Pediatric Neuroanesthesia and Critical Care

Induction of Anesthesia

The patient’s preoperative status dictates the appropriate technique and drugs for induction of anesthesia. General anesthesia can be induced with sevoflurane and nitrous oxide with oxygen. A nondepolarizing muscle relaxant is then administered after intravenous (IV) access has been established to facilitate intubation of the trachea. If the patient already has an IV catheter, anesthesia can be induced with a sedative-hypnotic drug such as propofol (2–4 mg/kg). It should be noted that neonates are vulnerable to propofol-induced hypotension that can persist for 30 minutes, especially when there is no surgical stimulation. Patients who are vomiting or have recently ingested food or fluids are at risk for aspiration pneumonitis and so should undergo rapid-sequence induction of anesthesia with propofol, followed immediately with a rapid-acting muscle relaxant and cricoid pressure.

Airway Management

Developmental changes in airway anatomy have a significant effect on the management of the pediatric airway. The infant’s larynx is funnel-shaped, being narrowest at the level of the cricoid ring. This feature puts the infant at risk for subglottic obstruction secondary to mucosal swelling after prolonged endotracheal intubation with a tight-fitting endotracheal tube. Cuffed endotracheal tubes can be used, but the cuff pressure should be checked frequently and adjusted to minimize tracheal injury. Since the infant’s trachea is relatively short, an endotracheal tube can easily migrate into a mainstem bronchus when the head is flexed, as it will be for a suboccipital approach to the posterior fossa or the cervical spine ( Fig. 20.5 ). Therefore, great care should be devoted to ensuring proper position of the endotracheal tube during tracheal intubation, and the anesthesiologist should auscultate both lung fields to rule out inadvertent intubation of a mainstem bronchus after final positioning of the patient. Nasotracheal tubes are best suited for the patient in the prone position, because they are easier to secure, less likely to kink at the base of the tongue when the head is a flexed and prevent pressure injury to the tongue.

Timing of tracheal extubation sometimes presents a challenge after neurosurgical procedures. Patients with Chiari malformation and brainstem surgery may exhibit intermittent postoperative apnea, vocal cord paralysis, or other irregularities before resuming a stable respiratory pattern. Significant airway edema and postoperative obstruction can complicate prone procedures or operations involving significant blood losses and large volume replacement. Preexisting pulmonary dysfunction, as in infants with bronchopulmonary dysplasia or older children with neuromuscular disease, may force delays in extubation because of respiratory insufficiency. In these cases, standard tracheal extubation criteria and the presence of an endotracheal tube air leak less than 20 cm H ₂ O can assist the clinician in appropriate decision-making. Lingual or supraglottic swelling can cause airway obstruction and can be diagnosed by direct laryngoscopy. When swelling is significant, postoperative lung ventilation in the intensive care unit, head-up positioning and gentle forced diuresis lead to improved conditions within 24 hours.

Positioning

The diminutive size of the neonate or infant requires careful preoperative planning to allow adequate access to the patient for both the neurosurgeon and anesthesiologist ( Table 20.5 ). Skull fixation with a Mayfield frame is standard practice in craniotomies and cervical spine surgery. However, infants and small children have thin skulls and are at increased risk for fractures and epidural hemorrhage. Therefore, age-appropriate fixation pins and tensions should be utilized. The prone position is commonly used for posterior fossa and spinal cord surgery. Although the sitting position has been used less often in pediatric patients, it may be appropriate for obese patients, who may be difficult to ventilate in the prone position. In addition to the physiologic sequelae of the prone position, a whole spectrum of compression and stretch injuries has been reported. Padding under the chest and pelvis can support the torso. It is important to ensure free abdominal wall motion because greater intra-abdominal pressure can impair ventilation, cause compression of the vena cava, and increase epidural venous pressure and bleeding. Soft rolls are generally used to elevate and support the lateral chest wall and hips to minimize the increases in abdominal and thoracic pressure. In addition, a Doppler probe can be applied to the chest without causing pressure sores.

Many neurosurgical procedures are performed with the head slightly elevated to facilitate venous and cerebrospinal fluid drainage from the surgical site. However, superior sagittal sinus pressures decrease with increasing head elevation and increase the likelihood of venous air embolism (VAE). Extreme head flexion can cause brainstem compression in patients with posterior fossa pathology, such as a mass lesion or Chiari malformation. Furthermore, significant rotation of the head can compress the jugular vein, which in turn impedes venous return leading to impairment of cerebral perfusion and increases in ICP and venous bleeding.

Vascular Access

Optimal intravenous access is mandatory prior to the start of surgery because access to the infant and small child during neurosurgical procedures can be limited. Typically two large-bore venous cannulas are sufficient for most craniotomies. Should initial attempts fail, central venous cannulation may be necessary. Cannulation of the femoral vein avoids the risk of pneumothorax associated with subclavian catheters and does not affect cerebral venous return. Furthermore, femoral catheters are more accessible during operations on the head. Since significant blood loss and hemodynamic instability can occur during craniotomies, an arterial catheter would provide direct blood pressure monitoring and sampling for blood gas analysis.

Maintenance of Anesthesia

Sevoflurane is the principal anesthetic for induction of anesthesia in infants and children followed by opioid and low-dose (0.2–0.5%) isoflurane. The incidence of awareness under anesthesia has been reported to be 0.8% in children, a value higher than in adults. Deep neuromuscular blockade with a nondepolarizing muscle relaxant is maintained to avoid patient movement and minimize the amounts of anesthetic agents needed. Patients who have undergone long-term anticonvulsant therapy will require larger doses of muscle relaxants and narcotics because of the enzymatic metabolism induced by these agents ( Fig. 20.6 ). Muscle relaxants should be withheld or their effects permitted to wear off when assessment of motor function during neurosurgery is planned.

Anesthetic-induced developmental neurotoxicity has been fully substantiated in preclinical reports, but the retrospective clinical studies have been inconclusive. It is well known that extremely and very low-birth-weight survivors have poor neurological and cognitive outcomes. Cerebral injury due to cerebral hypoperfusion, metabolic derangements, coexisting disease and surgery have the potential to worsen neurological outcomes in neonates and infants. Therefore, intraoperative hypotension, hypocarbia, oxygenation, glycemia and temperature control should be aggressively managed in these vulnerable patients.

Intraoperative Fluid and Electrolyte Management

Hemodynamic stability during intracranial surgery requires careful maintenance of intravascular volume and electrolytes. Sudden blood loss or venous air embolus can rapidly deteriorate to cardiovascular collapse. Therefore, normovolemia should be maintained throughout the procedure. Estimation of the patient’s blood volume is essential in determining the amount of allowable blood loss and the time to transfuse blood. Blood volume depends on the age and size of the patient, as delineated in Table 20.6 . Normal saline is commonly used as the maintenance fluid during neurosurgery because it is mildly hyperosmolar and should minimize cerebral edema. However, rapid infusion of large quantities of normal saline (> 60 mL/kg) can be associated with hyperchloremic acidosis. Given the relatively large blood volume of the neonate or infant, the maintenance rate of fluid administration depends on the weight of the patient ( Table 20.7 ). Significant blood loss is likely in most craniotomies in infants and children, so the maximum allowable blood loss should be determined in advance so the anesthesiologist will know when blood should be transfused to the patient. However, there are no guidelines for threshold in transfusing blood and the decision to transfuse should be dictated by the type of surgery, underlying medical condition of the patient, and potential for additional blood loss, both intraoperative and postoperative. Hematocrit values of 21–25% should provide some impetus for blood transfusion. Packed red blood cells (10 mL/kg) will raise the hematocrit by 10%. Initially, blood losses should be replaced with 3 mL of normal saline for each 1 mL of lost blood or a colloid solution, such as 5% albumin, equal to the blood loss. Depending on the extent and length of the surgical procedure and exposure of vascular beds, additional fluid administration at 3–10 mL/kg/h may be necessary.

Pediatric patients, particularly infants, are at particular risk for hypoglycemia. Small premature infants, who have limited reserves of glycogen and limited gluconeogenesis, require continuous infusions of glucose at 5–6 mg/kg/min to maintain serum levels. Surgery elicits a stress response, and children are generally able to maintain normal serum glucose levels without exogenous glucose administration. Since inadvertent hypoglycemia occurs in fasted infants and children, frequent glucose measurements are recommended. Limited evidence now suggests that tight glycemic control may reduce postoperative infections but still carry undue risk of hypoglycemia. ^,

Brain edema can be managed initially with hyperventilation and elevation of the head above the heart. Should these maneuvers fail, mannitol can be given at a dose of 0.25–1.0 g/kg IV. This agent will transiently alter cerebral hemodynamics and raise serum osmolality by 10–20 mOsm/kg. However, repeated dosing of mannitol can lead to extreme hyperosmolality, renal failure, and further brain edema. Furosemide is a useful adjunct to mannitol for decreasing acute cerebral edema and has been shown in vitro to prevent the rebound swelling due to mannitol. All diuretics interfere with the ability to use urine output as a guide to intravascular volume status.

Monitoring

Hemodynamic Monitoring

Patients undergoing major craniotomies and spine surgery are at risk of sudden hemodynamic instability due to hemorrhage, VAE, herniation syndromes, or manipulation of cranial nerves. The potential for cerebral hypoperfusion generally warrants placement of an arterial cannula for continuous blood pressure monitoring. The utility of central venous catheterization remains controversial. Cannulation of the jugular or subclavian vein with multiple-orifice catheters in adults is often preferred, particularly when VAE is anticipated. However, these multiple-orifice catheters are too large for infants and most small children, and are not used in pediatric settings. Furthermore, monitoring of the central venous pressure may not accurately reflect intravascular volume in small children. Therefore, the risks of a central venous catheter may outweigh its benefits. Even when VAE occurs, a single-orifice catheter is not often successful for aspirating air, presumably because of the high resistance of the small-gauge catheters used in these patients.

Venous air emboli have been detected during many craniotomies in infants and children, primarily because the head of a small child is large in relation to the rest of the body and rests above the heart in either the prone or supine position ( Fig. 20.7 ). Standard neurosurgical positioning often includes elevation of the patient’s head to optimize cerebral venous drainage. However, this maneuver can increase the risk for air entrainment into the venous system through open venous channels in bone and sinuses. Patients with cardiac defects and the potential for right-to-left shunting, such as patent foramen ovale and patent ductus arteriosus, are at risk for paradoxical air emboli, leading to cerebral and myocardial infarction. A precordial Doppler ultrasound device can detect minute VAE and should be routinely used in conjunction with an end-tidal carbon dioxide analyzer and arterial catheter in all craniotomies in order to detect VAE early, before significant hemodynamic instability develops. The Doppler probe is best positioned on the anterior chest, usually just over or to the right of the sternum at the fourth intercostal space (i.e., the nipple line). An alternative site on the posterior thorax can be used in infants in the prone position who weigh approximately 6 kg or less. In addition to the characteristic changes in Doppler sounds, sudden decreases in end-tidal CO ₂ , dysrhythmias, ischemic changes in the electrocardiogram, or a combination of these, can occur with VAE.