Pediatric Patients

PEDIATRIC PATIENTS

Cardiovascular function in infants and young chil-dren differs from that in adults. Stroke volume is relatively fixed, so that cardiac output is primarily dependent on heart rate. The immature hearts of neonates and infants often are less forgiving of pres-sure or volume overload. Furthermore, the functions of both ventricles are more interdependent, so that failure of one ventricle often precipitates failure of the other (biventricular heart failure).

Preoperative Evaluation

The potentially complex nature of congenital heart defects and their operative repair require close com-munication among the anesthesiologist, cardiolo-gist, and surgeon. The hemodynamic significance of the lesion and the planned surgical correction must be clearly understood. The patient’s condition must be optimized. Congestive heart failure and pulmo-nary infections should be treated. Prostaglandin E₁ infusion (0.05–0.1 mcg/kg/min) is used preopera-tively to prevent closure of the ductus arteriosus in infants dependent on ductal flow for survival.

Assessment of disease severity relies on both clinical and laboratory evaluation. Deterioration in infants may be manifested by increasing tachypnea, cyanosis, or sweating, particularly during feeding. Older children may complain of easy fatigability. In infants body weight is generally a good indication of disease severity, with the sickest children show-ing failure to thrive and reduced weight relative to expectations for age. Signs of congestive heart failureinclude failure to thrive, tachycardia, an S₃ gallop, weak pulses, tachypnea, pulmonary rales, and hepa-tomegaly. Cyanosis may be noted, but hypoxemia is best assessed by measurements of arterial blood gases and the hematocrit. In the absence of iron defi-ciency, the degree of polycythemia is related to the severity and duration of hypoxemia. Clubbing of the fingers is frequent in children with cyanotic defects. The evaluation should also search for other congeni-tal abnormalities, which are present in up to 30% of patients with congenital heart disease.

The results of echocardiography, heart catheter-ization, electrocardiography, and chest radiography should be reviewed. Laboratory evaluation typi-cally includes a complete blood count (with platelet count), coagulation studies, electrolytes, blood urea nitrogen, and serum creatinine. Measurements of ionized calcium and glucose are also useful in neo-nates and critically ill children.

Preinduction Period

A. Fasting

Fasting requirements vary according to the patient’s age and current guidelines. A preoperative intra-venous infusion that provides maintenance fluid requirements should be used in patients susceptible to dehydration, in those with severe polycythemia, and when excessive delays occur prior to surgery.

B. Premedication

Premedication varies according to age and cardiac and pulmonary reserves. Atropine, 0.02 mg/kg intramuscularly (minimum dose, 0.15 mg), has by tradition been given to pediatric cardiac patients to counteract enhanced vagal tone. Neonates and infants younger than 6 months of age may receive no premedication or given only atropine. Sedation is desirable in older patients, particularly those with cyanotic lesions (tetralogy of Fallot), as agitation and crying worsen right-to-left shunting. Patients older than 1 year may be given midazolam orally (0.5–0.6 mg/kg) or intramuscularly (0.08 mg/kg).

Induction of Anesthesia

A. Hemodynamic Anesthetic Goals

Obstructive lesions—Anesthetic managementshould strive to avoid hypovolemia, bradycardia,tachycardia, and myocardial depression. The opti-mal heart rate should be selected according to age; slow rates decrease cardiac output, whereas fast rates may impair ventricular filling. Mild cardiac depres-sion may be desirable in some hyperdynamic pa-tients, eg, those with coarctation of the aorta.

Shunts—A favorable ratio of pulmonary vascular resistance (PVR) to SVR should be maintained in the presence of shunting. Factors known to increase PVR such as acidosis, hypercapnia, hypoxia, enhanced sympathetic tone, and high mean airway pressures are to be avoided in patients with right-to-left shunting; hyperventilation (hypo-capnia) with 100% oxygen is usually effective in lowering PVR. Specific pulmonary vasodilators are not available; alprostadil (prostaglandin E ₁) or ni-troglycerin may be tried but they often cause sys-temic  hypotension.  Systemic  vasodilation  also worsens right-to-left shunting and should be avoid-ed; phenylephrine may be used to raise SVR. In-haled nitric oxide has no effect on systemic arterial pressure.  Conversely,  patients  with  left-to-right shunting benefit from systemic vasodilation and in-creases in PVR, although specific hemodynamic manipulation is generally not attempted.

B. Monitoring

Standard intraoperative monitors are generally used until the patient is anesthetized, although they may be “added” during the course of an inhaled induction in some patients. A large discrepancy between end-tidal and arterial CO₂ tensions should be anticipated in patients with large right-to-left shunts because of increased dead space. Following induction, intraarterial and central venous pressure monitoring are employed for thoracotomies and all procedures employing CPB. A 22- or 24-gauge catheter is used to enter the radial artery; 24-gauge catheters may be more appropriate for small neo-nates and premature infants. A cutdown may be necessary in some instances. The internal jugular or subclavian vein is generally used for central venous cannulation; if this approach is unsuccessful, a right atrial catheter may be placed intraoperatively by the surgeon. Pulmonary artery catheterization is almost never used in pediatric patients. TEE is invaluable for assessing the surgical repair following CPB. Ever smaller probes are yielding better resolution as the technology advances. Probes are currently available for patients as small as 3 kg. Intraoperative epicar-dial echocardiography is commonly used either in addition to or instead of TEE.

C. Venous Access

Venous access is desirable but not always necessary for induction. Agitation and crying are particularly undesirable in patients with cyanotic lesions and can increase right-to-left shunting. Intravenous access can be established after induction but before intu-bation in most patients. Subsequently, at least two intravenous fluid infusion portals are required; one is typically via a central venous catheter. Caution is necessary to avoid even the smallest air bubbles. Shunting lesions allow the passage of venous air into the arterial circulation; paradoxical embolism can occur through the foramen ovale even in patients without obvious right-to-left shunting. Aspiration prior to each injection prevents dislodgment of any trapped air at stopcock injection ports.

D. Route of Induction

To a major extent, the effect of premedication and the presence of venous access determine the induction technique. Intubation is facilitated by a nondepolarizing agent (rocuronium, 1.2 mg/kg, or pancuronium, 0.1 mg/kg) or, much less commonly, succinylcholine, 1.5–2 mg/kg. Pancuronium’s vagolytic effects are particularly useful in pediat-ric patients, but the agent itself is less often seen in North American hospitals.

Intravenous—Propofol (2–3 mg/kg), ketamine(1–2 mg/kg), fentanyl (25–50 mcg/kg), or sufentanil (5–15 mcg/kg) can be used for intravenous induc-tion. High-dose opioids may be suitable for very small and critically ill patients when postoperative ventilation is planned. Intravenous agents’ onset of action may be more rapid in patients with right-to-left shunting; drug boluses should be given slowly to avoid transiently high arterial blood levels. In con-trast, recirculation in patients with large left-to-right shunts dilutes arterial blood concentration and can delay the appearance of intravenous agents’ clinical effects.

Intramuscular—Ketamine, 4–10 mg/kg, is mostcommonly used, and onset of anesthesia is within 5 min. Coadministration with atropine helps prevent excessive secretions. Ketamine is a good choice for agitated and uncooperative patients as well as pa-tients with decreased cardiac reserve. Its safety with cyanotic lesions (particularly in patients with Fallot’s tetralogy) is well established. Ketamine does not ap-pear to increase PVR in children.

Inhalation—Sevoflurane is the most commonlyused volatile agent. The technique is the same as for noncardiac surgery, except for greater concerns about avoiding excessive anesthetic doses. Sevoflu-rane is particularly suitable for patients with good cardiac reserve. The safety of sevoflurane in patients with cyanotic heart disease and good cardiac reserve is now well established. Nitrous oxide may be used with inhalation inductions; its concentration should be limited to 50% in patients with cyanotic lesions. Nitrous oxide does not appear to increase PVR in pediatric patients. The uptake of inhalation agents may be slowed in patients with right-to-left shunts; in contrast, no significant effect on uptake is gener-ally observed with left-to-right shunting.

Maintenance Anesthesia

Following induction, opioids or inhalation anes-thetics are used for maintenance. Fentanyl and suf-entanil are the most commonly used intravenous agents, and isoflurane and sevoflurane the most commonly used inhalation agents. Some clinicians choose the anesthetic according to the patient’s hemodynamic responses. Isoflurane and sevoflu-rane may be more suitable than halothane (the most commonly used inhaled agent in years past) for most patients; in equivalent anesthetic doses, they cause less myocardial depression, less slow-ing of the heart rate, and more vasodilation than halothane. However, one can make a theoretical argument in favor of halothane over sevoflurane for patients with tetralogy of Fallot (and similarly obstructive lesions such as hypertrophic subaortic stenosis), where myocardial depression is much preferred over vasodilation.

Cardiopulmonary Bypass

The circuit and technique used are similar to those used for adults. Because the smallest circuit volume used is still about three times an infant’s blood vol-ume, blood is used to prime the circuit for neonatesand infants to prevent excessive hemodilution. CPB may be complicated by intracardiac and extracar-diac shunts and a very compliant arterial system (in very young patients); both tend to lower mean arterial pressure (20–50 mm Hg) and can impair systemic perfusion. High flow rates (up to 200 mL/ kg/min) may be necessary to ensure adequate per-fusion in very young patients. As noted previously, some evidence suggests that pH-stat management during CPB may be associated with better neuro-logical outcome in children who will undergo cir-culatory arrest. Weaning from CPB is generally not a problem in pediatric patients if the surgical repair is adequate; primary pump failure is unusual. Difficulty in weaning should prompt the surgeon to check the repair and search for undiagnosed lesions. Intraoperative echocardiography, together with measurement of the pressure and oxygen saturation within the various chambers, may reveal the prob-lem. Inotropic support may be provided by any of the agents used for adults. Calcium chloride is more often useful in critically ill young patients than in adults as children more often have impaired cal-cium homeostasis; ionized calcium measurements are invaluable in such cases. Close monitoring of glucose is required because both hyperglycemia and hypoglycemia may be observed. Dopamine and epinephrine are the most commonly used inotro-pes in pediatric patients. Addition of a phosphodi-esterase inhibitor is also useful when PVR or SVR is increased. Hypocapnia, systemic alkalosis, and a high inspired oxygen concentration should also be used to decrease PVR in patients with pulmonary hypertension; additional pharmacological adjuncts may include prostaglandin E₁ (0.05–0.1 mcg/kg/ min) or prostacyclin (1–40 mcg/kg/min). Inhalation nitric oxide may also be helpful for refractory pul-monary hypertension.

Children appear to have an intense inflamma-tory response during CPB that may be related to their blood being exposed to very large artificial surfaces relative to their size. Corticosteroids are often given to suppress this response. Many centers use modified ultrafiltration after weaning from CPB to partially correct the hemodilution but remove inflammatory vasoactive substances (cytokines); the technique takes blood from the aortic cannula and venous reservoir, passes it through an ultrafil-ter, and returns it to the right atrium.

Surgical correction of complex congenital lesions often requires a period of complete circula-tory arrest under deep hypothermia (deep hypother-mic circulatory arrest; DHCA). Following institution of CPB, cooling is accomplished by a combination of surface cooling and a cold perfusate. At a core tem-perature of 15°C, up to 60 min of complete circula-tory arrest may be safe. Ice packing around the head is used to delay rewarming and for surface cooling of the brain. Pharmacological brain protection is often attempted with methylprednisolone, 30 mg/kg, and mannitol, 0.5 g/kg. Following the repair, CPB flow is restarted and rewarming takes place.

Postbypass Period

Because of the large priming volumes used (often 200–300% of the patient’s blood volume), hemo-static defects from dilution of clotting factors and platelets are commonly seen after CPB in infants; in addition to heparin reversal, administration of fresh frozen plasma and platelets is often necessary.

Patients undergoing extensive or complicated procedures will generally remain intubated. Extu-bation may be considered for older, relatively healthy patients undergoing simple procedures such as clo-sure of a patent ductus or atrial septal defect or repair of coarctation of the aorta.