Podcast: Respiratory: anesthesia
- Indications for surgery of the respiratory system may include treatment of ethmoid hematoma Ethmoid: hematoma , guttural pouch epyema Guttural pouch: empyema , tympany Guttural pouch: tympany , or mycosis Guttural pouch: mycosis , and of fractures or infection Paranasal sinuses: bacterial sinusitis involving the facial sinuses as well as correction of dorsally displaced soft palate Soft palate: dorsal displacement , and epiglottic entrapment Larynx: epiglottic entrapment .
- In neonates, congenital abnormalities of the turbinates or nasal septum Nasal septum: defect will occasionally require surgery, although the guarded prognosis means this is frequently not an option.
- Tumors and fungal infections of the oronasal region are occasionally presented for surgical excision.
- Surgery of the larynx may be carried out for correction of laryngeal hemiplegia Larynx: hemiplegia , arytenoidectomy Larynx: arytenoidectomy , arytenoid chondritis Larynx: arytenoid chondritis , and correction of failed laryngoplasty Larynx: laryngoplasty .
- Tracheotomy Trachea: tracheotomy to relieve upper airway obstruction may be an emergency procedure.
- Surgical correction of diaphragmatic hernia is occasionally performed and the availability of intermittent positive pressure ventilation (IPPV) Anesthesia: ventilators - overview should be considered essential for this procedure to be safely carried out.
- 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.
- 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.