since laryngeal surgery is generally an elective procedure, the patients should present in good physical condition, however, as many of these cases will have come from racing stables, a careful clinical assessment should be made to detect the presence of respiratory disease.
  • When the surgery being carried out is an arytenoidectomy   Larynx: arytenoidectomy  , a tracheostomy may be required to improve access to the lumen of the larynx. This may be placed in the conscious horse prior to induction, after premedication with, for example, acepromazine   Cyclosporine  . The advantage of this is that if the tube is placed with the horse anesthetized and in dorsal recumbency, the skin incision may move in relation to the tracheostomy site when the horse stands and flexes its neck.
  • Anesthesia may be maintained using gaseous anesthetics via the tracheostomy tube for arytenoidectomy and other surgical procedures involving the larynx.
  • Acepromazine is a good premedicant   Anesthesia: premedication - overview  because of its long duration of action and its calming effect on the horse.
  • In cases of laryngeal hemiplegia   Larynx: hemiplegia  , or inflammation due to chondritis   Larynx: arytenoid chondritis  , the endotracheal tube may be difficult to pass through the partially occluded larynx. It is useful to have some smaller size tubes available as well as a stomach tube which can then be used as a guide to pass the endotracheal tube.
  • Anesthesia is usually maintained by means of gaseous anesthetics, eg halothane   Halothane  because of the long duration of many surgical procedures involving the larynx. However, the development of more reliable methods of intravenous anesthesia for maintenance   Anesthesia: maintenance - overview  means that these may be used more in the future with less side-effects than are seen with the inhaled anesthetics.
  • After the tieback procedure during laryngoplasty   Larynx: laryngoplasty  , ventriculectomy may be simultaneously carried out. This involves turning the patient from lateral to dorsal recumbency and extubating for a short time. During the period of extubation, intravenous anesthetics should be used to maintain depth of anesthesia and monitoring is particularly important at this time also. Reintubation is carried out as the surgeon provides suction to minimise aspiration of blood.
  • During recovery, it is advisable to leave the endotracheal tube in situor place an airway via the surgical incision until the horse is standing.
  • If a tracheotomy has not been performed, the equipment to perform one should be ready and available in case of emergency respiratory obstruction.

Surgery to the diaphragm

  • Diafragmatic herniation is a rare cause of abdominal crisis, accompanied by respiratory compromise, in the horse.
  • Preanesthetic preparation should include an assessment of the patient's acid-base status, electrolyte balance and the degree of respiratory compromise present.
  • Ventilation-perfusion mismatch, due to compression of lungs by abdominal viscera, may lead to severe hypoxemia and hypercapnia   Anesthesia: monitoring - respiratory management  developing.
  • The conscious horse with a diaphragmatic hernia may only be managing to stay alive due to the maximal respiratory effort which it is employing to breath. Administering an anesthetic will reduce or eliminate those efforts, so the use of IPPV   Anesthesia: ventilators - overview  should be considered essential for these cases.
  • Because the caudal mediastinum in the horse is often fenestrated, bilateral pneumothorax can develop once the abdominal cavity is opened for surgery. IPPV is essential to prevent atelectasis and further pulmonary collapse.
  • Postoperatively, careful monitoring for development of pneumothorax should be carried out. Thoracic auscultation   Thorax: auscultation  and percussion   Thorax: percussion  should be carried out several times daily for the first few days post-surgery and thoracic radiography may be carried out where there is suspicion of pneumothorax.
  • If the pneumothorax present is not interfering with respiratory rate or depth, it may not be necessary to evacuate residual air, but it can be left to be reabsorbed gradually over a period of 1-2 weeks.
  • The prognosis for surgical correction of diaphragmatic hernia in the adult horse is poor. In the foal, the prognosis is slightly better.

Further Reading

Publications

Refereed papers

Other sources of information

  • Hodgson D S & Dunlop C I (1990) General anesthesia for horses with specific problems. In: The Veterinary Clinics Of North America (3), 625-647.
  • Hall L W & Clarke K W (1983) Veterinary Anesthesia. 8th edn. Bailliere Tindall, UK.
  • Pascoe J R (1990) Pathophysiology of upper airway obstruction.In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 213-216.
  • Meagher D M (1990) Paranasal sinuses. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 223-227.
  • Meagher D M (1990) Ethmoid hematoma. In: Current Practice of Equine Surgery. Eds: N.A.White and J.N.Moore. J.B. Lippincott Company, Philadelphia. pp: 227-230.
  • Freeman D E (1990)Dorsal displacement of the soft palate.In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 230-236.
  • Ferraro G L (1990) Epiglottic entrapment. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 236-240.
  • Freeman D E (1990) Guttural pouch empyema. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 240-243.
  • Freeman D E (1990) Guttural pouch mycosis. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 243-249.
  • Freeman D E (1990) Guttural pouch tympany. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 249-251.
  • Ferraro G L (1990) Laryngeal hemiplegia. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 251-255.
  • Tulleners E P (1990) Arytenoidectomy. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 255-261.
  • Pascoe J R (1990) Tracheotomy and tracheostomy. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 261-264.
  • Honnas C M (1990) Diafragmatic hernia. In: Current Practice of Equine Surgery. Eds: White N A & Moore J N. J B Lippincott Company, USA. pp 267-269.
  • Dixon P (1991) Tracheostomy. In: Equine Practice. Ed: Edward Boden. The In Practice Handbooks, Balliere Tindall, UK. pp 81-97.