Pediatric Anesthesia Lecture Notes PDF

Anesthesia 2 – 3rd stage LECTURE FIVE (5) Pediatric Anesthesia It is often said that paediatric patients are ‗not simply small adults‘. The truth is that from the premature neonate to the near-adult adolescent, children are very diverse. Pediatric...

Anesthesia 2 – 3rd stage LECTURE FIVE (5) Pediatric Anesthesia It is often said that paediatric patients are ‗not simply small adults‘. The truth is that from the premature neonate to the near-adult adolescent, children are very diverse. Pediatric patients involve the fallowing age groups: - 1.Neonates (0–1 months). Up to 44 weeks post conception (includes premature neonates) 2.infants (1–12 months). From 44 weeks post conception – 1 year 3.toddlers (12–24 months). 4.young children (2–12 years of age). Pediatric anesthesia has differing anesthetic requirements. physiological, anatomic, and pharmacological characteristics of each group. Indeed, infants are at much greater risk of anesthetic morbidity and mortality than older children; risk is generally inversely proportional to age. Estimation of weight It is essential that every child is weighed prior to anaesthesia. This allows correct calculation of drug doses and selection of anaesthetic equipment. Weight can also be Page 55 of 104 Anesthesia 2 – 3rd stage estimated from the age of the child from standard growth charts, from the length of the child, or using this formula: Age of child Formula to estimate weight in kg 0-12 months (0.5 x age in months) +4 1-5 years (2x age in years) +8 6-12 years (3x age in years) +7 Anatomical & Physiological Differences 1-Respiratory System Differences The major anatomical differences affecting airway management in neonates and infants are: · Relatively large head and prominent occiput · Small mandible · Relatively large tongue · Short neck narrower nasal passages, an anterior and cephalad larynx, a longer epiglottis, and a shorter trachea. These anatomic features make neonates and young infants obligate nasal breathers until about 5 months of age · Soft tracheal cartilages, easily compressed. These differences predispose to airway obstruction, particularly if the child‘s head is placed on a pillow, or the soft tissues on the floor of the mouth are compressed, or the head is hyperextended. Ideally, maintain the child‘s head in a neutral position, or slightly extended. Page 56 of 104 Anesthesia 2 – 3rd stage Anatomical differences affecting the larynx include:  A high, anterior position of the larynx (level of C3-4 in infants compared to C5-6 in adults)  A long, U-shaped epiglottis  The narrowest part of the airway is at the cricoid cartilage (below the vocal cords). o The narrowest part of the airway in adults is at the vocal cords. o Adeno-tonsillar hypertrophy is common in children 3 – 8 years of age. o Airway obstruction may develop after induction of anaesthesia; an oropharyngeal may help to maintain a patent airway. o Take care when passing nasopharyngeal, nasotracheal and nasogastric tubes in these children. Children aged 5-13 years may have loose teeth; take note of loose teeth at your pre- assessment visit Page 57 of 104 Anesthesia 2 – 3rd stage The major physiological differences in respiratory system  Faster respiratory rate  Lower lung compliance  Greater chest wall compliance  Lower functional residual capacity  high metabolic rate and oxygen consumption Note: children tidal volume is relatively fixed (5-7 ml.kg-1), and the infant can only increase minute ventilation by increasing respiratory rate.  Apnoeas are particularly common in premature and ex-premature infants, so monitor all babies for apneas after surgery; until they are 60 weeks post conception. If a mechanical ventilator is used, select the appropriate tidal volume and respiratory rate for age – pressure control ventilation is preferred 2-Cardiovascular considerations  Residual fetal circulation  Noncompliant left ventricle : so increase in cardiac output is achieved through an increase in heart rate (Heart-rate-dependent cardiac output)  Faster heart rate: It is important to avoid bradycardia. This should be treated rapidly if it occur; the most common cause is hypoxia Lower blood pressure Page 58 of 104 Anesthesia 2 – 3rd stage  Activation of the parasympathetic nervous system by anesthetic overdose, or hypoxia can quickly trigger bradycardia and profound reductions in cardiac output.  Bradycardia that can lead to hypotension, asystole, and intraoperative death.  The immature heart is more sensitive to depression by volatile anesthetics and to opioid-induced bradycardia.  The main causes of neonatal bradycardia and cardiac arrest during anesthesia are: -  Respiratory causes: - airway obstruction, bronchospasm, inadequate O2 delivery.  Pharmacology causes: -inhalation agents, succinylcholine, anticholinesterase.  Metabolic causes: -hypothermia, anemia, hypoglycemia.  Children are more susceptible than adults to cardiac arrhythmias, hyperkalemia, masseter spasm, and malignant hyperthermia associated with succinylcholine.  When a child experiences cardiac arrest following administration of succinylcholine, immediate treatment for hyperkalemia should be instituted Page 59 of 104 Anesthesia 2 – 3rd stage 3. Metabolism & Temperature Regulation Differences. Neonates promote greater heat loss to the environment and liable to hypothermia Because: - 1. Thin skin. 2. Low fat content. 3. Greater surface area relative to weight. 4. Inadequately warmed operating rooms (cold theater) 5. Prolonged wound exposure. 6. Administration of room temperature intravenous or irrigation fluid. 7. Dry anesthetic gases 8. Effects of anesthetic agents on temperature regulation center. Even mild degrees of hypothermia can cause perioperative problems which including: - Delayed awakening from anesthesia. Cardiac irritability and arrest. Respiratory depression. Altered responses to anesthetics and Neuromuscular blockers, and other agents. 4-Renal & Gastrointestinal Function Differences  The total body water is about 80% of body weight at birth, gradually decreasing with age. fluid loss is more critical problem to them.  Immature kidney function increases the importance of meticulous attention to fluid administration in the early days of life  Neonates also have a relatively increased incidence of gastroesophageal reflux.  The immature liver conjugates drugs and other molecules less readily early in life. Page 60 of 104 Anesthesia 2 – 3rd stage 5-Glucose Homeostasis Differences  Neonates have relatively reduced glycogen stores, predisposing them to hypoglycemia. 6-Pharmacological Differences: The main difference is prolonging the clinical duration of action of drugs such as thiopental and fentanyl. this Because: - 1. larger pediatric intravascular and extracellular fluid compartments compare with adult. Neonates and infants have a proportionately greater total water content than adults (50–60%). 2.Immaturity of hepatic biotransformation pathways, 3.Decreased protein for drug binding. 4.Smaller muscle mass in neonates prolongs or delaying redistribution of some drugs such as thiopental and fentanyl. Volatile anaesthetic  Neonates are more sensitive to volatile agents than older children  The minimum alveolar concentration (MAC) values are decreased in neonates but increased in infants and children compared to adults. Sedatives and hypnotics Children are particularly sensitive to sedative and hypnotic drugs such as barbiturates and benzodiazepines due to the Immature hepatic biotransformation and Decreased protein binding so these drugs should be used with caution, in weight appropriate doses, titrated according to effect. Page 61 of 104 Anesthesia 2 – 3rd stage Muscle relaxants Neonates and infants are more sensitive to non-depolarizing neuromuscular blocking drugs because Immature neuromuscular junction. A normal loading dose is given but subsequent doses should be reduced Page 62 of 104 Anesthesia 2 – 3rd stage LECTURE SIX (6) Pediatric Anesthesia  Preoperative Preparation  All children should be visited preoperatively by the anaesthetist responsible for caring for them in the perioperative period.  There is an increased incidence of airway problems during anaesthesia  children are more at risk of laryngeal spasm, breath-holding and bronchospasm  in the postoperative period the chance of post-intubation croup is increased.  It is extremely important that the child is weighed before arrival in theatre, because body weight is the simplest and most reliable guide to drug dosage.  Veins suitable for insertion of a cannula should be identified.  Morbidity and mortality caused by aspiration of gastric contents are extremely rare in children undergoing elective surgery.  Prolonged periods of starvation in children, especially the very young infant, are harmful.  These children, who have a rapid turnover of fluids and a high metabolic rate, are at risk of developing hypoglycaemia and hypovolaemia.  Solids (including breast and formula milk) should not be given for at least 6 h before the anticipated start of induction. Page 63 of 104 Anesthesia 2 – 3rd stage  In the emergency setting, e.g. the child who has sustained trauma shortly after ingesting food, it is probably best (if possible) to wait 4 h before inducing anaesthesia. Clearly, in this situation risk–benefit judgements have to be made.  If it is surgically possible to wait 4 h, an i.v. infusion of a glucose-containing solution such as 5% dextrose with 0.9% NaCl, must be commenced and, if necessary, appropriate fluid resuscitation undertaken.  Intravenous Induction  The same induction sequence can be used as in adults: a rapid-acting barbiturate (eg, thiopental, 3 mg/kg in neonates, 5–6 mg/kg in infants and children) or propofol (2–3 mg/kg) followed by a non-depolarizing muscle relaxant (eg, rocuronium, cisatracurium, atracurium, mivacurium, or succinylcholine).  Atropine should be given intravenously prior to succinylcholine.  It is important that children are accompanied into the anaesthetic room by someone with whom they are familiar.  The appropriate monitoring should be applied as soon as possible after the start of anaesthesia.  When inhalational induction is planned, clear, scented plastic masks are much more acceptable to small children than the traditional Rendell–Baker rubber masks.  Clear masks allow respiration and the presence of vomitus to be observed.  An alternative to using a mask is cupping the hands over the face of the child while holding the T-piece, It is important to ensure that the flow of fresh gas is directed away from the child‘s eyes because anaesthetic gases may be irritant.  When using a face mask, it is important that the soft tissue behind the chin is not pushed backwards by the fingers, thereby obstructing the airway. The anaesthetist’s fingers should rest only on the mandible. Page 64 of 104 Anesthesia 2 – 3rd stage  Airway Management  The Jackson–Rees modification of the Ayre‘s T-piece is the breathing system used traditionally for children under 20 kg in weight.  It has been designed to be lightweight with a minimal apparatus dead space. The apparatus may be used for both spontaneous and controlled ventilation  The open-ended reservoir bag is used for manually controlled ventilation. This mode of ventilation is especially useful in the neonate and infant.  Laryngeal mask airway (LMA) should be used only when it is planned that the child is to breathe spontaneously during surgery. It follows that it is unwise to use the device when neuromuscular blocking drugs are used.  It is mandatory to intubate the trachea during artificial ventilation.  Neonates with a tracheal tube must undergo artificial ventilation in order to reduce the work of breathing.  Infants have a head which is large and a neck which is short relative to the size of the body. Instead of placing a pillow under the head, it is usually necessary to place a small pad or pillow under the torso.  Tracheal intubation For children over 1 year:  Appropriate tube internal diameter (ID) can be approximately estimated by the formula: age / 4 + 4. Page 65 of 104 Anesthesia 2 – 3rd stage  Appropriate tube length in cm. can be approximately estimated by the formula: age / 2 + 12 oral (+15 for nasal). In infants:  Appropriate tube ID sizes for preterm: