Emily McCobb1 and Sheilah A. Robertson2 1 Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, Massachusetts, USA 2 Lap of Love Veterinary Hospice Inc., Lutz, Florida, USA Young animals have physiologic and anatomic characteristics that can make them challenging to anesthetize. In addition to their small size, these patients tend to have limited body system reserves, can be more sensitive to the sedative effects of some agents, such as opioids, and are vulnerable to anesthetic complications, such as hypothermia. Fortunately, with careful attention to these considerations, most pediatric patients can be anesthetized safely and successfully. In cats and dogs, the neonatal period extends from birth to weaning (4 to 6 weeks of age) and the pediatric period can be defined as the period between 6 weeks (which marks the end of the neonatal period) and 12 weeks of age [1,2]. Puppies and kittens can be defined as dogs and cats less than 6 months of age. The most important physiologic differences for foals exist between 48 and 72 h after birth, but they are considered physiologically mature by 4 to 6 weeks of age [3]. Ruminants (cattle, sheep, and goats) and camelids are considered neonatal until 4 weeks of age and physiologically mature by 4 to 6 weeks of age. Because of their relatively small size, lack of physiologic reserves, incomplete immunity, and susceptibility to the stress of handling and separation from their dam, all puppies and kittens should be considered a special category of anesthesia patient until at least 3 months of age. The risks of anesthesia are not increased in young healthy animals undergoing elective procedures which are performed rapidly and efficiently [4]. In contrast, a large study in general veterinary practices found that the risk of anesthetic death in cats weighing less than 2 kg was over 15 times greater than that of cats between 2 and 6 kg [5]. In addition, the risk of anesthetic death was higher in dogs weighing < 5 kg compared to dogs weighing 5–15 kg suggesting size is a risk factor that is independent of age [6]. Foals less than 1 month of age have increased risk of mortality during anesthesia [7,8]. Early spay‐neuter is an accepted practice for population control and is endorsed by the American Veterinary Medical Association [9] and the Association of Shelter Veterinarians [10,11]. The rationale for early spay‐neuter has been reviewed elsewhere [12–15] but, in addition to the benefits for population management, surgery in young, healthy animals is generally associated with good outcomes and, at least in cats, may be less painful [16]. For elective procedures (sterilization), it is preferred that puppies and kittens are ≥ 1 kg. However, puppies and kittens may present at a very early age and under 1 kg for emergency procedures such as abdominal surgery, diaphragmatic hernia, fractures, and other accidental injuries, and good outcomes can be achieved with careful anesthetic and analgesic management. Foals and young ruminants are not commonly anesthetized for castration during the pediatric period, but ruminants may present for routine husbandry procedures such as disbudding. Emergency surgeries in foals such as patent urachus surgery or septic joints are common. These species may present at a young age for the correction of congenital abnormalities (e.g., limb malformations) or with traumatic injuries such as fractures. The unique physiologic features of pediatric cats and dogs have been reviewed [17,18]. Pediatric patients of all species have limited reserves of all body systems and a limited ability to respond to events that challenge homeostasis. The role of a high resting basal metabolic rate in determining the pediatric patient’s response to anesthesia should not be underestimated and can lead to fast metabolism of drugs and the need for redosing. Anatomic characteristics that are relevant to anesthesia include a high body surface area‐to‐mass ratio and a high lean body mass compared to body fat. These features can predispose the pediatric patient to hypothermia, which is perhaps the most devastating complication of anesthesia for young animals. In addition, the tissues are delicate requiring gentle handling. The pediatric heart is relatively small for the size of the patient, ventricular compliance is low, and stroke volume is relatively fixed. Low cardiac reserves are accompanied by a high cardiac index (volume of blood pumped per minute indexed to body size: L/min/m2) leading to an overall decreased ability to compensate for or respond to fluid losses and hypotension. Because the stroke volume of a pediatric patient is relatively fixed, these patients are particularly dependent on heart rate for the maintenance of cardiac output (heart rate × stroke volume) and organ perfusion [17,19]. The neonate is unable to increase cardiac contractility in response to increased demand, unlike the adult [20]. Because cardiac output is determined by heart rate, any decrease in heart rate in a pediatric patient can have serious consequences, and it should be noted that one of the most common causes of bradycardia is hypothermia. In addition to the lack of reserves, the autonomic nervous system, especially sympathetic control, is thought to be poorly developed in pediatric patients leading to poor vasomotor control and responses. Baroreceptors are immature until 12 weeks of age leading to an inadequate response to a fall in blood pressure [17]. Lastly, persistent fetal circulation may be present in some pediatric patients leading to right to left anatomical shunting and hypoxemia, especially if systemic hypotension develops. Like the pediatric cardiovascular system, the pediatric respiratory system is also characterized by a lack of reserve capacity. While tidal volumes are similar to those of the adult animal (10–15 mL/kg), the pediatric respiratory rate is two to three times higher in order to provide an appropriate minute ventilation (tidal volume × respiratory rate) [21]. An important clinical consequence of a faster respiratory rate is a shorter induction time in neonates when inhalant agents are used, although this effect may be counteracted by their high cardiac output and may be less obvious when using today’s less soluble inhalant agents such as isoflurane and sevoflurane. With a high resting metabolic rate, a high oxygen demand, and high baseline minute ventilation, pediatric patients are at increased risk of developing hypoxemia and have limited ability to adapt to decreases in oxygen delivery. The heart is entirely dependent on aerobic metabolism; therefore, any decrease in oxygen supply results in rapid decompensation. The nares and tracheas of pediatric patients are small and susceptible to obstruction; the small airways and alveoli are also prone to collapse [17]. The extremely pliant rib cage increases the work of breathing in pediatric patients which can predispose to ventilation fatigue and further increase the risk of hypoxemia. Ventilation of pediatric patients should be monitored and supported when necessary. Airway anatomy can pose a challenge. The pediatric patient has small nares and the tongue is proportionally large for the size of the mouth. Intubation can be difficult, and when performed, great care must be taken to avoid airway trauma resulting in postoperative swelling and obstruction. However, the small trachea creates increased resistance to airflow, and intubation and ventilatory support may be necessary. By 8 weeks of age most physiologic processes related to renal and hepatic function should be relatively normal, although organ reserves may still be limited. The cytochrome P450 enzyme system is immature after birth but develops during the neonatal period [22–24]. After the age of 6 weeks, there is no rationale for altering drug doses in animals to account for hepatic clearance [25]. Herbivores have better‐developed drug metabolism and excretion at younger ages than do dogs and cats. Hepatic development is largely mature in foals by 2 weeks of age. However, hepatic glycogen stores are low and gluconeogenesis is decreased, factors which increase the risk for hypoglycemia, especially if the patient is fasted for more than a short period. Pediatric dogs and cats and very young foals (under 4 days of age) have immature renal function, which can result in delayed clearance of renally metabolized drugs, potentially increasing their duration of action. Young animals have decreased glomerular filtration rates, decreased renal blood flow, and decreased concentrating ability. Decreased renal clearance rates may also increase the risk of fluid overload in the young patient [24]. The blood–brain barrier is immature in young animals and therefore profound sedative effects may be noted with drugs such as opioids and benzodiazepines. In foals under 3 weeks of age, diazepam is profoundly sedating, and this may in part be due to the increased permeability of the blood–brain barrier. In this age group, benzodiazepines will induce deep sedation and muscle relaxation, with patients often becoming recumbent [26]. In contrast, when used in older foals and adults, benzodiazepines often result in excitement and ataxia but not recumbency. Pain management in young animals is very important in order to prevent permanent changes to the central nervous system [27,28]. The decreased function of the thermoregulatory center contributes to the risk of hypothermia. Young patients have less capacity for hematopoiesis [29]. In addition, they have lower levels of erythropoietin, lower packed cell volume, and fewer circulating red blood cells. This resulting anemia means they are less able to compensate for hemorrhage. Moreover, hematopoiesis does not commence until after 15 weeks of age [30]. Other hematologic variabilities include decreased albumin, blood urea nitrogen, and creatinine, as well as increased phosphorus. Pediatric patients are likely more susceptible to the effects of stress than adults. Studies in neonatal rat pups demonstrate that the increased plasticity of the neonatal brain may increase vulnerability to stress and anxiety disorders later in life [27]. Pediatric animals may be presented for surgery during their critical socialization period. It is essential to protect these patients from the risk of fear imprinting when a bad experience could translate into a lifetime of fearful or fractious behavior during veterinary visits. Puppies and kittens should be gently handled. Littermates should be housed together during transport if possible and should be kept together in the cage until the time of premedication. If still with their dam, they should not be separated from her. Fasting times for younger animals should be short (under 2 h) to account for their decreased glycogen stores.
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Neonatal and Pediatric Patients
Introduction
Definition of pediatric and neonatal periods
Anesthetic risk for young patients
Circumstances warranting anesthesia
Patient characteristics
Cardiovascular system
Respiratory system
Renal and hepatic systems
Central nervous system
Hematologic system
Anesthetic management
Patient preparation
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