CHAPTER 23 Anesthesia in the Pediatric Patient
Anesthetizing newborn or very young patients is not uncommon in small animal practice and, when necessary, presents the veterinarian with a unique challenge. Anesthetic procedures may be performed for the correction of life-threatening congenital abnormalities or complications arising during the first weeks of life (e.g., fractures, congenital abnormalities, diaphragmatic hernia), diagnostic procedures, or early spay/neuter surgeries. Cosmetic procedures (e.g., tail docking, ear cropping) may also be performed at this age, but fortunately many of these procedures have become less popular because of ethical and humane concerns.
Several definitions have been given for the neonatal and pediatric phases as they pertain to anesthesia. Small animal patients are considered neonates for the purposes of anesthesia for the first 6 weeks and pediatric patients until 12 weeks old. During the first 12 weeks and beyond, there is a continuum of developmental changes. It is important to consider the impact of these physiologic and anatomic changes on anesthetic management of these patients. In addition, it is important to note that the rate of organ maturity varies among species (dogs, cats), breeds, and individuals; therefore time estimates for organ maturity and development should be viewed only as guidelines and not values with absolute meaning. The organ systems of greatest importance for anesthesia include the cardiovascular, respiratory, hepatic, renal, thermoregulatory, and central nervous systems. Although we normally think of the central nervous system as being relatively immature in neonates, it is now recognized that early nociceptive and painful events can lead to long-term alterations in central pain processing. Therefore it is important that we not only think of the central nervous system in terms of depression and anesthesia but also in terms of pain and nociception.
The cardiovascular system undergoes enormous alterations shortly after birth, from fetal to adult circulation. These changes normally occur without incident in the first days of life, but remnants of fetal circulation may persist (e.g., patent ductus arteriosus, patent foramen ovale), and other congenital heart defects may be detected as heart murmurs during routine auscultation. If a significant murmur is present, additional diagnostics and/or medical intervention may be required before anesthesia.
In general, higher heart rates and lower blood pressure should be expected for the first 4 to 12 weeks compared with adult animals. The cardiovascular system is characterized as a low-pressure system resulting from the lower myocardial contractile mass, decreased ventricular compliance, and a relatively immature autonomic nervous system found in young animals. Normal awake mean blood pressures can be 20 to 40 mm Hg lower than those in adult animals. Consequently lower blood pressures are often tolerated during anesthesia in neonates compared with adults. There are no good guidelines as to what an acceptable blood pressure is in anesthetized neonate and pediatric patients. Current guidelines are based on the lower blood pressure limits for renal autoregulation in adults. The pressure limits for renal autoregulation in neonate and pediatric patients have not been established. However, the authors will often accept mean blood pressures above 40 to 50 mm Hg in neonates and above 50 to 60 mm Hg in pediatrics as being adequate.
The low ventricular compliance may lead to less tolerance for changes (increases and decreases) in preload. Preload is frequently increased by the administration of intravascular fluids and should be maintained without administering excessive fluids to optimize cardiac output and organ perfusion. The immaturity of the autonomic nervous system leads to an impaired ability to change systemic vascular resistance, as well as cardiac contractility in response to cardiovascular challenges (i.e., dehydration and blood loss). Hence the neonate is often incapable of compensating for the vasodilatory effects of commonly used anesthetic drugs (i.e., inhalant anesthetics, acepromazine). In addition, drugs normally used to support the cardiovascular system through their effects on the autonomic nervous system, such as sympathomimetics (dopamine, dobutamine) and parasympatholytics (atropine, glycopyrrolate), are less effective.
Neonates also have higher oxygen consumption, and hence relative cardiac output is greater. Because the neonate and pediatric patient have little means to increase cardiac output through changes in cardiac contractility or alterations in preload, the preservation of cardiac output primarily depends on the maintenance of a relatively elevated heart rate and low system vascular resistance. Drugs that impact these parameters, such as α-2 agonists, are probably best avoided.
Hemoglobin concentrations are usually lower than those of adults because hematopoietic potential does not reach adult levels until around 2 to 3 months. Erythrocyte production is lower with a shorter lifespan. Further, hemodilution is present as a result of an expanding blood volume during that time. However, at birth, hemoglobin levels are actually high with a high percentage of fetal hemoglobin (70% to 80%), and then they decrease as a result of low erythrocyte production.
The respiratory system changes from neonate to adult are gradual but significant. Pediatric patients have a twofold to threefold higher tissue oxygen demand relative to body weight compared with adults. Pediatric animals commonly have a higher resting respiratory rate because of the elevated oxygen demand. The tidal volume of neonates is similar to adults, but the functional residual capacity (FRC) is lower. The reduced FRC may hasten the development of hypoxia during breath holding in neonates compared with adults.
The work of breathing in neonates is increased as a result of elevated airflow resistance caused by the narrow-diameter airways and the pliable chest wall (increased compliance). Neonates are more susceptible to the muscle-relaxing effects of anesthetics. This contributes to respiratory fatigue and hypoventilation. Assisted ventilation is often necessary in pediatric patients, and capnography can be useful for assessing the ventilatory status of the animal. Finally, ventilatory control in neonates is relatively immature with a decreased responsiveness of the carotid body chemoreceptors to hypoxemia. The neonate and pediatric patient is more prone to developing hypoxia and hypercapnia in the perianesthetic period, and careful attention to respiration, proper respiratory monitoring, and preoxygenation before the induction of anesthesia are all recommended to minimize these risks.
The hepatic microsomal and cytochrome P450 enzyme systems are functionally immature for the first 4 to 5 months in pediatric patients. This can lead to slowed elimination of highly metabolized drugs such as nonsteroidal antiinflammatory drugs (NSAIDs) and benzodiazepines. In the case of benzodiazepines, slowed metabolism is likely to be of little clinical significance (for moderately prolonged mild sedation) if only a single dose is used in the perianesthetic period. However, with the repeat administration of drugs with potentially significant side effects (such as NSAIDs), accumulation and toxicities are possible. In general, it is common practice to avoid drugs requiring major hepatic metabolism in neonates or to significantly increase the dosing intervals.
Lower concentrations of plasma albumin are also present for the initial 8 to 12 weeks of life. This can be an important consideration for highly protein-bound drugs, which may exert a greater clinical effect as a result of the higher quantities of unbound or active drug in the plasma. However, this is rarely observed clinically with the commonly used anesthetic-related drugs. This may be in part because most intravenous anesthetic drugs are titrated to effect rather than administered as a single intravenous bolus dose.
Neonates and pediatric patients have little glycogen storage capacity. Therefore excessive fasting before or delays in feeding after anesthesia should be avoided to prevent hypoglycemia. Renal function is also less developed for the first 2 to 8 weeks and is characterized by a decreased glomerular filtration rate, low renal blood flow, and a low concentrating ability. Urine-specific gravity is lower, whereas urine protein and glucose may be higher. In general, this means that during this period of life, animals may not be capable of tolerating large fluid loads and do not have the ability to conserve fluids when faced with decreased intake. Careful attention to hydration status is very important in the perianesthetic period. Because it is sometimes more difficult to assess hydration in neonates based on clinical signs such as skin tenting, serial weight assessments and careful assessment of the volume of fluid administered versus the volume of fluid lost should be made.
Pediatric patients have a large surface area–to–body mass ratio and lack significant insulating body fat, predisposing them to hypothermia. In the perianesthetic period, young patients are even more susceptible to hypothermia because of the effects of the anesthetic agents on the thermoregulatory center and loss of peripheral vasomotor tone. During anesthesia, the production of heat is reduced (decreased muscular activity, inability to shiver), and the loss of heat through conduction, convection, evaporation, and radiation is facilitated. All these factors make hypothermia in the perianesthetic period likely. The use of techniques to minimize heat loss (e.g., warm tables, insulation, minimize use of cold fluids, active rewarming—forced warm air, circulating water blankets) should be used preemptively throughout the perianesthetic period. Side effects associated with hypothermia include cardiovascular alterations (bradycardia, hypotension, decreased cardiac output, and arrhythmias), prolonged recovery times and drug metabolism, as well as increased infection rate and decreased wound healing.
The central nervous system matures progressively during the first 6 to 8 weeks, with the central nervous system of cats maturing more rapidly than dogs. The peripheral nervous system of dogs may take up to a year to fully develop. Very little is known about the functional level (i.e., consciousness, sensory, and motor coordination) of the central nervous system in dogs and cats. For example, initially it was believed that neonates were incapable of feeling pain. However, it is now known that early pain experiences, such as dewclaw removal, may be associated with an exaggerated pain response later in life in some animals. Pain is generally defined as a sensory and/or an emotional experience associated with tissue damage, and perception of this sensation occurs in the cortex of mammals. Because neonates have questionable cortical activity and sensory coordination, it was believed that neonates do not “feel” pain. However, nociception, the physiological components leading to the sensation and the “feeling” of pain, is active when tissue damage occurs at any stage of life. Nociceptive activity alone can lead to a stress response subsequently leading to alterations in many body systems (i.e., elevated sympathetic tone, increased release of stress hormones, increased metabolic rate). In neonates, acute severe or chronic pain can alter immediate, and perhaps long-term, central and peripheral sensory processing, leading to an exaggerated pain response later in life. Therefore it is important to use analgesics for “painful” procedures (i.e., dewclaw removal, tail cropping), even in the very young patient, to limit the effects of nociceptive activity on the developing central nervous system.
Neonates also appear to have exaggerated responses to many anesthetic drugs. It is unclear whether these exaggerated responses are caused by pharmacokinetic alterations as a result of differences in body water and fat composition or related to an immaturity of the blood-brain barrier.