Anaesthesia for the Brachycephalic Patient
World Small Animal Veterinary Association Congress Proceedings, 2017
Louise O’Dwyer, MBA, BSc (Hons), VTS (Anesthesia & ECC) DipAVN (Medical & Surgical), RVN
Clinical Support Manager, Anaesthesia & ECC, VetsNow, Manchester, UK

Brachycephalics are commonly encountered within veterinary practice and require special consideration for anaesthesia due to the anatomical abnormalities which feature in these breeds. These abnormalities include: stenotic nares, an elongated soft palate, laryngeal collapse, hypoplastic trachea, and laryngeal saccule eversion. These abnormalities have been grouped together into what is known as brachycephalic syndrome.

Anaesthesia of dogs suffering from brachycephalic obstructive airway syndrome (BOAS) should be managed using a minimal sedative premedication, pre-oxygenation and rapid intubation following induction. The problem with these dogs is often in the recovery period, as they reach a point where they are sufficiently ‘light’ that they will start to chew the ET tube, but not sufficiently awake that they can adequately maintain a patent airway. It is essential in these patients to realise a rapid recovery with minimal hangover, so appropriate drugs should be selected to achieve this - this means avoiding long-acting drugs (such as acepromazine) or using only very low doses, and perhaps choosing a maintenance agent with a fast recovery profile (e.g., sevoflurane; although we use desflurane for these cases, as recovery is even faster).

Irrespective of the species, upper airway abnormalities result in a reduction in airway diameter and an associated increase in upper airway resistance. To compensate for the latter, a greater negative intrathoracic pressure is created to generate adequate inspiratory airflow. In addition to the increase in the work of breathing, the dynamic pressure changes can further exacerbate the collapse of upper airway structures into the air passages and further increase the airway resistance. In severe cases, airway dysfunction is associated with inflammation and oedema of the pharyngeal tissues. In addition to the respiratory problems, cardiovascular and gastrointestinal abnormalities may be present in brachycephalic animals.

Of the abnormalities listed above, the most commonly seen with brachycephalic breeds are stenotic nares and an elongated soft palate. Stenotic nares is a condition in which the nostrils are malformed. These nostrils are narrow and sometimes collapse inward during inhalation making it difficult for the patient to breathe through their nose. An elongated soft palate is a condition where the soft palate is too long and the tip of it protrudes into the airway and interferes with inspiration of air into the lungs. Brachycephalic breeds tend to learn to compensate for these respiratory insufficiencies, but sedation and anaesthesia remove these compensatory mechanisms. It then becomes the job of the anaesthetist to monitor and protect the airway.

Adequate pre-anaesthesia planning for these patients is essential. The aim of the preoperative evaluation is to determine if there is any disease present that will affect the uptake, action, metabolism, elimination, and safety of the anaesthetic. Primarily the cardiopulmonary, nervous, renal and hepatic are the systems of greatest concern. The history and physical examination are the best determinants of disease. Laboratory tests should only be performed on the basis of history/physical examination. It has been shown that the use of extensive laboratory screening has not improved outcome in human or veterinary medicine.

The reference ranges, ‘normal’ for laboratory tests are presumed to be within ±2 standard deviations of the mean, and therefore 5% of normal animals fall outside this range. The upper and lower values do not represent a cut off between normal and disease. Indeed, a normoglycaemic diabetic, or an animal with cirrhosis with normal hepatic enzymes and bilirubin will seem unremarkable on screening and therefore fall into the ‘normal’ category when that is far from the case.

Brachycephalic breeds are particularly prone to airway obstruction during the perianesthetic period. They are prone to obstruct and die if left unattended after having been given sedatives or anesthetic drugs. Short-acting agents that leave little residual drug effect should allow these dogs to wake up rapidly and get back airway control. These cases are high risk and should be labor intensive to ensure a good outcome.

Deep sedation of these patients can be associated with excessive relaxation of the upper airway muscles and worsened airway obstruction. Unless a patient is aggressive or dangerous to you, use lower doses of premedications. Also note that analgesic agents should always be used for surgical procedures. Opioids are the most frequently used pre-anaesthetic analgesic agents. Opioids are not contraindicated simply because the patient is brachycephalic. Although it is thought that opioids cause respiratory depression, this is more of a dose-dependent issue. Opioids commonly used for pre-medication include: methadone, morphine, and buprenorphine. The premedication also frequently involves a sedative component in the form of an alpha-2 agonist such as medetomidine, a tranquilizer such as acepromazine, or a benzodiazepine such as diazepam or midazolam. Unlike phenothiazines and benzodiazepines, medetomidine will also provide analgesia. When combined with other medications in the premedication, dexmedetomidine may even provide sufficient analgesia and muscle relaxation for minor surgical procedures to be performed.

After proper premedication has been administered it is recommended that brachycephalic patients be “pre­oxygenated” prior to the administration of induction drugs. Administration of 100% oxygen before induction of anaesthesia prolongs the time to onset of arterial hypoxaemia. This technique increases the body’s oxygen stores, primarily in the functional residual capacity (FRC) of the lungs. Pre-oxygenation should only occur if it is not overly stressful to the patient.

When intubating a brachycephalic patient, expect to use a much smaller endotracheal tube than typically used for other similarly sized patients. Carefully select a wide variety of sizes, but be ready with 2 tubes smaller than what you estimate to be the right size. A laryngoscope is a necessary tool for intubation, as the amount of redundant tissue in the pharynx may reduce the visibility of the laryngeal opening.

While under anesthesia, patients can be maintained with inhaled anesthetic such as isoflurane or sevoflurane in 100% oxygen. Sevoflurane is metabolized faster than isoflurane allowing for a faster recovery. This may be an attractive choice when anesthetizing a brachycephalic patient. All inhalant anesthetic agents produce an agent­ and dose-dependent reduction in myocardial contractility, systemic vascular resistance, and cardiac preload with subsequent reductions in mean arterial pressure (MAP) and cardiac output in a dose-dependent manner; therefore, vaporizer settings should be kept as low as possible, whilst maintaining an appropriate depth of anaesthesia.

Brachycephalic dogs have been shown to have a higher resting vasovagal tone than other breeds of dogs, which may predispose them to bradycardia. They are at risk of an arrhythmia termed sinus arrest. A sinus arrest is a pause between two normal complexes that is greater than two times the normal R to R interval due to lack of sinus node discharge. It will create on the ECG rhythm strip irregular pauses. These can be normal incidental findings, especially in brachycephalic breeds due to increased vagal tone associated with inspiration. Therefore, treatment is usually only started if the patient is clinical due to decreased cardiac output. Frequently, if the pauses are long enough, the heart's natural defense system will “escape” out of the normal pathway and initiate a beat from another region of the heart. These are then called “escape beats” or “escape rhythms” if it is a series of beats. These escape beats can either originate from around the AV node (called junctional escapes) or from the ventricles (called ventricular escapes). These beats can be differentiated from premature beats because they will occur after the normal sinus beat would have occurred.

In addition, dogs with brachycephalic airway obstructive syndrome may also have functional and anatomic abnormalities of the gastrointestinal tract, which may predispose them to regurgitation or vomiting in the perioperative period. The postoperative surgical complication rate of brachycephalic dogs in the perioperative period has been reported to be as high as 12%, with 5% developing severe dyspnoea or death. Overall, the major concerns related to anaesthesia of the patient with brachycephalic airway syndrome are the development of airway obstruction (partial or complete) at any time in the anaesthetic period (from preoperative sedation to full recovery) and the predisposition to bradycardia and regurgitation.

The recovery period is an important time for the anaesthetist to stay vigilant about patient monitoring. Appropriate postoperative medications should be administered, taking into account the level of pain anticipated from the surgery performed. Note that acepromazine has no analgesic properties and is not considered an adequate postoperative medication if a painful procedure has been performed. We want recovery to be smooth and stress free. Because brachycephalics can sometimes desaturate during recovery, a portable pulse oximeter is a useful tool when in recovery. Brachycephalic patients should be recovered in sternal recumbency with their head slightly elevated. Avoid overly aggressive initial stimulation, as this may trigger swallowing only to be followed by a relapse into unconsciousness when the stimulation is removed. It is important to have additional induction agent and additional endotracheal tubes ready in recovery in the event that airway obstruction occurs and re-intubation is needed. Recovering these patients with supplemental oxygen is advisable; an effective option is the placement of nasal oxygen catheter(s) during recovery. A nasopharyngeal tube can be placed and connected directly to an oxygen source to allow delivery of oxygen to the nasopharynx during recovery.

In summary, brachycephalic breeds have anatomical abnormalities that require the anaesthetist to carefully monitor breathing and any airway disturbances. However, proper premedication, vigilant monitoring in the preoperative to recovery stages, as well as a stress-free induction and recovery, can make working with these patients less challenging and more rewarding.

 

Speaker Information
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Louise O'Dwyer, MBA, BSc (Hons), VTS (Anesthesia & ECC), DipAVN (Medical & Surgical), RVN
VetsNow
Manchester, UK


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