anestesia de paciente pediátrico

8/11/2019 Anestesia de Paciente Peditrico

1/4

Proceeding of the NAVC

North American Veterinary ConferenceJan. 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


2/4

Small Animal - Anesthesia

53

ANESTHESIA OF THE PEDIATRIC PATIENT

Victoria M. Lukasik, DVM, DACVASouthwest Veterinary Anesthesiology and the

Southern Arizona Animal Pain CenterTucson, AZ

Puppies and kittens are considered neonates until 4 to5 weeks of age and are classified as pediatric until about

12 weeks old. Anesthesia of neonates may be necessary forurgent correction of congenital anomalies or for necessary(not elective) diagnostics. Anesthesia of pediatric patientsmay be undertaken for elective early ovariohysterectomy orcastration or for urgent procedures. The physiology ofneonates and pediatric patients is different in several wayscompared to adult dogs and cats. These differences need tobe understood to formulate optimal anesthetic plans thatincorporate balanced drug combinations and appropriatedosing of those drugs. Postoperative analgesia for electiveprocedures and peri-operative analgesia for urgent situationsare a very important part of the anesthetic experiencebecause a life long over sensitivity (alloydinia) can develop ifpain in neonatal and pediatric patients is not managed

appropriately. Studies in infant boys undergoing circumcisionindicate that a sensitivity to pain can be precipitated at even afew days of age if proper analgesia is not provided.

THERMOREGULATIONSmaller size, minimal fat reserves, and a higher body

surface area to mass ratio all contribute to the developmentof hypothermia during the entire peri-anesthetic period.Of the three most common anesthetic complications(hypothermia, hypotension, hypoventilation), hypothermia isthe easiest to document without the aid of expensiveequipment. All that is needed is a hand held thermometer.

Rectal temperature is usually 1 to 2 F lower than coretemperature due to loss of muscle tone under anesthesia.

It may be lower during procedures that expose the peri-rectaltissues: caudal abdominal, perineal, etc. Tympanicmembrane temperatures can be very accurate because themiddle ear shares the same vascular supply as thehypothalamus. However, ear thermometers can betechnically challenging to properly use in most species.Esophageal readings reflect the temperature of the greatvessels. Other methods of monitoring temperature: oral,axillary, and skin surface are not accurate.

Almost all patients that are sedated or anesthetized willlose body temperature. The exceptions are the adult Nordicbreeds of dog (Husky, Malamute, Samoyed, etc.) that mayactually become hyperthermic under general anesthesia.

Some patients develop hypothermia so severe that normal

physiology is wrecked. The adverse effects of inadvertenthypothermia include: (1) immune system depression:impaired leukocyte mobility and phagocytosis, (2) decreasedT-cell antibody production, (3) depressed non-specific hostdefenses, (4) post-operative infection rate is increased tothree times the normal rate in patients experiencing mildintra-operative hypothermia, (5) coagulopathy, which isindependent of clotting factor levels (more severe in factordeficient patients), (6) blood viscosity is increased andsludging can occur, (7) systemic vascular resistance andafterload are increased, (8) myocardium is depressed andmore prone to arrhythmias and hypoxia, (9) CO2production isdecreased and may lead to alkalemia, (10) respiratory drive

is diminished, (11) physiologic response to hypoxemia andhypercarbia is blunted, (12) CNS: delayed recovery fromanesthesia, confusion, stupor, or coma, (13) hyperglycemiadue to catecholamine release, (14) hypovolemia due to colddiuresis (may be seen as profound hypotension after rewarming), (15) MAC is decreased approximately 5% pedegree centigrade below normal body temperatureanesthetic overdose may easily occur, (16) drug metabolismcan be significantly delayed, and (17) liver metabolism can

be greatly decreased, leading to drug toxicity.Skin surface temperature rises and falls with theenvironmental temperature. Core body temperature is closelyregulated by the hypothalamus; however, this regulation isimmature and inefficient in pediatric patients. There are threetissue layers designed to insulate the body and prevent healoss. These consist of the skin, subcutaneous fat, and hairThese layers are more or less efficient in different patientsdepending upon their thickness. Overall, most pediatricpatients lack thickness in all three layers. Heat transferthrough the insulating layers and to the environment occursin two stages. In stage one, heat is transferred from the coreto the skin. In stage two, heat is lost to the environment byradiation, conduction, convection, and evaporation.

In awake animals, there are several reactions to coldBehavioral reactions include seeking shelter and curling upPhysiologic reactions also occur. These include piloerectionvasoconstriction, and shivering. Piloerection increases thedepth of insulation by forming a stagnant layer of air aroundthe animal. Vasoconstriction of the skin arterioles andarteriovenous anastamoses limits heat loss from theextremities. Shivering increases heat production in all musclegroups. There is also a chemical excitation for heaproduction. This includes the release of epinephrinenorepinephrine, and thyroxin. Pediatric patients lack theability to effectively respond to cold using these physiologicmechanisms. The anesthetic drugs effect thethermoregulatory center and all compensatory reactions are

further blunted or abolished during sedation and anesthesia.Causes of inadvertent hypothermia include: (1) generaanesthesia: all anesthetics decrease the threshold fothermoregulatory vasoconstriction, (2) basal metabolic rate isdecreased, (3) muscle tone is decreased, (4) operating roomtemperature is often well below body temperature, (5) skinprep solutions at room temperature and evaporation, (6) coldirrigation solutions at room temperature and evaporation(7) IV fluids at room temperature, (8) exposed seroussurfaces: evaporation, (9) prolonged surgical procedurespatients become more unstable and continue to cool asanesthesia time increases, and (10) patient becomes weduring 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 asmuch 200% to 600% at a time when lung and circulatoryfunction may not be optimal. Post-operative shivering alsoincreases intraocular pressure, increases intracraniapressure, and increases wound pain. Hemorrhage may beincreased by the disruption of clots. Carbon dioxideproduction is greatly increased and may cause acidemiaVentilation may be decreased, leading to hypoxemia (tissuehypoxia). 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.org

www.ivis.org


3/4

The North American Veterinary Conference 2005 Proceedings

54

The prevention of inadvertent hypothermia is moredesirable than trying to re-warm patients once they becomecold. Effective re-warming cannot happen unless at least60% of body surface area is in contact with an external heatsource. Desirable methods for preventing inadvertenthypothermia include: (1) controlling ambient temperature:

keep the OR temperature at least 75F, (2) insulate patientsusing bubble wrap, plastic wrap, or warm blankets, (3) warmskin prep and irrigation solutions, avoid alcohol (4) warm all

intravenous fluids, (5) humidify and heat inspired gasses byusing 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 externalheat source, but they are not desirable due to the potentialfor thermal injury or electrocution. Radiant heaters or heatlamps (French Fry lamps) cannot be easily regulated andcan cause severe thermal injury to the skin. Electric heatingpads and electric heating boards can develop hot spots orbecome wet and shock/electrocute a patient. Hot water

bottles can be used provided that they are not above 107Fand are removed when they become cool.

It is important to monitor a patients temperature closelybecause of the possibility of overshoot. Hyperthermia duringsurgery or re-warming can occur because the blood vesselsin the periphery are vasodilated due to the anesthetic drugs.Heat is easily transferred to the core when peripheral vesselsare vasodilated. The adverse effects of hyperthermia are alsonumerous and can be detrimental to a patients well being.

CARDIOVASCULAR AND PULMONARY PHYSIOLOGYWith the first breath of life, a profound and necessary

change in circulatory physiology takes place. Expansion ofthe pulmonary tissues by inflation of the alveoli creates the

supportive structure necessary for pulmonary circulation tooccur. Pulmonary vascular resistance decreases dramaticallyand the exchange of oxygen and carbon dioxide by the lungscommences. There is a much higher metabolic demand foroxygen in the first few weeks of life, approximately threetimes greater compared to adults. An increased respiratoryrate helps to meet this increased demand for oxygen. Anydecrease in respiratory rate or depth, which is very commonduring anesthesia, will have an effect upon tissueoxygenation. Cardiac output is dependant upon heart rate.Induced bradycardia may profoundly affect cardiac outputand blood pressure. Increases in preload and afterload arepoorly tolerated and blood loss as little as 5 ml/kg canprecipitate profound hypotension. Hematopoesis does not

effectively begin until two to three months of age, furtherlimiting the pediatric patients ability to withstand hemorrhage.Blood loss needs to be prevented by adequate surgicalhemostasis or treated aggressively before severe physiologicinsult occurs due to tissue hypoxia.

PREPARATION FOR GENERAL ANESTHESIAPediatric patients have minimal glycogen stores in the liver

and should be minimally fasted. Approximately four hours offof food is sufficient for gastric emptying. Water should bewithdrawn when the pre-medication is given. Laboratorytesting for young, healthy patients should include a packedcell volume (PCV), total plasma solids (TPS), blood urea

nitrogen (BUN), and blood glucose. This minimal laboratoryevaluation is designed to aid in the recognition of diseaseprocesses not related to the surgical problem. Premedicationwith a balanced drug combination is the most desirable(Table 1) Combining drugs from different classes will enableindividual drug doses to be reduced, limiting unwanted sideeffects 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, untimaximal drug effects have occurred. This may be as long as60 minutes after SQ injection. The environmentatemperature in the pre-surgical holding area should berelatively warm or an external heat source should be suppliedbecause hypothermia is common after sedation. The patienshould also be placed on towels or shredded paper to absorburine or feces.

It is important to have all necessary drugs and equipmenready in advance. This includes the appropriate breathingcircuit, correct size endotracheal tube and reservoir bagchanging CO2 absorbent if necessary, leak checking theanesthesia machine, and ensuring an adequate oxygen andliquid anesthetic supply. Being prepared for any complication

before drug administration is the key to a successfuanesthetic. This includes knowing the dose and route oadministration of any appropriate reversal drugs. (Table 3)

In general, non-rebreathing circuits (Bain, Norman elbowJackson-Reese, Ayers T-piece, etc.) are recommended fopatients with lean body weights less than 5 kg andrebreathing or circle systems (Wye, Universal-F, etc.) areused in patients weighing more than 5 kg. Pre-inductionsupport should include pre-oxygenation via facemask, IVfluids, pre-emptive analgesia, external heat source, andproper padding. Removing the rubber diaphragm from themask may prevent some of the resistance to it.

INDUCTION TO GENERAL ANESTHESIA

General anesthesia is a reversible process that inducesimmobilization, muscle relaxation, unconsciousness, andfreedom from pain. Induction of general anesthesia inpediatric patients is best accomplished using injectabledrugs, rather than by the administration of inhalant anestheticby facemask.(Table 2) Injectable inductions are preferredbecause they allow a more rapid loss of consciousness, lesspatient struggling, earlier control of the airway, and lessdanger of injury to the patient and staff. Popular drugs for IVinduction include propofol and the combination of diazepamand ketamine. The author prefers a 2:1 volume ratio odiazepam:ketamine dosed at 0.5 to 1 ml/10 kg (diazepam0.165 to 0.33 mg/kg and ketamine 1.65 to 3.3 mg/kg) toreduce muscle stiffness. Ketamine causes a central release

of catecholamines resulting in tachycardia, increased cardiacoutput, and increased blood pressure. In catecholaminedepleted patients, ketamine will act as a direct myocardiadepressant and decrease cardiac output. Etomidate may alsobe used, especially in patients with cardiovascular instability.

Immediately after anesthetic induction, the endotracheatube is placed and the cuff inflated. Avoid overinflation of theendotracheal tube cuff, as tracheal crush injury or trachearupture may occur. The patients respiratory rate, heart rateand rhythm, MM color, CRT and other monitoring parametersare checked immediately after anesthesia induction and aintervals of 5 minutes or less throughout anesthesia.


www.ivis.org

www.ivis.org


4/4

Small Animal - Anesthesia

55

MAINTENANCEMost patients are maintained on inhalant anesthetics and

they are preferred to injectable drugs in pediatric dogs andcats. The most commonly used inhalant anesthetic today isisoflurane. Halothane, sevoflurane, and desflurane are alsoused with some frequency. All inhalant anesthetics causesome degree of vasodilation, hypotension, myocardialdepression, and respiratory depression. Other adverseeffects include nausea, vomiting, ileus, and cardiac

arrhythmias. Modern inhalant anesthetics undergo very littlehepatic metabolism. Elimination is via the lung, so awakeningis usually rapid after discontinuing inhalant administration.

RECOVERYPatients need to be supported and monitored in the

immediate post-operative period. It is also vitally important toprovide appropriate analgesia.(Table 4) The administration oanalgesics to pediatric patients is often overlooked oinappropriately under dosed because these patients do noshow overt outward signs of pain due to survival instinctsThe need for post-operative analgesia cannot be overlookedbecause of the pediatric patients lack of overt, outward signs

of pain. Analgesic protocols should be devised based uponthe invasiveness of the surgical procedure and theanticipated degree of pain postoperatively. Analgesics shouldbe provided for a minimum of 48 to 72 hours

Table 1. Drugs Used for Premedication.Balanced premedication combinations usually include one drugfrom each group based upon individual patient needs.

Group Drug Dose Route Comments

Anticholinergics Glycopyrrolate 0.01 0.02 IM, IV Does not cross blood-brainbarrier

Atropine 0.02 to 0.04 IM, IV Therapy for profoundbradycardia

Tranquilizers Midazolam 0.1 to 0.3 IM, IV IM uptake rapid andcomplete

Diazepam 0.1 0.4 IV IV only, IM uptake notreliable

Xylazine 0.5 2 IM, IV Not in patients under 12weeks

Medetomidine 0.005 0.02 IM More potent than xylazine

Acepromazine 0.005 0.03 IM Not in patients under 8weeks 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


www.ivis.org

www.ivis.org

anestesia de paciente pediátrico

Documents