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Close window to return to IVIS Proceeding of the NAVC North American Veterinary Conference Jan. 8-12, 2005, Orlando, Florida Reprinted in the IVIS website with the permission of the NAVC http://www.ivis.org/ Published in IVIS with the permission of the NAVC Small Animal - Anesthesia ANESTHESIA OF THE PEDIATRIC PATIENT Victoria M. Lukasik, DVM, DACVA Southwest Veterinary Anesthesiology and the Southern Arizona Animal Pain Center Tucson, AZ Puppies and kittens are considered neonates until 4
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    Proceeding of the NAVC  North American Veterinary Conference Jan. 8-12, 2005, Orlando, Florida   Reprinted in the IVIS website with the permission of the NAVC http://www.ivis.org/  Close window to return to IVIS  Small Animal - Anesthesia 53 ANESTHESIA OF THE PEDIATRIC PATIENT Victoria M. Lukasik, DVM, DACVA Southwest Veterinary Anesthesiology and the Southern Arizona Animal Pain Center Tucson, AZ Puppies and kittens are considered neonates until 4 to 5 weeks of age and are classified as pediatric until about 12 weeks old. Anesthesia of neonates may be necessary for urgent correction of congenital anomalies or for necessary (not elective) diagnostics. Anesthesia of pediatric patients may be undertaken for elective early ovariohysterectomy or castration or for urgent procedures. The physiology of neonates and pediatric patients is different in several ways compared to adult dogs and cats. These differences need to be understood to formulate optimal anesthetic plans that incorporate balanced drug combinations and appropriate dosing of those drugs. Postoperative analgesia for elective procedures and peri-operative analgesia for urgent situations are a very important part of the anesthetic experience because a life long over sensitivity (alloydinia) can develop if pain in neonatal and pediatric patients is not managed appropriately. Studies in infant boys undergoing circumcision indicate that a sensitivity to pain can be precipitated at even a few days of age if proper analgesia is not provided. THERMOREGULATION Smaller size, minimal fat reserves, and a higher body surface area to mass ratio all contribute to the development of hypothermia during the entire peri-anesthetic period. Of the three most common anesthetic complications (hypothermia, hypotension, hypoventilation), hypothermia is the easiest to document without the aid of expensive equipment. All that is needed is a hand held thermometer. Rectal temperature is usually 1 °  to 2 °  F lower than core temperature due to loss of muscle tone under anesthesia. It may be lower during procedures that expose the peri-rectal tissues: caudal abdominal, perineal, etc. Tympanic membrane temperatures can be very accurate because the middle ear shares the same vascular supply as the hypothalamus. However, ear thermometers can be technically challenging to properly use in most species. Esophageal readings reflect the temperature of the great vessels. Other methods of monitoring temperature: oral, axillary, and skin surface are not accurate.  Almost all patients that are sedated or anesthetized will lose body temperature. The exceptions are the adult Nordic breeds of dog (Husky, Malamute, Samoyed, etc.) that may actually become hyperthermic under general anesthesia. Some patients develop hypothermia so severe that normal physiology is wrecked. The adverse effects of inadvertent hypothermia include: (1) immune system depression: impaired leukocyte mobility and phagocytosis, (2) decreased T-cell antibody production, (3) depressed non-specific host defenses, (4) post-operative infection rate is increased to three times the normal rate in patients experiencing mild intra-operative hypothermia, (5) coagulopathy, which is independent of clotting factor levels (more severe in factor deficient patients), (6) blood viscosity is increased and sludging can occur, (7) systemic vascular resistance and afterload are increased, (8) myocardium is depressed and more prone to arrhythmias and hypoxia, (9) CO 2  production is decreased and may lead to alkalemia, (10) respiratory drive is diminished, (11) physiologic response to hypoxemia and hypercarbia is blunted, (12) CNS: delayed recovery from anesthesia, confusion, stupor, or coma, (13) hyperglycemia due to catecholamine release, (14) hypovolemia due to cold diuresis (may be seen as profound hypotension after re-warming), (15) MAC is decreased approximately 5% per degree centigrade below normal body temperature: anesthetic overdose may easily occur, (16) drug metabolism can be significantly delayed, and (17) liver metabolism can be greatly decreased, leading to drug toxicity. Skin surface temperature rises and falls with the environmental temperature. Core body temperature is closely regulated by the hypothalamus; however, this regulation is immature and inefficient in pediatric patients. There are three tissue layers designed to insulate the body and prevent heat loss. These consist of the skin, subcutaneous fat, and hair. These layers are more or less efficient in different patients depending upon their thickness. Overall, most pediatric patients lack thickness in all three layers. Heat transfer through the insulating layers and to the environment occurs in two stages. In stage one, heat is transferred from the core to the skin. In stage two, heat is lost to the environment by radiation, conduction, convection, and evaporation. In awake animals, there are several reactions to cold. Behavioral reactions include seeking shelter and curling up. Physiologic reactions also occur. These include piloerection, vasoconstriction, and shivering. Piloerection increases the depth of insulation by forming a stagnant layer of air around the animal. Vasoconstriction of the skin arterioles and arteriovenous anastamoses limits heat loss from the extremities. Shivering increases heat production in all muscle groups. There is also a chemical excitation for heat production. This includes the release of epinephrine, norepinephrine, and thyroxin. Pediatric patients lack the ability to effectively respond to cold using these physiologic mechanisms. The anesthetic drugs effect the thermoregulatory center and all compensatory reactions are further blunted or abolished during sedation and anesthesia. Causes of inadvertent hypothermia include: (1) general anesthesia: all anesthetics decrease the threshold for thermoregulatory vasoconstriction, (2) basal metabolic rate is decreased, (3) muscle tone is decreased, (4) operating room temperature is often well below body temperature, (5) skin prep solutions at room temperature and evaporation, (6) cold irrigation solutions at room temperature and evaporation, (7) IV fluids at room temperature, (8) exposed serous surfaces: evaporation, (9) prolonged surgical procedures: patients become more unstable and continue to cool as anesthesia time increases, and (10) patient becomes wet during anesthesia: urine, flush, or bathed. It is in the best interest of all patients (except those needing deliberate hypothermia) to be kept normothermic pre-operatively, during anesthesia and post-operatively. Post-operative shivering will increase oxygen consumption by as much 200% to 600% at a time when lung and circulatory function may not be optimal. Post-operative shivering also increases intraocular pressure, increases intracranial pressure, and increases wound pain. Hemorrhage may be increased by the disruption of clots. Carbon dioxide production is greatly increased and may cause acidemia. Ventilation may be decreased, leading to hypoxemia (tissue hypoxia). Hypothermia must be differentiated form post-operative pain, which may also cause shivering. Published in IVIS with the permission of the NAVC Close window to return to IVIS www.ivis.orgwww.ivis.org  The North American Veterinary Conference – 2005 Proceedings 54 The prevention of inadvertent hypothermia is more desirable than trying to re-warm patients once they become cold. Effective re-warming cannot happen unless at least 60% of body surface area is in contact with an external heat source. Desirable methods for preventing inadvertent hypothermia include: (1) controlling ambient temperature: keep the OR temperature at least 75 °  F, (2) insulate patients using bubble wrap, plastic wrap, or warm blankets, (3) warm skin prep and irrigation solutions, avoid alcohol (4) warm all intravenous fluids, (5) humidify and heat inspired gasses by using an “artificial nose” like the Humid-Vent ®  (Gibeck; Upplands Vaesby, Sweden) (6) use circulating hot water blankets at 105 °  to 107 °  F, (7) forced air heat exchange blanket like the Bair Hugger  ® (Arizant; Eden Prairie, MN, USA), and (8) keep patients dry or actively dry them post-operatively: hand held blow dryer. There are other available methods for providing an external heat source, but they are not desirable due to the potential for thermal injury or electrocution. Radiant heaters or heat lamps (“French Fry” lamps) cannot be easily regulated and can cause severe thermal injury to the skin. Electric heating pads and electric heating boards can develop hot spots or become wet and shock/electrocute a patient. Hot water bottles can be used provided that they are not above 107 °  F and are removed when they become cool. It is important to monitor a patient’s temperature closely because of the possibility of overshoot. Hyperthermia during surgery or re-warming can occur because the blood vessels in the periphery are vasodilated due to the anesthetic drugs. Heat is easily transferred to the core when peripheral vessels are vasodilated. The adverse effects of hyperthermia are also numerous and can be detrimental to a patient’s well being. CARDIOVASCULAR AND PULMONARY PHYSIOLOGY With the first breath of life, a profound and necessary change in circulatory physiology takes place. Expansion of the pulmonary tissues by inflation of the alveoli creates the supportive structure necessary for pulmonary circulation to occur. Pulmonary vascular resistance decreases dramatically and the exchange of oxygen and carbon dioxide by the lungs commences. There is a much higher metabolic demand for oxygen in the first few weeks of life, approximately three times greater compared to adults. An increased respiratory rate helps to meet this increased demand for oxygen. Any decrease in respiratory rate or depth, which is very common during anesthesia, will have an effect upon tissue oxygenation. Cardiac output is dependant upon heart rate. Induced bradycardia may profoundly affect cardiac output and blood pressure. Increases in preload and afterload are poorly tolerated and blood loss as little as 5 ml/kg can precipitate profound hypotension. Hematopoesis does not effectively begin until two to three months of age, further limiting the pediatric patient’s ability to withstand hemorrhage. Blood loss needs to be prevented by adequate surgical hemostasis or treated aggressively before severe physiologic insult occurs due to tissue hypoxia. PREPARATION FOR GENERAL ANESTHESIA Pediatric patients have minimal glycogen stores in the liver and should be minimally fasted. Approximately four hours off of food is sufficient for gastric emptying. Water should be withdrawn when the pre-medication is given. Laboratory testing for young, healthy patients should include a packed cell volume (PCV), total plasma solids (TPS), blood urea nitrogen (BUN), and blood glucose. This minimal laboratory evaluation is designed to aid in the recognition of disease processes not related to the surgical problem. Premedication with a balanced drug combination is the most desirable. ( Table 1 ) Combining drugs from different classes will enable individual drug doses to be reduced, limiting unwanted side effects while still providing optimal stress reduction and pre-emptive analgesia.  After premedication, patients should be placed in a quiet, warm environment and be observed, but undisturbed, until maximal drug effects have occurred. This may be as long as 60 minutes after SQ injection. The environmental temperature in the pre-surgical holding area should be relatively warm or an external heat source should be supplied because hypothermia is common after sedation. The patient should also be placed on towels or shredded paper to absorb urine or feces. It is important to have all necessary drugs and equipment ready in advance. This includes the appropriate breathing circuit, correct size endotracheal tube and reservoir bag, changing CO 2  absorbent if necessary, leak checking the anesthesia machine, and ensuring an adequate oxygen and liquid anesthetic supply. Being prepared for any complication before drug administration is the key to a successful anesthetic. This includes knowing the dose and route of administration of any appropriate reversal drugs. ( Table 3 ) In general, non-rebreathing circuits (Bain, Norman elbow, Jackson-Reese, Ayers T-piece, etc.) are recommended for patients with lean body weights less than 5 kg and rebreathing or circle systems (Wye, Universal-F, etc.) are used in patients weighing more than 5 kg. Pre-induction support should include pre-oxygenation via facemask, IV fluids, pre-emptive analgesia, external heat source, and proper padding. Removing the rubber diaphragm from the mask may prevent some of the resistance to it. INDUCTION TO GENERAL ANESTHESIA General anesthesia is a reversible process that induces immobilization, muscle relaxation, unconsciousness, and freedom from pain. Induction of general anesthesia in pediatric patients is best accomplished using injectable drugs, rather than by the administration of inhalant anesthetic by facemask.( Table 2 ) Injectable inductions are preferred because they allow a more rapid loss of consciousness, less patient struggling, earlier control of the airway, and less danger of injury to the patient and staff. Popular drugs for IV induction include propofol and the combination of diazepam and ketamine. The author prefers a 2:1 volume ratio of diazepam:ketamine dosed at 0.5 to 1 ml/10 kg (diazepam 0.165 to 0.33 mg/kg and ketamine 1.65 to 3.3 mg/kg) to reduce muscle stiffness. Ketamine causes a central release of catecholamines resulting in tachycardia, increased cardiac output, and increased blood pressure. In catecholamine depleted patients, ketamine will act as a direct myocardial depressant and decrease cardiac output. Etomidate may also be used, especially in patients with cardiovascular instability. Immediately after anesthetic induction, the endotracheal tube is placed and the cuff inflated. Avoid overinflation of the endotracheal tube cuff, as tracheal crush injury or tracheal rupture may occur. The patient’s respiratory rate, heart rate and rhythm, MM color, CRT and other monitoring parameters are checked immediately after anesthesia induction and at intervals of 5 minutes or less throughout anesthesia. Published in IVIS with the permission of the NAVC Close window to return to IVIS www.ivis.orgwww.ivis.org  Small Animal - Anesthesia 55 MAINTENANCE Most patients are maintained on inhalant anesthetics and they are preferred to injectable drugs in pediatric dogs and cats. The most commonly used inhalant anesthetic today is isoflurane. Halothane, sevoflurane, and desflurane are also used with some frequency. All inhalant anesthetics cause some degree of vasodilation, hypotension, myocardial depression, and respiratory depression. Other adverse effects include nausea, vomiting, ileus, and cardiac arrhythmias. Modern inhalant anesthetics undergo very little hepatic metabolism. Elimination is via the lung, so awakening is usually rapid after discontinuing inhalant administration. RECOVERY Patients need to be supported and monitored in the immediate post-operative period. It is also vitally important to provide appropriate analgesia.( Table 4 ) The administration of analgesics to pediatric patients is often overlooked or inappropriately under dosed because these patients do not show overt outward signs of pain due to survival instincts. The need for post-operative analgesia cannot be overlooked because of the pediatric patient’s lack of overt, outward signs of pain. Analgesic protocols should be devised based upon the invasiveness of the surgical procedure and the anticipated degree of pain postoperatively. Analgesics should be provided for a minimum of 48 to 72 hours.  Table 1. Drugs Used for Premedication.  Balanced premedication combinations usually include one drug from each group based upon individual patient needs. Group Drug Dose Route Comments  Anticholinergics Glycopyrrolate 0.01 – 0.02 IM, IV Does not cross blood-brain barrier  Atropine 0.02 to 0.04 IM, IV Therapy for profound bradycardia Tranquilizers Midazolam 0.1 to 0.3 IM, IV IM uptake rapid and complete Diazepam 0.1 – 0.4 IV IV only, IM uptake not reliable Xylazine 0.5 – 2 IM, IV Not in patients under 12 weeks Medetomidine 0.005 – 0.02 IM More potent than xylazine  Acepromazine 0.005 – 0.03 IM Not in patients under 8 weeks or in dehydration Opioid Analgesics Morphine 0.05 – 0.25 IM Emesis common Hydromorphone 0.03 – 0.075 IM, IV Good analgesic Oxymorphone 0.03 – 0.075 IM, IV Good analgesic Buprenorphine 0.01 – 0.05 IM For mild pain only Butorphanol 0.2 to 0.4 IM, IV Very poor analgesia, good sedative properties Table 2. Induction Drugs Drug(s) Dose mg/kg Route Comments Propofol 1 – 4 IV Apnea and hypotension common Diazepam/Ketamine 0.15-0.3/1.5-3 IV Retain laryngeal reflexes Etomidate 1 - 2 IV Good in unstable cardiac patients Table 3. Reversal Drugs Drug Dose mg/kg Route Drug Class Reversed Flumazenil 0.1 IV Benzodiazepines  Atipamezole 0.2 to 0.4 IM Alpha-2 Agonists Naloxone 0.01 to 0.1 IM, IV Opioids: all analgesia reversed Table 4. Analgesic Drugs given SQ to patients less than 4 weeks of age Species Mild pain Dose mg/kg Moderate to severe pain Dose mg/kg Canine Oxymorphone 0.02 – 0.05 Oxymorphone 0.05 – 0.1 Morphine 0.2 – 0.5 Morphine 0.5 - 1 Methadone 0.2 – 0.5 Methadone 0.5 - 1 Buprenorphine 0.005 – 0.01 Fentanyl 0.005 – 0.01 Cats Oxymorphone 0.02 – 0.05 Oxymorphone 0.05 – 0.1 Morphine 0.05 – 0.1 Morphine 0.1 – 0.3 Buprenorphine 0.005 – 0.01 Published in IVIS with the permission of the NAVC Close window to return to IVIS www.ivis.orgwww.ivis.org
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