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Pediatric Neuroanesthesia
Overview
Advances in neurosurgery and neuromonitoring have dramatically improved the outcome in neurologically injured patients. Most of these advanced technologies have been utilized in the adult population and have also been extrapolated to children with improved perioperative outcome. Pediatric neurosurgical patients present a set of inherent challenges because of their developing and maturing neurologic and physiologic status apart from the associated central nervous system (CNS) disease process. This chapter revisits the anesthetic management and perioperative care of children undergoing neurosurgical procedures.
Pediatric Neurophysiology
In children, accurate data in relation to normal neurophysiologic variables is limited and mostly derived from the adult data. Cerebral blood flow (CBF), in pediatric population, varies with age. It is lower in premature infants (12 mL/100 g/min) and full-term neonates (23–40 mL/100 g/min), and higher in infants and older children, than the CBF of adults (50 mL/100 g/min). From the age of 6 months to 3 years, the CBF is 90 mL/100 g/min, and it is 100 mL/100 g/min at the age of 3–12 years. Changes in CBF lead to alterations in cerebral blood volume and the intracranial volume, which further affects the intracranial pressure (ICP). CBF is coupled tightly with the metabolic demand known as cerebral metabolic rate of oxygen (CMRO 2 ), and both increase proportionally after birth. In children, the CMRO 2 is higher than that in adults (5.2 vs. 3.5 mL/100 g/min) and hence, less tolerant to hypoxia. Neonates have a lower CMRO 2 (3.5 mL/100 g/min) with a relative tolerance to hypoxemia.
The autoregulation range of blood pressure in normal newborns is between 20 and 60 mm Hg, which is a very narrow range. The autoregulatory slope drops and rises significantly at the lower and upper limits of the curve, respectively. Sudden hypotension and hypertension at either end of the autoregulatory curve places the neonate at risk for cerebral ischemia and intraventricular hemorrhage, respectively.
According to the Monro–Kellie doctrine , the brain tissue, blood, and cerebrospinal fluid (CSF) are enclosed inside the rigid skull. An increase in volume of any one of these three components, with increase in ICP, will result in a compensatory reduction of other components. In infants, open fontanel and cranial sutures lead to a compliant intracranial space. The mass effect of a large space-occupying lesion can be masked by a slow increase in the size of skull. Hence, infants presenting with intracranial hypertension (ICH) may have a well-advanced pathology. Moreover, the size of the skull in small children may not increase at a similar pace to accommodate rapid changes in the intracranial volume, e.g., after head injury. In such conditions, the ICP may increase as it occurs in adults.
A large percentage of cardiac output is directed to brain in infants and children. This is because the head accounts for a large percentage of the body surface area and blood volume. This aspect places the infant at a higher risk for significant hemodynamic instabilities during neurosurgical procedures.
Currently, no data are available with regard to effects of different anesthetic drugs on CSF dynamics, CBF, and cerebral metabolism. The response of drugs on neurophysiology in children has been presumed to same as those in the adults. The anesthetic requirement in children may vary with the age. Neonates and premature infants may have reduced anesthetic requirement as compared to the older children owing to immaturity of the CNS and blood–brain barrier, presence of maternal progesterone, and elevated level of endorphins. In neonates and infants, induction of anesthesia [inhalational or intravenous (IV)] is more rapid as compared to that in adults as the ratio of alveolar ventilation to functional residual capacity is more, the blood–gas partition coefficient for volatile anesthetics is low, and the cardiac output is greater as compared to the adults.
Children have anatomically different airway than the adults, which amounts to encountering a difficult airway during endotracheal intubation. Moreover, infants are more prone to rapid deoxygenation as the oxygen consumption is very high in this age group. Such complicated anatomy and physiology makes the airway management more difficult in these children.
General Principles of Pediatric Neuroanesthesia
Preoperative Evaluation and Preparation
The evaluation of a pediatric neurosurgical patient should include history and physical examination pertaining to the conditions that require special anesthetic considerations. The neurologic status can be assessed from the evidence of raised ICP, altered sensorium, and cranial nerve palsies. Infants with ICH might present with irritability, lethargy, decreased consciousness, failure to feed, bulging fontanale, and cranial enlargement. In children, it may present with early morning headache, vomiting without nausea, diplopia, papilledema, and in very late stage, the Cushing’s triad composed of hypertension, bradycardia, and respiratory changes. The conscious level can be ascertained from age-specific Glasgow Coma Scale (GCS) score. Frequent episodes of vomiting may lead to dehydration and electrolyte imbalances, and increases the risk of aspiration. Hence, serum electrolyte should be determined to identify abnormalities of sodium and potassium following vomiting. Hemoglobin or hematocrit level, typing and cross-matching of blood if the loss is expected to be considerable, should be done and blood should be kept ready before shifting the child to the operating room. Additional studies include electrocardiography (ECG), coagulation profile, as well as renal and hepatic function, as and when required. Children with pituitary tumors should undergo complete endocrine evaluation.
Premedication
In children with suspected increase in ICP, sedative premedication must be avoided as the medications decrease respiratory drive resulting in hypercapnia and further increase in ICP. However, in patients with normal ICP such as those scheduled for repair of vascular lesions like arteriovenous malformations (AVMs) may be sedated so as to allay preoperative anxiety and avoid hypertension thus, preventing rupture of the abnormality. Oral benzodiazepines such as midazolam may be beneficial for small children as they provide sedation without respiratory depression but should be administered under supervision. Some reports suggest that midazolam is not associated with respiratory depression even in children with reduced intracranial compliance. Rather, it (midazolam) reduces anxiety and hence helps separating the child from the parents. It also reduces crying and associated cardiovascular changes, which may further lead to raised ICP. Midazolam also reduces the requirement of analgesics.
Intraoperative Management
Induction of Anesthesia
The goal of anesthetic induction is to avoid increase in ICP owing to associated hypoxia, hypercapnia, and volatile anesthetic–induced increases in CBF. An IV induction with thiopentone or propofol and neuromuscular blockade to facilitate endotracheal intubation is ideal in children with raised ICP. All IV induction agents, except ketamine, would cause a reduction in ICP. However, in children without IV access or with difficult IV access, inhalational induction by facemask with sevoflurane should be preferred as crying or struggling may lead to further increase in ICP. After the IV access is secured, a bolus of thiopentone (1–2 mg/kg) or propofol may be given to prevent the pressure responses of tracheal intubation. Furthermore, the inhalational technique may subsequently be changed to an IV induction. All volatile anesthetics cause an increase in CBF, and hence the ICP. Therefore, ventilation should be controlled as early as possible and mild hyperventilation is to be instituted to prevent rise in ICP. Children at risk for aspiration should undergo rapid-sequence anesthetic induction with thiopentone or propofol followed by rapid-acting muscle relaxant such as succinylcholine or rocuronium.
Maintenance of Anesthesia
Low exhaled concentrations of inhalational agents with mild hyperventilation does not attribute to increased ICP. Hence, anesthesia is maintained either with low end-tidal volatile agents (minimum alveolar concentration, MAC <1) or with total IV anesthesia (TIVA) along with short-acting opioids (fentanyl or remifentanil), with or without nitrous oxide, and controlled ventilation. Halothane is a potent cerebral vasodilator and causes maximum increase in ICP among all the volatile anesthetic agents currently in use. Sevoflurane an agent for induction has almost replaced halothane as far as pediatric neuroanesthesia is concerned. Isoflurane, sevoflurane, or desflurane are all used for maintenance of anesthesia during the neurosurgical procedures. Sevoflurane provides smooth induction followed by a rapid recovery. Recovery is also rapid following desflurane anesthesia. However, nitrous oxide should avoided as it causes increase in ICP and cerebral metabolism. Nitrous oxide also increases the size of the gas-filled space after craniotomy with a propensity to occurrence of significant pneumocephalus and consequent increase in ICP following surgery. Neuromuscular blockade with nondepolarizing muscle relaxants is achieved to prevent patient movement and to minimize the amount of anesthetics required. Children on chronic antiepileptic medications require large doses of muscle relaxants and narcotics due to enzymatic induction in liver. The muscle relaxant should be withheld when assessment of motor function is carried out, e.g., during spinal cord surgery. Fentanyl is the most commonly used opioid, but its half-life increases with repeated dosing. It requires hepatic metabolism, which is immature in premature infants. Hence, the sedative and respiratory depressive effects of fentanyl may be prolonged in these children. Remifentanil is a newer narcotic agent, cleared rapidly by the plasma esterases. The rapid recovery associated with its use may be accompanied by delirium and inadequate analgesia. Hence, it requires supplementation of other analgesics such as morphine during postoperative pain management. Opioids have not been shown to increase ICP in patients undergoing controlled ventilation.
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