51Comparative Anesthesia and Analgesia – Community Medicine Settings
Emily McCobb
Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, Massachusetts, USA
Introduction
Over the past 10 years, the scope of medicine practiced within animal shelters and other settings within the animal welfare industry has greatly evolved, as evidenced by the establishment of shelter medicine as an American Board of Veterinary Practitioners specialty in 2014 [1]. Veterinarians working in animal shelters and associated clinics provide care to a variety of patients, including animals in the shelter, newly adopted animals, and animals belonging to members of the public, as well as animals involved in cruelty investigations. Within shelter populations, many animals present with unknown health status and a range of behavior challenges such as fear, anxiety, and aggression. Patients may include victims of animal cruelty or neglect. Spay and neuter of shelter animals prior to adoption is standard policy, and these patients include pediatric and juvenile as well as geriatric patients [2–8]. The care provided must balance the needs of the individual animal with those of the entire population, often while working with budget or equipment limitations.
High‐quality high‐volume spay‐neuter
In addition to caring for shelter animals, many organizations also provide low‐cost spay and neuter services for members of the public [9]. Most organizations employ high‐quality high‐volume spay‐neuter (HQHVSN) practice which the Association of Shelter Veterinarians (ASV) defines as “efficient surgical initiatives that meet or exceed veterinary medical standards of care in providing accessible, targeted sterilization of large numbers of cats and dogs to reduce their overpopulation and subsequent euthanasia [10].” While many spay or neuter patients are young and healthy (ASA I), members of the public may also present higher risk patients such as geriatric or brachycephalic dogs and cats for surgery. Morbidity and mortality rates in sterilization programs are remarkably low [11], due to thorough staff training, short procedure times, and the large numbers of young and healthy patients, although the impact of a balanced anesthesia protocol cannot be discounted. Thoughtful, practical, and reliable protocols for all aspects of the patient’s perianesthetic care are essential to allow safe and efficient processing of large numbers of patients [10,12].
HQHVSN programs can be successful in a variety of settings including standing clinics, “MASH style” or mobile units, and field clinics, and operate both domestically and internationally (Fig. 51.1). In such settings, the available equipment may be limited, and flexibility and creativity are often required to maximize the patient experience while minimizing resource utilization. Available drugs may also be limited. Many HQHVSN programs rely on total injectable anesthesia [13–18].
Clinical situations requiring anesthesia
Whether in the shelter or at a community clinic, a variety of patients present for anesthesia in shelter medicine.
HQHVSN – healthy dogs, cats, and rabbits
A large proportion of the patients in shelter medicine anesthesia are healthy dogs, cats, and rabbits presenting for elective spaying or neutering. It is the standard of care for shelter animals to be sterilized prior to adoption and this policy may even be legally required in some states. In addition, spay‐neuter outreach to local communities as well as Trap Neuter Return (TNR) programs for free roaming cats are essential to reduce shelter intake and thus shelter euthanasia [9]. The ASV has published guidelines for spay‐neuter programs [11,12], and a textbook devoted to HQHVSN practice is also available [19].
Figure 51.1 Mobile spay‐neuter clinic.
Preoperative considerations
Each program determines which patients are suitable candidates for the spay‐neuter clinic, depending on staff training and clinic capabilities. According to the ASV guidelines, a veterinarian should make the final decision regarding acceptance of any patient for surgery based on historical and physical examination findings [10]. Veterinarians must weigh the risks and benefits of anesthetizing patients with mild infectious or noninfectious medical conditions, such as upper respiratory tract disease, parasite infestation, or subclinical heartworm infection against the risk of the animal not returning for surgery. Patient history should include current health status, current clinical signs of disease, current medications and supplements, previous vaccinations, pre‐existing medical conditions, and previous adverse reactions. In addition, clients should be instructed to appropriately withhold food from their animals prior to surgery. Fasting times and other preanesthetic recommendations are the same as for patients of similar species and age presenting for anesthesia in other clinical settings. Clients must be informed of surgical and anesthetic risks and must consent to the anticipated procedures.
Prior to anesthesia, a brief screening physical exam should be performed on every patient with a focus on the cardiorespiratory systems. Animals that are anxious or unsocialized should be sedated as soon as the physical examination is completed. Feral animals are examined after anesthesia induction. Routine preanesthetic laboratory screening is not generally performed in HQHVSN clinics for apparently healthy patients and indeed is likely unnecessary for healthy patients presenting for anesthesia in most circumstances [20]. Routine laboratory screening testing for patients with normal physical exam findings is unlikely to change the patient’s ASA status, recommended anesthetic protocol, or affect patient outcome after anesthesia [20,21]. Using a spectrum of care approach [22], the costs of any diagnostic tests must be carefully weighed against potential benefits.
Drug protocols for HQHVSN
Selecting an appropriate protocol for an HQHVSN program depends on several factors such as the number and type of patients presenting each day, technician skill level, drug availability, and the timing of and competence in performing various surgical and anesthetic techniques. A variety of protocols are safe, humane, efficient, and cost effective but all must provide the following: (1) analgesia, (2) stress reduction or anxiolysis, (3) immobility and muscle relaxation, and (4) controlled, reversible depression of the central nervous system resulting in unconsciousness. Numerous cost‐effective protocols, combining multiple anesthetic and analgesic drugs, including injectable and inhalant agents, exist for achieving balanced anesthesia in pediatric and adult patients [13–17,23–35]. A few examples of common protocols are given in Table 51.1. Anesthetic drugs should be calculated for each individual patient using an actual or estimated (for intractable patients) body weight. The use of a chart that expresses drug doses as a function of body weight may help prevent calculation errors [10]. However, when using a dose chart, caution is advised for patient weights at both extremes of the range provided (i.e., very small and very large patients). In these cases, dosing based on body surface area or metabolic scaling is recommended to improve accuracy (i.e., larger patients usually require lower drug doses on a per weight basis). For situations in which an accurate body weight cannot be obtained prior to drug administration, such as programs serving community cats, safety is increased by using reversible agents, avoiding drugs that result in marked cardiorespiratory depression, and estimating body weight as accurately as possible. Close physical monitoring of the patients following drug administration can also allow timely recognition of adverse physiologic side effects.
Equipment
The anesthesia equipment used in HQHVSN clinics is generally the same as that used in any veterinary practice setting. However, in a high‐volume setting, special consideration should be given to equipment use and maintenance to enhance patient safety. All anesthesia equipment including endotracheal tubes, laryngoscopes, anesthesia machines, and monitors should be prepared and checked daily prior to use [40,41]. A machine safety checklist is available in Chapter 6. Any time the anesthesia circuit is changed (such as switching back and forth from a rebreathing to a non‐rebreathing system), the machine checklist should be repeated to ensure that the circuit has been connected correctly. Anesthesia equipment in high‐volume programs undergoes intense use, often by multiple personnel, which can put additional strain on equipment. Heavily used equipment in high‐volume programs should be serviced frequently. Anesthesia machines and monitors should be maintained in accordance with manufacturers’ recommendations through regularly performed in‐house procedures or outsourcing to equipment service companies [41].
Table 51.1 Common injectable anesthesia protocols used in HQHVSN clinics.
| Protocol | Dose/Recipe | Comments | Reference |
|---|---|---|---|
| Hydromorphone–acepromazine | Hydromorphone 0.1 mg/kg,Acepromazine 0.05 mg/kg IM | Common premedication for dogs; can also be used for catsInduction can be via Telazol®, propofol, or ketamine–midazolam combinations | [36] |
| TTDex | Combine 2.5 mL Butorphanol (10 mg/mL) and 2.5 mL Dexmedetomidine (0.5 mg/mL) into one bottle of Telazol®Administer at 0.035 mL/kg IM to provide surgical anesthesia | Used for cats and dogsDose recommendations range from 0.01 mL/kg to 0.035 mL/kg IM, depending on level of sedation/anesthesia required | [37,38] |
| DKB | Dexmedetomidine 10–20 μg/kg,Ketamine 3–10 mg/kg,Buprenorphine 0.01–0.02 mg/kg IM | Many variations for cats, rabbits, and dogsButorphanol can also be substituted for buprenorphine which provides superior sedation but less analgesia | [17,38] |
| TKX | Combine 4 mL of Ketamine (100 mg/mL) and 1 mL of Xylazine (100 mg/mL) into one bottle of Telazol®Administer 0.25 mL IM per “average” (~ 4 kg) cat | May be used in some feral cat programsDoes not provide lasting analgesia | [39] |
| Quad combination | Ketamine 60 mg/m2,Midazolam 3 mg/m2,Buprenorphine μg/m2,Dexmedetomidine 300 μg/m2 IM | Common protocol for cats in the UKBody surface area = (10.4 × BW0.67)/100 (a smart phone app is available) | [14] |
Carbon dioxide absorbents should be checked and changed regularly; high patient volumes dictate increased frequency for changing and cleaning of canisters. Soda lime should be changed after an appropriate period of use according to canister size. Anesthesia circuits, masks, and endotracheal tubes should be cleaned by soaking in a dilute chlorhexidine solution, taking care to rinse thoroughly. A waste gas scavenging system should be used; both active and passive systems are acceptable. Use of charcoal canisters in high‐volume clinics is discouraged as they are acceptable for only short periods of time, typically < 6–8 h or as determined by the weight gain of the canister, and are variably effective [10,42].
Monitoring and patient support
Each individual patient should be carefully monitored from the time of drug administration until recovery. Identification of changes in vital parameters is essential for accurate assessment [43]. The most reliable means to ensure ongoing patient assessment and safety during anesthesia is vigilant “hands‐on” observation by trained staff [10]. Monitoring should involve assessment of various combinations of vital parameters using an objective means of monitoring in accordance with current veterinary anesthesia monitoring guidelines [44,45]. Options include but are not limited to pulse detection via palpation or Doppler ultrasound, auscultation of the heartbeat, pulse oximetry, capnography, and blood pressure monitoring. The use of pulse oximetry is highly encouraged because this modality provides an objective auditory and visual means of determining the presence of a pulse, pulse rate, and adequacy of oxygenation [46]. Use of pulse oximetry monitors has been associated with a decreased risk of anesthetic death in cats [47].
While some patients will oxygenate adequately while breathing room air, oxygen supplementation, including pre‐ and postoperatively, is recommended for high‐risk patients such as brachycephalic animals as well as those that are frail, ill, or in advanced stages of pregnancy. In addition, oxygen supplementation should be immediately available as needed for all heavily sedated or anesthetized patients. Ventilation can be accomplished in anesthetized patients by connecting the anesthesia machine to a rebreathing circuit with a functional carbon dioxide absorbent or to a non‐rebreathing circuit with appropriate oxygen flow rates. Alternatively, a manual resuscitator (e.g., Ambu® bag) connected to an oxygen source with a regulator can be used as a means of oxygen supplementation and ventilation. Ventilation is compromised when carbon dioxide absorbents are depleted or exhausted; therefore, vigilance in monitoring carbon dioxide absorbent adequacy is necessary. Capnography is a useful tool for judging adequacy of ventilation, detecting carbon dioxide absorbent exhaustion or other causes of rebreathing, and assessing patient circulatory status [48].
General anesthesia may be maintained using a mask, endotracheal tube, or laryngeal mask airway device. When balanced anesthesia is achieved using injectable drug protocols, routine intubation of all patients undergoing spay‐neuter surgery should be considered [10]. However, the benefits of intubation (i.e., protection of the airway, ability to ventilate, and reduced waste gas exposure for staff) should be weighed against the risks, particularly airway trauma, with some species being more likely to sustain tracheal injuries (rabbits, cats). If intubation is performed as a standard part of an anesthetic protocol, having an anesthesia team that possesses the required skills for this maneuver is critical to success [49]. The ability to perform intubation when medically indicated, including rapid intubation in emergency situations, is a requirement for all spay‐neuter programs [10,49–52]. The anesthetist must verify the placement of the endotracheal tube by direct visualization or via capnography. Specific patients that may benefit from intubation include all brachycephalic breeds, overweight or obese patients, animals in advanced stages of gestation, patients with pre‐existing severe upper respiratory disease, and patients for which procedures are anticipated to require a long duration of anesthesia (e.g., greater than 30 min).
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