Problems In Pediatrics Anesthesia

Co-ordinator:- Dr. Anshul Jain (M.D)Speaker:- Dr. Sushil kr. patel

Introduction

• The needs of infants and young children differ greatly from those of adults.

• Pediatric patients, especially neonates and infants younger than 6 months of age, have anatomic and physiologic differences that place them at higher risk of anesthetic complications than adults.

• Differences in responses to pharmacologic agents in this population further add to the complexity of administering anesthesia to these patients. Many diseases present exclusively or with greater frequency in this age group.

Developmental Considerations• A preterm infant is one born before 37 weeks gestation; a postmature infant is one

born after 42 weeks gestation. • Any infant born at less than 2500 g is considered a low-birth-weight infant. • Plotting weight against gestational age allows classification into three general

categories:a) small for gestational age, b) appropriate for gestational agec) large for gestational age • Infants who are small or large for gestational age often have developmental

problems or difficulties associated with maternal disease . • Careful physical and neurologic examination at birth allows a fairly accurate

estimate of gestational age.• The anesthesiologist should be aware of this type of evaluation so that potential

problems can be anticipated. • Obtaining a perinatal history of problems during pregnancy (e.g., maternal drug

abuse, maternal infection, eclampsia, diabetes) or during and after delivery (e.g., fetal distress, meconium aspiration, prematurity, postdelivery intubation) is also valuable for assessing possible anesthetic implications that may require specific considerations during and after anesthetic management.

Airway differences –Infant Vs Adult Airway anatomy make the potential for technical airway difficulties greater in

infants than in teenagers or adults. The airway of infants differs in five ways:- (1) the relatively large size of the infant's tongue in relation to the oropharynx

increases the likelihood of airway obstruction and technical difficulties during laryngoscopy

(2) the larynx is located higher (more cephalic) in the neck, thus making straight blades more useful than curved blades

(3) the epiglottis is shaped differently, being short, stubby, omega shaped, and angled over the laryngeal inlet; control with the laryngoscope blade is therefore more difficult

(4) the vocal cords are angled, so a “blindly” passed endotracheal tube may easily lodge in the anterior commissure rather than slide into the trachea

(5) the infant larynx is funnel shaped, the narrowest portion occurring at the cricoid cartilage .

adult larynx is cylindrical and the infant larynx is funnel shaped

Conti...

• In infants or young children, an endotracheal tube that easily passes the vocal cords may be tight in the subglottic region because of the relatively greater proportional narrowing at the level of the cricoid cartilage.

• for this reason that uncuffed endotracheal tubes have generally been preferred for children younger than 6 years.

Age-specific considerations Airway differences –Infant Vs Adult

Big head , small bodyTongue/Epiglottis relatively largerGlottis more superior, at level of C3 (vs C4 or 5)Cricoid ring narrower than vocal cord aperture

Respiratory System• Compared with older children and adults, neonates and infants have less efficient

ventilation because of weak intercostal and diaphragmatic musculature (due to a paucity of type I fibers), horizontal and more pliable ribs, and a protuberant abdomen.

• Respiratory rate is elevated in neonates and gradually falls to adult levels by adolescence. • Tidal volume and dead space per kilogram remain constant during development. • A relative paucity of small airways increases airway resistance. Alveolar maturation is not

complete until late childhood (about 8 years of age). • Alveoli increase in number and size until the child is approximately 8 years old. Further

growth is manifested as an increase in size of the alveoli and airways. • The work of breathing is increased and respiratory muscles easily fatigue. The small and

limited number of alveoli in neonates and infants reduces lung compliance; in contrast, their cartilaginous rib cage makes their chest wall very compliant.

• The combination of these two characteristics promotes chest wall collapse during inspiration and relatively low residual lung volumes at expiration. The resulting decrease in functional residual capacity (FRC) is important because it limits oxygen reserves during periods of apnea (eg, intubation) and readily predisposes neonates and infants to atelectasis and hypoxemia.

• This may be exaggerated by their relatively higher rate of oxygen consumption.• Hypoxic and hypercapnic ventilatory drives are not well developed in neonates and

infants. In fact, unlike in adults, hypoxia and hypercapnia depress respiration in these patients.

Developmental Changes of the Rib Cage

Reproduced from - R. S. Litman: Pediatric Anesthesia – The Requisites in Anesthesiology, Elsevier Mosby 2004

Cardiovascular System• Birth and the initiation of spontaneous ventilation initiate circulatory changes, permitting

neonates to survive in an extrauterine environment. • Fetal circulation is characterized by high pulmonary vascular resistance, low systemic

vascular resistance (placenta), and right-to-left shunting of blood through the foramen ovale and ductus arteriosus.

• The onset of spontaneous ventilation at birth is associated with decreased pulmonary vascular resistance and increased pulmonary blood flow. As the left atrial pressure increases, the foramen ovale functionally closes.

• Anatomic closure of the foramen ovale occurs between 3 months and 1 year of age, although 20% to 30% of adults have probe-patent foramen ovales.

• Functional closure of the ductus arteriosus normally occurs 10 to 15 hours after birth, with anatomic closure taking place in 4 to 6 weeks. Constriction of the ductus arteriosus occurs in response to increased arterial oxygenation that develops after birth.

• Ductus arteriosus may reopen during periods of arterial hypoxemia.• During this critical period, the infant readily reverts from the adult circulation to a fetal

type of circulation; this state is called transitional circulation. • Many factors (hypoxia, hypercapnia, anesthesia-induced changes in peripheral or

pulmonary vascular tone) can affect this precarious balance and result in a sudden return to the fetal circulation. When such a “flip-flop” occurs, pulmonary artery pressure increases to systemic levels, blood is shunted past the lungs via the patent foramen ovale, and the ductus arteriosus may reopen and allow blood to shunt at the ductal level

cvs cont..... • Care must be directed to keeping the infant warm, maintaining normal arterial

oxygen and carbon dioxide tension, and minimizing anesthetic-induced myocardial depression.

• The myocardial structure of the heart, particularly the volume of cellular mass devoted to contractility, is significantly less developed in neonates than in adults.

• These differences, as well as developmental changes in contractile proteins, produce a leftward displacement of the cardiac function curve and less compliant ventricles.

• This developmental immaturity of myocardial structures accounts for the tendency toward biventricular failure, sensitivity to volume loading, poor tolerance of increased afterload, and heart rate–dependent cardiac output.

• Another issue is that cardiac calcium stores are reduced because of immaturity of the sarcoplasmic reticulum; consequently, neonates have a greater dependence on exogenous (ionized) calcium and probably increased susceptibility to myocardial depression by potent inhaled agents that have calcium channel blocking activity.

Kidneys

• Renal function is markedly diminished in neonates and further diminished in preterm infants because of low perfusion pressure and immature glomerular and tubular function .

• Nearly complete maturation of glomerular filtration and tubular function occurs by approximately 20 weeks after birth, although it is somewhat delayed in preterm infants. Complete maturation of renal function takes place by about 2 years of age.

• Thus, the ability to handle free water and solute loads may be impaired in neonates, and the half-life of medications excreted by means of glomerular filtration will be prolonged .

Liver• At term, the functional maturity of the liver is somewhat incomplete. Most enzyme

systems for drug metabolism are developed but not yet induced (stimulated) by the agents that they metabolize.

• As the infant grows, the ability to metabolize medications increases rapidly for two reasons: (1) hepatic blood flow increases and more drug is delivered to the liver, and (2) the enzyme systems develop and are induced.

• The cytochrome P450 system is responsible for phase I drug metabolism of lipophilic compounds. This system reaches approximately 50% of adult values at birth, thus suggesting an overall reduced capacity for drug metabolism.

• However, this generalization does not hold true for all lipophilic medications, and the ability of neonates to metabolize some drugs is dependent on specific individual drug cytochromes. CYP3A is generally present at adult values at birth, whereas other cytochromes are absent or reduced.

liver....• Phase II reactions involve conjugation, which makes the drug more water soluble

to facilitate renal excretion. These reactions are often impaired in neonates and result in jaundice (decreased bilirubin breakdown) and long drug half-lives (e.g., morphine and benzodiazepines). Some of these reactions do not achieve adult activity until after 1 year of age.

• A preterm infant's liver has minimal glycogen stores and is unable to handle large protein loads. This difference accounts for the neonate's tendency toward hypoglycemia and acidemia and for the failure to gain weight when the diet contains too much protein.

• Additionally, plasma levels of albumin and other proteins necessary for binding of drugs are lower in full-term newborns (and are even lower in preterm infants) than in older infants .

• This condition has clinical implications regarding neonatal coagulopathy (e.g., the need for vitamin K at birth), as well as for drug binding and pharmacodynamics, in that the lower the albumin value, the less protein binding of some drugs with resultant greater levels of unbound drug (unbound drug is the portion available to cross biologic membranes).

• In addition, binding of some drugs to albumin may be altered in the presence of hyperbilirubinemia in the neonatal period.

Gastrointestinal System

• At birth, gastric pH is alkalotic; by the second day of life, pH is in the normal physiologic range for older children.

• The ability to coordinate swallowing with respiration does not fully mature until infants are 4 to 5 months of age, thus resulting in a high incidence of gastroesophageal reflux in newborns.

• This problem is particularly common in preterm infants.• In general, if a developmental problem occurs within the gastrointestinal system,

symptoms will occur within 24 to 36 hours of life; upper intestinal abnormalities are manifested as vomiting and regurgitation, whereas lower intestinal abnormalities produce abdominal distention and failure to pass meconium

Thermoregulation• Infants are especially vulnerable to hypothermia because of the large ratio of

body surface area to weight, the thinness of the skin, and a limited ability to cope with cold stress.

• Cold stress will result in increased oxygen consumption and can cause metabolic acidosis. A preterm infant is more susceptible because of even thinner skin and limited fat stores.

• The infant may compensate by means of shivering and nonshivering (cellular) thermogenesis. The minimal ability to shiver during the first 3 months of life makes cellular thermogenesis (metabolism of brown fat) the principal method of heat production.

• It is very important to address all aspects of possible heat loss during anesthesia, as well as during transport to and from the operating room. Placing the baby on a warming mattress and warming the operating room (80°F or warmer) reduce heat lost by conduction.

• Keeping the infant in an incubator, covered with blankets, minimizes heat lost through convection. The head should also be covered.

• Hot air blankets are the most effective means of warming children. Anesthetic agents can alter many thermoregulatory mechanisms, particularly nonshivering thermogenesis in neonates.

Hematology• Characteristics of fetal hemoglobin influence oxygen transport. • fetal hemoglobin has a P50 (the partial pressure of oxygen at which

hemoglobin is 50% saturated) of 19 mm Hg compared with 26 mm Hg for adults, which results in a leftward shift of the fetal oxyhemoglobin dissociation curve.

• Subsequent increased affinity of hemoglobin for oxygen manifests as decreased oxygen release to peripheral tissues. This decreased release is offset by increased oxygen delivery provided by the increased hemoglobin concentrations characteristic of neonates .

• By 2 to 3 months of age, however, physiologic anemia results. After 3 months, there are progressive increases in erythrocyte mass and hematocrit. By 4 to 6 months, the oxyhemoglobin dissociation curve approximates that of adults.

• In view of the decreased cardiovascular reserve of neonates and the leftward shift of the oxyhemoglobin dissociation curve, it may be useful to maintain the neonate's hematocrit closer to 40% than 30%, as is often accepted for older children.

• Calculation of the estimated erythrocyte mass and the acceptable erythrocyte loss provides a useful guide for intraoperative blood replacement.

Conti.....

• The need for routine preoperative hemoglobin determinations is controversial.

• Routine preoperative hemoglobin determinations in children younger than 1 year of age results in the detection of only a small number of patients with hemoglobin concentrations less than 10 g/dL, and this rarely influences management of anesthesia or delays planned surgery.

• Because of the potential benefit of identifying anemia during infancy, routine preoperative hemoglobin testing may be justifiable only in this age group.

Pharmacology and Pharmacodynamics

• The response of infants and children (and particularly neonates) to medications is modified by many factors: body composition, protein binding, body temperature, distribution of cardiac output, functional maturity of the heart, maturation of the blood-brain barrier, the relative size (as well as functional maturity) of the liver and kidneys, and the presence or absence of congenital malformations.

• The body compartments (fat, muscle, water) change with age .• Total body water content is significantly higher in preterm than in term infants and

in term infants than in 2-year-olds. Fat and muscle content increases with age.• These alterations in body composition have several clinical implications for

neonates: (1) A drug that is water soluble has a larger volume of distribution and usually

requires a larger initial dose (mg/kg) to achieve the desired blood level (e.g., most antibiotics, succinylcholine);

(2) because there is less fat, a drug that depends on redistribution into fat for termination of its action will have a longer clinical effect (e.g., thiopental); and

(3) drug that redistributes into muscle may have a longer clinical effect (e.g., fentanyl,

for which, however, saturation of muscle tissue has not been demonstrated.

Pharmaco...

In addition to these very basic concepts, other important factors play a role in the neonate's response to medications:

(1) delayed excretion secondary to the larger volume of distribution (2) immature hepatic and renal function, and (3) altered drug excretion caused by lower protein binding. All of these factors lead to

clinically important neonatal patient-to-patient variability in pharmacokinetics and pharmacodynamics.

• Older children tend to have mature renal and hepatic function, with normal adult values for protein, fat, and muscle content. A greater proportion of cardiac output is diverted to the liver and kidneys—which also weigh more in relation to body mass—in older children than in infants.

• These factors usually mean that the half-life of most medications in children older than 2 years is shorter than in adults or equivalent.

• In general, most medications will have a prolonged elimination half-life in preterm and term infants, a shortened half-life in children older than 2 years up to the early teen years, and a lengthening of half-life in those approaching adulthood.

Preoperative Considerations

• Children frequently present for surgery with evidence—a runny nose with fever, cough, or sore throat—of a coincidental viral upper respiratory tract infection (URTI).

• Attempts should be made to differentiate between an infectious cause of rhinorrhea and an allergic or vasomotor cause.

• A viral infection within 2–4 weeks before general anesthesia and endotracheal intubation appears to place the child at an increased risk for perioperative pulmonary complications, such as wheezing (10-fold), laryngospasm (5-fold), hypoxemia, and atelectasis. This is particularly likely if the child has a severe cough, high fever, or a family history of reactive airway disease.

• The decision to anesthetize children with URTIs remains controversial and depends on the presence of other coexisting illnesses, the severity of URTI symptoms, and the urgency of the surgery.

• If surgery cannot be deferred, consideration should be given to an anticholinergic premedication, mask ventilation, humidification of inspired gases, and a longer-than-usual stay in the recovery room.

• The risk of perioperative complications is greatest in the presence of acute infection but remains increased for 2-6 weeks after URTI.

• Airway reactivity is increased for up to 6-8 weeks following an URTI. Children undergoing major surgery may have increased perioperative complications, particularly infective complications.

• Most adverse perioperative events are easily manageable and have no lasting effect.

• Induction of anaesthesia with an inhalation agent (sevoflurane, halothane) is associated with more respiratory complications compared to induction with propofol; induction with thiopentone is associated with the highest risk of respiratory complications

• . Respiratory complications are higher when neuromuscular blocking agents are not reversed.

• Those with mild URTI have clear rhinorrhea, appear otherwise healthy, and have clear lungs to auscultation and no fever. Overtly sick children have fever >38°C, purulent nasal discharge, productive cough and are ill-appearing with signs of pulmonary involvement.

• Most children with mild URTI can be safely anaesthetized without significant morbidity and those children with more severe symptoms should have elective surgeries postponed for at least 4 weeks.

Laboratory Tests• Few, if any, preoperative laboratory results have been deemed cost effective.

Some pediatric centers require no preoperative laboratory tests in healthy children undergoing minor procedures.

• this places more responsibility on the anesthesiologist, surgeon, and pediatrician to correctly identify those patients who should have preoperative testing for specific surgical procedures.

• Most asymptomatic patients with murmurs do not have significant cardiac pathology.

• Innocent murmurs may occur in more than 30% of normal children.• They are usually soft, short systolic ejection murmurs that are best heard along

the left upper or left lower sternal border without significant radiation. • Innocent murmurs at the left upper sternal border are due to flow across the

pulmonic valve (pulmonic ejection) whereas those at the lower left border are due to flow from the left ventricle to the aorta (Still's vibratory murmur).

• An echocardiogram should be obtained if the patient is symptomatic (eg, poor feeding, failure to thrive, or easy fatigability); the murmur is harsh, loud, holosystolic, diastolic, or radiates widely; or pulses are either bounding or markedly diminished

NPO Guidelines

• Clear liquids –2 hours

• Breast milk –4 hours

• Formula or milk –6 hours

• Light meal –6-7 hours

• Full meal –8 hours

Monitoring• Monitoring requirements for infants and children are generally similar to adults with some

minor modifications.• Noninvasive blood pressure monitors have proved to very reliable. • A precordial stethoscope provides an inexpensive means of monitoring heart rate, quality

of heart sounds, and airway patency. • Small pediatric patients have a smaller allowable margin of error. Pulse oximetry and

capnography assume an even greater monitoring role in pediatric patients because hypoxia from inadequate ventilation is a major cause of perioperative morbidity and mortality.

• In neonates, the pulse oximeter probe should preferably be placed on the right hand or earlobe to measure preductal oxygen saturation.

• End-tidal CO2 analysis allows assessment of the adequacy of ventilation, confirmation of endotracheal tube placement, and early warning of malignant hyperthermia. Nonetheless, the small tidal volumes and rapid respiratory rates of small infants can present difficulties with some capnograph models.

• Flow-through (mainstream) analyzers are usually less accurate in patients weighing less than 10 kg. Even with aspiration (sidestream) capnographs, the inspired (baseline) CO2 can appear falsely elevated and the expired (peak) CO2 can be falsely low.

• The degree of error depends on many factors but can be minimized by placing the sampling site as close as possible to the tip of the endotracheal tube, using a short length of sampling line, and lowering gas-sampling flow rates (100–150 mL/min).

Conti....• Temperature must be closely monitored in pediatric patients because of a higher

risk for malignant hyperthermia and the potential for both iatrogenic hypothermia and hyperthermia.

• Hypothermia can be prevented by maintaining a warm operating room environment (26°C or higher), warming and humidifying inspired gases, using a warming blanket and warming lights, and warming all intravenous fluids.

• Invasive monitoring (eg, arterial cannulation, central venous catheterization) requires considerable expertise and extreme caution.

• All air bubbles should be removed from pressure tubing and only small volume flushes should be used to prevent air embolism, inadvertent heparinization, and fluid overload.

• Urinary output is an important measure of volume status.• Neonates who are premature or small for gestational age, who have received

hyperalimentation, or whose mothers are diabetic may be prone to hypoglycemia. • These infants should have frequent serum glucose determinations: levels < 30

mg/dL in the neonate and < 40 mg/dL in older children indicate hypoglycemia.

Premedication

• The need for premedication must be individualized according to the underlying medical conditions, the length of surgery, the desired induction of anesthesia, and the psychological makeup of the child and family. Premedication is not normally necessary for the usual 6-month-old infant but is warranted for a 10- to 12-month-old who is afraid to be separated from parents.

• Oral midazolam is the most commonly administered premedication . An oral dose of 0.25 to 0.33 mg/kg (maximum, 20 mg) generally results in a very compliant child who will separate from parents without crying.

• Premedications may also be administered intramuscularly, intravenously, rectally, sublingually, or nasally. Although most of these routes are effective and reliable, each has drawbacks.

• Oral or sublingual premedication does not hurt but may have a slow onset or be spit out; drug taste and child cooperation are the main determinants of success.

• Intramuscular medications hurt and may result in a sterile abscess. Intravenous medications may be painful during injection or at the start of infusion. Rectal medications sometimes make the child feel uncomfortable, cause defecation, and occasionally burn.

Premedication....• Nasal medications can be irritating, although absorption is rapid. • Midrange doses of intramuscular ketamine (2 to 4 mg/kg) combined with

atropine (0.02 mg/kg) and midazolam (0.05 mg/kg), or oral ketamine (4 to 6 mg/kg) combined with atropine (0.02 mg/kg) and midazolam (0.5 mg/kg, maximum of 20 mg), will result in a deeply sedated child

• This combination is generally reserved for children who refuse oral premedication or those in whom lighter premedication regimens have failed in the past. Higher doses of intramuscular ketamine (up to 10 mg/kg) combined with atropine and midazolam may be administered to children with anticipated difficult venous access or in whom an intravenous line is necessary for induction (e.g., infants with congenital heart disease) to provide better conditions for insertion of an intravenous line.

• Anticholinergic drugs are not routinely administered intramuscularly to children because they are painful on administration and do not significantly reduce laryngeal reflexes during induction of anesthesia.

• On the other hand, atropine (0.02 mg/kg) administered orally or intramuscularly less than 45 minutes before induction does reduce the incidence of hypotension during induction with potent inhaled anesthetics, but only in infants younger than 6 months.

Inhalational Anesthetics• Neonates, infants, and young children have relatively higher alveolar ventilation

and lower FRC compared with older children and adults. • This higher minute ventilation-to-FRC ratio with relatively higher blood flow to

vessel-rich organs contributes to a rapid rise in alveolar anesthetic concentration and speeds inhalation induction.

• Furthermore, the blood/gas coefficients of volatile anesthetics are lower in neonates than in adults, resulting in even faster induction times and potentially increasing the risk of overdosing.

• The minimum alveolar concentration (MAC) for halogenated agents is higher in infants than in neonates and adults .

• Unlike other agents, sevoflurane has the same MAC in neonates and infants. For unknown reasons, use of nitrous oxide in children does not augment the effects (lower MAC requirements) of desflurane and to some extent sevoflurane as it does for other agents.

Inhalation cont..• The blood pressure of neonates and infants tends to be more sensitive to volatile

anesthetics, probably because of not fully developed compensatory mechanisms (eg, vasoconstriction, tachycardia) and an immature myocardium that is very sensitive to myocardial depressants.

• As with adults, halothane also sensitizes the heart to catecholamines; the maximum recommended dose of epinephrine in local anesthetic solutions during halothane anesthesia is 10 micro g/kg.

• Cardiovascular depression, bradycardia, and arrhythmias are significantly less with sevoflurane than with halothane.

• Halothane and sevoflurane are least likely to irritate the airway and cause breath holding or laryngospasm during induction .

• Volatile anesthetics appear to depress ventilation more in infants than in older children. Sevoflurane is associated with the least respiratory depression. Prepubertal children are at much less risk for halothane-induced hepatic dysfunction than adults.

• Overall, sevoflurane appears to have a greater therapeutic index than halothane and has become a preferred induction agent in pediatric anesthesia.

Conti.....

• The rate of emergence is fastest following desflurane and sevoflurane anesthesia, but both agents are associated with an increased incidence of agitation or delirium upon emergence, particularly in young children.

• Because of the these, many clinicians switch to either isoflurane or halothane for maintenance anesthesia following a sevoflurane induction .

• speed of emergence from halothane and isoflurane anesthesia appears to

be similar for procedures lasting less than 1 h.

Nonvolatile Anesthetics• Based on weight, infants and young children require larger doses of propofol because of

a larger volume of distribution compared to adults. Children also have a shorter elimination half-life and higher plasma clearance for propofol.

• Whereas recovery from a single bolus is not appreciably different from adults, recovery following a continuous infusion may be more rapid. For the same reasons, children may require higher rates of infusion for maintenance of anesthesia (up to 250micro g/kg/min)

• Propofol is not recommended for sedation of critically ill pediatric patients in the intensive care unit (ICU). The drug has been associated with higher mortality compared to other agents, and a controversial "propofol infusion syndrome" has been described.

• Its essential features are metabolic acidosis, hemodynamic instability, hepatomegaly, rhabdomyolysis, and multiorgan failure. Although appearing primarily in critically ill children, this rare syndrome has been reported in adults and in patients undergoing long-term propofol infusion (> 48 h) for sedation at high doses (> 5 mg/kg/h.

• Children require relatively higher doses of thiopental compared to adults. The elimination half-life is shorter and the plasma clearance is greater than in adults.

• In contrast, neonates, particularly those depressed at birth, appear to be more sensitive to barbiturates and have less protein binding, a longer half-life, and impaired clearance. The thiopental induction dose for neonates is 3–4 mg/kg compared to 5–6 mg/kg for infants.

Nonvolatile Anesthetics conti...... Opioids appear to be more potent in neonates than in older children and adults. Possible

explanations include easier entry across the blood–brain barrier, decreased metabolic capability, or increased sensitivity of the respiratory centers.

Morphine sulfate should be used with caution in neonates because hepatic conjugation is reduced and renal clearance of morphine metabolites is decreased.

• The cytochrome P-450 pathways mature at the end of the neonatal period. Older pediatric patients have relatively high rates of biotransformation and elimination as a result of high hepatic blood flow.

Sufentanil, alfentanil, and, possibly, fentanyl clearances may be higher in children than in adults. Remifentanil clearance is increased in neonates and infants but elimination half-life is unaltered compared to adults.

Neonates and infants may be more resistant to the hypnotic effects of ketamine, requiring slightly higher doses than adults; pharmacokinetics do not appear to be significantly different from adults.

• The combination of ketamine and fentanyl is more likely to cause hypotension in neonates and young infants than ketamine and midazolam. Etomidate has not been studied adequately in pediatric patients less than 10 years old; its profile in older children is similar to adults.

Midazolam has the fastest clearance of all the benzodiazepines; however, midazolam clearance is significantly less in neonates than in older children. Moreover, the combination of midazolam and fentanyl can cause profound hypotension

Muscle Relaxants• All muscle relaxants generally have a shorter onset (up to 50% less) in pediatric

patients because of shorter circulation times than adults.• Intravenous succinylcholine (1–1.5 mg/kg) has the fastest onset. Infants require

significantly higher doses of succinylcholine (2–3 mg/kg) than older children and adults because of the relatively larger volume of distribution (extracellular space). This discrepancy disappears if dosage is based on body surface area.

• With the notable exclusion of succinylcholine, mivacurium, and possibly cisatracurium, infants require significantly less muscle relaxant than older children.

The response of neonates to nondepolarizing muscle relaxants is quite variable. Immaturity of the neuromuscular junction (particularly in premature neonates) tends to increase sensitivity, whereas a disproportionately large extracellular compartment dilutes drug concentration.

• The relative immaturity of neonatal hepatic function prolongs the duration of action for drugs that depend primarily on hepatic metabolism (eg, pancuronium, vecuronium, and rocuronium).

• In contrast, atracurium and cisatracurium, which do not depend on hepatic biotransformation, reliably behave as intermediate acting muscle relaxants. Breakdown of mivacurium also does not appear to be significantly altered in neonates.

MR Conti......

• Children are more susceptible than adults to cardiac arrhythmias, hyperkalemia, rhabdomyolysis, myoglobinemia, masseter spasm, and malignant hyperthermia after administration of succinylcholine. If a child unexpectedly experiences cardiac arrest following administration of succinylcholine, immediate treatment for hyperkalemia should be instituted.

• For this reason, succinylcholine is best avoided for routine elective surgery in children and adolescents. Unlike in adult patients, profound bradycardia and sinus node arrest can develop in pediatric patients following the first dose of succinylcholine without atropine pretreatment. Atropine (0.1 mg minimum) must therefore always be administered prior to succinylcholine in children.

• Generally accepted indications for succinylcholine in children are rapid sequence induction with a full stomach, laryngospasm. Intramuscular succinylcholine (4–6 mg/kg) can be used for the latter; in this situation, atropine (0.02 mg/kg intramuscularly) should be administered at the same time to prevent bradycardia.

• Some clinicians advocate intralingual administration (2 mg/kg in the mid-line to avoid hematoma formation) as an emergency alternate route.

• Some clinicians consider rocuronium (0.6 mg/kg) to be the drug of choice for routine intubation in pediatric patients with intravenous access because it has the fastest onset of nondepolarizing neuromuscular blocking agents .

MR cont.......

Higher doses of rocuronium (0.9–1.2 mg/kg) may be used for rapid sequence induction but a prolonged duration (up to 90 min) should be expected. Rocuronium is the only nondepolarizing neuromuscular blocker that can be given intramuscularly (1.0–1.5 mg/kg) but requires 3–4 min for onset.

• Mivacurium, atracurium, or cisatracurium may be preferred agents in young infants, particularly for short procedures, because these three drugs consistently display short to intermediate duration. Mivacurium is typically used for procedures lasting 10–15 min, whereas atracurium or cisatracurium is usually used for procedures lasting more than 30 min.

• Because of the extreme variability in response, the doses of long-acting muscle relaxants used for infants should be titrated carefully, starting with one third to one half of the usual dose administered to older children.

Antagonism of neuromuscular blockade in all neonates and small infants, even if they have recovered clinically, because any increase in the work of breathing may cause fatigue and respiratory failure. Useful signs of reversal are the ability to lift the legs and arms and recovery of the train-of-four response to peripheral nerve stimulation.

• reversed with neostigmine (0.03–0.07 mg/kg) or edrophonium (0.5–1 mg/kg) along with an anticholinergic agent (glycopyrrolate 0.01 mg/kg or atropine 0.01–0.02 mg/kg).

Emergence & RecoveryPediatric patients are particularly vulnerable to two postanesthetic complications: laryngospasm

and postintubation croup. Laryngospasm is a forceful, involuntary spasm of the laryngeal musculature caused by stimulation

of the superior laryngeal nerve .• It may occur at induction, emergence, or any time in between without an endotracheal tube.

Laryngospasm is more common in young pediatric patients (almost 1 in 50) than in adults, being highest in infants 1–3 months old.

• Laryngospasm at the end of a procedure can usually be avoided by extubating the patient either awake (opening the eyes) or while deeply anesthetized (spontaneously breathing but not swallowing or coughing); both techniques have advocates.

• Extubation during the interval between these extremes, however, is generally recognized as hazardous. A recent URTI predisposes patients to laryngospasm on emergence.

Treatment of laryngospasm includes gentle positive- pressure ventilation, forward jaw thrust, intravenous lidocaine (1–1.5 mg/kg), or paralysis with intravenous succinylcholine (0.5–1 mg/kg) or rocuronium (0.4 mg/kg) and controlled ventilation.

• Intramuscular succinylcholine (4–6 mg/kg) remains an acceptable alternative in patients without intravenous access and in whom more conservative measures have failed.

• Laryngospasm is usually an immediate postoperative event but may occur in the recovery room as the patient wakes up and chokes on pharyngeal secretions.

• For this reason, recovering pediatric patients should be positioned in the lateral position so that oral secretions pool and drain away from the vocal cords.

Conti...Postintubation Croup• Croup is due to glottic or tracheal edema. Because the narrowest part of the

pediatric airway is the cricoid cartilage, this is the most susceptible area.• Croup is less common with endotracheal tubes that are uncuffed and small

enough to allow a slight gas leak at 10–25 cm H2O. • Postintubation croup is associated with early childhood (age 1–4 years), repeated

intubation attempts, large endotracheal tubes, prolonged surgery, head and neck procedures, and excessive movement of the tube (eg, coughing with the tube in place, moving the patient's head).

• Intravenous dexamethasone (0.25–0.5 mg/kg) may prevent formation of edema, and inhalation of nebulized racemic epinephrine (0.25–0.5 mL of a 2.25% solution in 2.5 mL normal saline) is an effective treatment.

• Although postintubation croup is a complication that occurs later than laryngospasm, it almost always appears within 3 h after extubation.

Regional Anesthesia• The primary uses of regional techniques in pediatric anesthesia have been to

supplement and lower general anesthetic requirements and provide good postoperative pain relief.

• Caudal blocks have proved useful in a variety of surgeries, including circumcision, inguinal herniorrhaphy, hypospadias repair, anal surgery, clubfoot repair, and other subumbilical procedures.

• Contraindications include infection around the sacral hiatus, coagulopathy, or anatomic abnormalities. The patient is usually lightly anesthetized or sedated and placed in the lateral position.

• For pediatric caudal anesthesia, a short bevel 22-gauge needle can be used.• Loss of resistance should be assessed with saline, not air, because of the latter's

possible association with hemodynamically significant air embolism.• After the characteristic "pop" that signals penetration of the sacrococcygeal

membrane, the needle is lowered and advanced only a few more millimeters to avoid entering the dural sac or the anterior body of the sacrum.

• Aspiration is used to check for blood or cerebrospinal fluid; local anesthetic can then be slowly injected; a 2-mL test dose of local anesthetic with epinephrine (1:200,000) helps exclude intravascular placement.

Regional Anesthesia conti....

• Many anesthetic agents have been used for caudal anesthesia in pediatric patients, with 1% lidocaine (up to 7 mg/kg for an epinephrine-containing solution) and 0.125–0.25% bupivacaine (up to 2.5 mg/kg) being most common.

• Ropivacaine 0.2% (up to 2 mg/kg) can provide analgesia similar to bupivacaine but with less motor blockade.

• The volume of local anesthetic required depends on the level of blockade desired, ranging from 0.5 mL/kg for a sacral block to 1.25 mL/kg for a midthoracic block. Single-shot injections generally last 4–12 h.

• Placement of 20-gauge caudal catheters with continuous infusion of local anesthetic (eg, 0.125% bupivacaine or 0.1% ropivacaine at 0.2–0.4 mg/kg/h) or an opioid (eg, fentanyl 2micro g/mL at 0.6micro g/kg/h) allows prolonged anesthesia and postoperative analgesia.

• Complications are rare but include local anesthetic toxicity from prolonged continuous infusions or intravascular injection (eg, seizures, hypotension, dysrhythmias), spinal blockade, and respiratory depression.

• Postoperative urinary retention does not appear to be a problem following single-

dose caudal anesthesia.

Perioperative Fluid Requirements• Meticulous fluid management is required in small pediatric patients because of

extremely limited margins of error. • A programmable infusion pump or a buret with a microdrip chamber should be used

for accurate measurements. • Drugs are flushed through low dead-space tubing to minimize unnecessary fluid

administration.• Fluid overload is diagnosed by prominent veins, flushed skin, increased blood

pressure, decreased serum sodium, and a loss of the folds in the upper eyelids. Fluid therapy can be divided into maintenance, deficit, and replacement requirements.

1)Maintenance Fluid Requirements- Maintenance requirements for pediatric patients can be determined by the formula , 4:2:1 rule: 4 mL/kg/h for the first 10 kg of weight, 2 mL/kg/h for the second 10 kg, and 1 mL/kg/h for each remaining kilogram. The choice of maintenance fluid remains controversial.

• A solution such as D5½NS with 20 mEq/L of potassium chloride provides adequate dextrose and electrolytes at these maintenance infusion rates.

• D51/4NS may be a better choice in neonates because of their limited ability to handle sodium loads. Neonates require 3–5 mg/kg/min of a glucose infusion to maintain euglycemia (40–125 mg/dL); premature neonates require 5–6 mg/kg/min.

2)Deficits-In addition to a maintenance infusion, any preoperative fluid deficits must be replaced as rule 4:2:1

• In contrast to adults, infants respond to dehydration with decreased blood pressure but without increased heart rate.

• Preoperative fluid deficits are typically administered with hourly maintenance requirements as 50% in the first hour and 25% in the second and third hours.

• Large quantities of dextrose-containing solutions are avoided to prevent hyperglycemia. Preoperative fluid deficits are usually replaced with a balanced salt solution (eg, lactated Ringer's ) or ½ normal saline. Compared with lactated Ringer's injection, normal saline has the disadvantage of promoting hyperchloremic acidosis.

• If a child is thought to be at risk for hypoglycemia, 5% dextrose in 0.45% normal saline should be administered by “piggyback” infusion at maintenance rates. This minimizes the chance of a bolus administration of glucose and satisfies the concern for unrecognized hypoglycemia or accidental hyperglycemia.

Conti....

• For most children, lactated Ringer's solution is the only fluid required.

3)Replacement Requirements- Replacement can be subdivided into blood loss and third-space loss.

Blood Loss-The blood volume of premature neonates (100 mL/kg), full-term neonates (85–90 mL/kg), and infants (80 mL/kg) is proportionately higher than that of adults (65–75 mL/kg).

• An initial hematocrit of 55% in the healthy full-term neonate gradually falls to as low as 30% in the 3-month-old infant before rising to 35% by 6 months.

• Hemoglobin (Hb) type is also changing during this period: from a 75% concentration of HbF (high oxygen affinity, low PaO2, poor tissue unloading) at birth to almost 100% HbA (low oxygen affinity, high PaO2, good tissue unloading) by 6 months.

Conti...

• Blood loss is typically replaced with non-glucose-containing crystalloid (eg, 3 mL of lactated Ringer's injection for each milliliter of blood lost) or colloid solutions (eg, 1 mL of 5% albumin for each milliliter of blood lost) until the patient's hematocrit reaches a predetermined lower limit.

• In premature and sick neonates, this may be as high as 40% or 50%, whereas in healthy older children a hematocrit of 20–26% is generally well tolerated.

• Because of their small intravascular volume, neonates and infants are at an increased risk for electrolyte disturbances (eg, hyperglycemia, hyperkalemia, and hypocalcemia) that can accompany rapid blood transfusion.

• Platelets and fresh frozen plasma 10–15 mL/kg should be given when blood loss exceeds 1–2 blood volumes. One unit of platelets per 10 kg weight raises the platelet count by about 50,000/L. The pediatric dose of cryoprecipitate is 1 U/10 kg weight.

Third-Space Loss- These losses are impossible to measure and must be estimated by the extent of the surgical procedure. One popular guideline is 0–2 mL/kg/h for relatively atraumatic surgery (eg, strabismus correction) and up to 6–10 mL/kg/h for traumatic procedures (eg, abdominal abscess). Third-space loss is usually replaced with lactated Ringer's .

Hypovolemia

• Hypotension is a late finding in pediatric patients (children may maintain a normal blood pressure until 35% of blood volume is lost).

• Tachycardia is sensitive but not specific indicator.• Prolonged capillary refill (> 2 seconds), especially when combined with

tachycardia, is more specific, although it may be difficult to measure.• Cold skin and decreased urine output may be present. Weak pulses,

mottling, cyanosis, and impaired consciousness may all precede hypotension. In fact, hypotension is an ominous sign in pediatric patients

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medical status mortalityASA I normal healthy patient without organic, biochemical,

or psychiatric disease0.06-0.08%

ASA II mild systemic disease with no significant impact on daily activity e.g. mild diabetes, controlled hypertension, obesity .

Unlikely to have an impact0.27-0.4%

ASA III severe systemic disease that limits activity e.g. angina, COPD, prior myocardial infarction

Probable impact 1.8-4.3%

ASA IV an incapacitating disease that is a constant threat to life e.g. CHF, unstable angina, renal failure ,acute MI, respiratory failure requiring mechanical ventilation

Major impact 7.8-23%

ASA V moribund patient not expected to survive 24 hours e.g. ruptured aneurysm

9.4-51%

ASA VI brain-dead patient whose organs are being harvested

ASA Physical Status Classification System

For emergent operations, you have to add the letter ‘E’ after the classification.