Klaus Hopster1 and Eileen Hackett2 1 Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, 382 West Street Road, Kennett Square, PA, 19348, USA 2 Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, 930 Campus Road, Ithaca, NY, 14853, USA Maintaining respiratory system function is critical to safe anesthetic management. Hence, surgery of the respiratory tract adds challenges and necessitates cooperation between the surgical and anesthetic team especially when managing horses with pre‐existing respiratory disease. Sedation and anesthesia may further impair respiratory system function, and thoughtful selection of protocols, careful application, and monitoring are imperative. Horses undergoing surgery of the respiratory system have often undergone extensive examination prior to admission. Clinical signs of respiratory disorders are varied, and range in extremes from fulminant respiratory distress to minor performance limiting wind issues. Prior to planning a sedation and anesthetic operative protocol, it is important to consider the history, signalment, physical examination parameters, and diagnostic information available. Upper airway obstruction is typically accompanied by increased airflow turbulence and breathing sounds, in addition to bradypneic breathing patterns. Static disorders that contribute to upper airway obstruction include subepiglottic or nasopharyngeal cysts, nasal septum enlargement, nasal edema, arytenoid chondritis, epiglottitis, and tracheal stenosis. Static disorders are generally identified with flexible endoscopy or via radiographic examination. Conditions that contribute to increased inspiratory negative pressure and dynamic airway collapse include recurrent laryngeal neuropathy, laryngeal dysplasia, epiglottic retroversion, nasopharyngeal collapse, and tracheal collapse. Identification of dynamic disorders often requires exercising endoscopic examination. Assessments of the lower airway for pulmonary and pleural space conditions typically begin with cardiopulmonary auscultation and rebreathing examination. Resting tachypnea or an altered respiratory pattern, especially one associated with increased abdominal effort, could signify lung disease. Diagnostics could include tracheal wash, bronchoalveolar lavage, thoracic ultrasonography, and thoracic radiographs. Arterial blood gas analysis is an important diagnostic element when altered pulmonary function is suspected. Diaphragmatic integrity can be assessed with thoracic ultrasound or radiography. A lung biopsy could be considered if interstitial pulmonary disease is identified. In addition to the primary gas exchange functions of the respiratory system, thermoregulation, local immunity, filtration of inhaled gases, and protection of the respiratory tract from aspiration of swallowed substances are important (David and Marshall 2012). Anatomically and physiologically, the respiratory system can be divided into gas conduction and gas exchanging segments (Weibel 2017). The upper airways of the horse, including the nasopharynx, trachea, bronchi, and bronchioles, facilitate gas conduction into the lungs. In the lungs, gas exchange is dependent on alveolar ventilation, pulmonary perfusion, and gaseous diffusion (Petersson and Glenny 2014). The diaphragm and intercostal muscles provide force needed for ventilation. The pumping action of these respiratory muscles results in large pressure changes in the airway. On inhalation, pressures are negative, resulting in movement of air from outside the nares to inside the lungs (Nason et al. 2012). On exhalation, pressure in the upper airway becomes positive, driving the air out of the lungs against the atmospheric pressure. In certain upper airway regions, rigid support by bone or cartilage is incompatible with other functions, such as swallowing. The larynx and pharynx are therefore supported by upper airway dilator muscles. On inhalation, these muscles begin to contract just before the diaphragm contracts and remain active during inhalation for the purpose of opening and stiffening the airway before subatmospheric pressure is created (Parente 2018). When functioning, these muscles provide appropriate tension to prevent dynamic collapse of tissues during inhalation. Most sedatives, including alpha‐2‐agonists, and most general anesthetics, cause a certain degree of muscle relaxation (Manneveau et al. 2018). This decrease in muscle tone leads to a smaller, more compliant, airway conduction system, resulting in a reduction of airflow and an increase in respiratory work (Petersson and Glenny 2014). Upper airway impedance is further affected by head and neck position. In resting horses, a neutral head position results in an angle between the head and neck >100°. Reducing this angle can increase the work of breathing due to an increase in impedance (Parente 2018). When horses maintain a low head position, as with sedation, resulting nasal edema and narrowing of the nasal passage could further increase airway impedance. Multiple strategies have been developed to overcome the increase in impedance resulting from decreased upper airway diameter during sedation and anesthesia. In some circumstances, endotracheal intubation or tracheotomy could be necessary (Mahmood and Wahidi 2016). Positive pressure ventilation can help reduce the work of breathing, and delivering helium gas mixtures can reduce turbulent flow and airway resistance, improving ventilation and gas exchange. Numerous studies suggest that people with lower airway obstruction benefit from helium gas mixtures during mechanical ventilation (Truebel et al. 2019). Diffusion is the primary mode of gas transport in lung capillaries and due to differences in diffusion efficiency among gases, hypoxemia is often recognized prior to hypoventilation (Spaeth and Friedlander 1967). In amenable non‐intubated or standing equids with recognized hypoxemia, intranasal or mask supplementation with flow‐by oxygen can be attempted. Flow‐by oxygen supplementation efficacy has been evaluated in horses and foals. Healthy foals experience a dose‐dependent increase in inspired oxygen fraction and arterial oxygen partial pressure with nasal cannula insufflation (Wong et al. 2010). Similar increases in inspired oxygen fraction and arterial oxygen partial pressure have been observed in healthy horses and those with lung disease during nasal cannula insufflation (Wilson et al. 2006). A short duration of intranasal oxygen supplementation prior to induction of general anesthesia has been shown to moderately increase arterial oxygenation measured immediately after induction in adult horses undergoing elective surgery (van Oostrom et al. 2017). Multiple lower airway disorders, such as pulmonary edema, acute respiratory distress syndrome, and bacterial or viral pneumonia, can lead to ventilation‐perfusion and diffusion impairments that affect pulmonary gas exchange (Mellor and Beausoleil 2017). As respiratory function is frequently depressed with most sedative and anesthetic agents, co‐existing respiratory diseases can impair function even further (Auckburally and Nyman 2017). Most sedatives and tranquilizers like acepromazine and alpha‐2‐agonists have some, although often minimal, respiratory depressant effects that may be potentiated by other opioid or anesthetic drugs. Alpha‐2‐agonists have been shown to impair pulmonary perfusion and arterial oxygenation, and increase pulmonary shunting in standing, sedated, and anesthetized horses (Marntell et al. 2005). These detrimental effects can be partially limited by adding acepromazine to the sedation protocol, which has been shown to subsequently improve pulmonary perfusion (Marntell et al. 2005). General anesthesia and changes induced by positioning in lateral or dorsal recumbency also contribute to an impairment of ventilation and perfusion matching, leading to worsening respiratory function especially in patients with co‐existing respiratory disease (Hopster et al. 2017a,b). Upper respiratory surgery performed in the standing sedated horse is common and represents a special challenge. The horse must be sufficiently sedated to tolerate the surgical manipulation but must also maintain a safe standing position without significant compromise to breathing. The length and complexity of the surgical procedure often dictates whether sedation is administered as a bolus or variable rate infusion. Sedation is primarily achieved via administration of alpha‐2‐agonists. Combination of alpha‐2‐agonists with opioid agents results in synergistic sedative and analgesic effects (Corletto et al. 2005). The authors recommend placing a jugular catheter prior to surgery, which allows for repeated dosing or infusions, in addition to providing venous access if complications are encountered. Local anesthesia of the surgical site is often necessary prior to surgery in the standing, sedated horse. Common methods of local anesthetic delivery in equine airway surgery include topical mucosal splash blocks, local infiltration, and regional perineural injection. Local anesthetics should be applied approximately 5–10 minutes prior to the surgical procedures. As tracheotomy is typically a short procedure, horses are typically sedated with a single intravenous bolus of xylazine, detomidine, or romifidine. This may be combined with morphine (e.g. 0.05 mg/kg) or butorphanol (e.g. 0.02 mg/kg) to take advantage of the sedative and analgesic synergy of alpha‐2‐agonists and opioids. This combined approach could inhibit the superficial hypersensitivity associated with alpha‐2‐agonists and decrease tracheal sensitivity due to opioid mediated cough‐suppression, resulting in improved patient compliance during tracheal manipulation (Kamei et al. 1989). Low dosages of opioids are recommended when being used for sedation enhancement as these drugs can lead to excitement and mild headshaking and bobbing in horses (Dönselmann Im Sande et al. 2017) which could make these procedures challenging. Local anesthetic application for tracheotomy and tracheostomy is typically achieved via subcutaneous infiltration of 3–4 ml lidocaine (2%) per 100 kg bwt. Local anesthetic can be injected directly along ventral midline in the region of planned tracheotomy, or in a U‐shaped infiltration cranial and lateral to the surgical site. When the tracheotomy is done prior to performing surgery under anesthesia as for example an arytenoidectomy, the endotracheal tube can be introduced through this site prior to anesthetic induction to maintain control of the airway and facilitate mechanical ventilation during recumbency (Figure 4.1). Note that the size of the endotracheal tube may be of a smaller diameter than an orotracheal tube increasing resistance and work of breathing in the spontaneously breathing horse. In horses with life‐threatening acute upper airway obstruction, emergency tracheotomy may be required for treatment. In some cases, surgical preparation and local anesthesia is forgone in the interest of immediate control of the airway. For this reason, we often clip, prepare, and inject local anesthetic in horses in which development of upper airway obstruction is a concern (e.g. prior to recovery following sinus surgery). We have also found it helpful to have a tracheotomy kit, including a scalpel and tracheotomy tube, hung on the stall of horses considered to be at risk for upper airway obstruction, as this can save time and confusion. If a horse has collapsed as a result of airway obstruction, insertion of a nasal or oral endotracheal tube could be lifesaving and potentially more rapid than surgical intervention. In this instance, if multiple personnel are present, and it is determined interventions can be achieved safely, one person could intubate while another could begin the tracheotomy. Figure 4.1 This photograph depicts a draft breed horse with an endotracheal tube positioned via tracheotomy prior to induction of anesthesia. Pre‐placement of the endotracheal tube simplifies connection to the rebreathing circuit during surgical positioning. Multiple surgical upper airway procedures are performed in the standing horse with flexible endoscopic guidance (Hawkins and Andrews‐Jones 2001) using specially designed scissors, hooked knives, electrocautery loops, or where appropriate lasers. Examples of these procedures include epiglottic entrapment correction, subepiglottic cyst removal, ventriculectomy, ventriculocordectomy, auditory tube diverticulotomy, and nasopharyngeal mass removal. Sedation may be performed as previously mentioned. Local anesthetic can be applied as a mucosal splash block by injecting via the endoscopic working channel under visual control, ensuring the area of planned excision is well coated (Figure 4.2). Topical local anesthetic can be delivered to this region without endoscopic control via a nasopharyngeal catheter inserted to the level of the medial ocular canthus (Colbath et al. 2017). Swallowing during nasopharyngeal administration was shown to be important for distribution of the topical agent.
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Anesthetic Management for Surgery of the Respiratory Tract
Introduction
Pre‐procedural Evaluation
Anatomical and Physiological Considerations
Sedation for Standing Upper Airway Surgeries
Tracheotomy/Tracheostomy
Intraluminal Nasopharyngeal and Laryngeal Procedures
Prosthetic Laryngoplasty
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