Disorders of Cardiac Rhythm and Conduction in Newborns

Sinus Bradycardia

Sinus bradycardia is defined as heart rate less than 60 beats per minute (bpm) in older children and adults. However, in newborns, the resting heart rate is typically faster with a normal resting heart rate between 90 and 160 bpm, with intermittent decreases to as low as 70 bpm during rest or sleep.

Sinus bradycardia generally is physiologic in infants with a structurally normal heart and is secondary to transient increases in vagal tone. This increase in vagal tone is almost always related to an underlying process, such as increased intracranial pressure, an acute abdominal process (e.g., necrotizing enterocolitis), suctioning of an endotracheal tube, gastric reflux, apnea of prematurity, or medications (e.g., sedatives, beta blockers).

Most episodes of sinus bradycardia in neonates are episodic and related to an abrupt increase in vagal tone that may result in sinus pauses or dropped heart beats, resulting in heart rate decreasing to 40-50 beats per minute. Episodic abrupt decreases in the heart rate with an otherwise normal heart are caused by a secondary process and almost never have a primary cardiac etiology, but the pauses can be relatively long lasting, up to several seconds.

The initial evaluation of bradycardia involves obtaining an electrocardiogram (ECG) to ensure that there is no evidence of atrioventricular (AV) block or other primary cause such as a cardiac channelopathy as the cause of the bradycardia. The primary method used to assess the overall function of the sinus node in a neonate is a 24-hour cardioscan monitor (Holter) that records every electrical cardiac impulse during the monitoring period. This monitor is used to evaluate the infant's heart rate variability as well as maximum and minimum heart rates. Even though rarely there are ways to demonstrate that sinus bradycardia is caused by increased vagal tone and is not secondary to sinus node disease, administration of a vagal nerve antagonist such atropine (muscarinic-cholinergic blocking agent) increases heart rate almost immediately if the bradycardia is secondary to vagotonia, although this is only rarely clinically indicated. There is minimal or no heart rate increase to atropine in those infants and children with primary sinus node dysfunction. Patients with sinus node dysfunction also typically have bradycardia that persists throughout the day and may only reach a maximum heart rate of 100-130 beats per minute.

Treatment of sinus bradycardia is rarely required. Evaluation for an underlying cause (e.g., seizures, reflux, or apnea) should be undertaken, particularly in cases of episodic bradycardia. If a neonate has a slow underlying rate but has no evidence of hemodynamic compromise, in general, no intervention is required. A pacemaker may be placed if there is persistent hemodynamically significant sinus node dysfunction, but this is exceptionally rare in the neonatal population.

Sinus Tachycardia

Sinus tachycardia is defined as increase in the sinus rate above 160-180 bpm in infants. It is usually a normal physiologic response to anemia, hyperthyroidism, fever, agitation, infection, or medications to name just a few causes. It is rarely caused by a primary cardiac etiology, unless there is underlying cardiac dysfunction such as seen with myocarditis.

It is important to ensure that an elevated heart rate is truly sinus tachycardia and is not caused by an abnormal cardiac rhythm such as supraventricular tachycardia (SVT). The typical maximum heart rate in a “normal” infant is 220 bpm minus the age of the patient. Critically ill neonates may sometimes exceed this rate; sinus tachycardia at rates greater than 220 bpm should warrant evaluation for the presence of pathology.

Some other findings that suggest a pathologic mechanism of tachycardia are:

• 1.

  The lack of heart rate variability

• 2.

  Abnormal P-wave morphology or axis (normally upright in leads I and aVF) on ECG

• 3.

  Prolongation of PR interval

• 4.

  Very rapid increases or decreases in heart rate

• 5.

  Abnormal cardiac function on echocardiogram

The best method for initial evaluation of sinus tachycardia typically involves obtaining an ECG to ensure that there is no evidence of a pathologic arrhythmia. Similar to sinus bradycardia, a 24-hour Holter can be helpful in ruling out SVT in questionable cases when one evaluates it for heart rate variability, episodes of pauses, and/or changes in the P-wave axis.

Treatment of sinus tachycardia is directed at treating the underlying cause. Neonates with high resting heart rates generally do not require any treatment but should be closely evaluated to make sure they do not have a secondary cause such as infection or anemia.

Premature Atrial Contractions

Premature atrial contractions (PACs) are early depolarizations of atrial tissue distinct from the sinus node. As the impulse originates from the area different than the sinus node, the P wave will often have a different morphology than the “normal” P wave from the sinus node.

Premature atrial contractions usually conduct through the AV node, bundle of His, and right and left bundle branches to the ventricles. If atrial activation occurs early enough that the AV node or bundle of His is refractory, the PACs will not be conducted to the ventricle; this is commonly referred to as a “blocked” PAC. In this case, the premature P wave is seen without a following QRS complex. When this occurs clinically, these block PACs are often read as bradycardia, which is not an accurate description of what is occurring electrically in the heart. If a PAC activation wavefront passes through the AV node, but either the right or left bundle branch is still refractory, then it will be followed by a wide QRS (aberrant ventricular conduction). This can be mistaken for a premature ventricular contraction, but close observation will show atrial activation before the QRS, thus making the diagnosis of a premature atrial contraction. All three of these examples, conducted, nonconducted, and aberrantly conducted PACs, may be present in the same patient.

Premature atrial contractions are commonly seen in healthy newborns and may occur frequently (>200 per hour) in an individual patient. In patients with isolated PACs, they may occur in a pattern of two PACs in a row, which is referred to as an atrial couplet. If every other beat is a PAC, then this pattern is referred to as atrial bigeminy . Atrial couplets and bigeminy are benign and make no impact on the individual's prognosis when compared with those with isolated PACs as long as no evidence of atrial tachycardia is present. Rarely, PACs in a bigeminal pattern will block (i.e., every other beat is a nonconducted PAC), thus resulting in bradycardia. It is important to evaluate the ECG in fetuses and neonates with bradycardia to ensure that this pattern is not the cause of their “bradycardia.” This pattern is especially common in fetuses that are referred for episodes of extreme bradycardia with intermittent episodes of normal conduction. These fetuses are usually not in distress, and it is worthwhile to explain these circumstances to other care providers before any rush decisions are made concerning early or emergent delivery.

Isolated PACs usually are benign and do not require any therapy. In most newborns, PACs will resolve in the first few months of life and cause no hemodynamic compromise or clinical symptoms. A follow-up visit with a pediatric cardiologist may be recommended at 4-6 months postdelivery to ensure resolution.

Atrial Ectopic Tachycardia

When three or more consecutive PACs occur in an infant or neonate at a rate faster than 120 beats per minute, the term atrial ectopic tachycardia (AET) is used. Atrial ectopic tachycardia is typically the result of an increased automaticity of atrial myocardium. With this mechanism, there is abnormal firing of atrial tissue originating outside the sinus node. With an automatic focus, there are typically “warming up” and “cooling down” periods for the tachycardia. This frequently involves rapid increases or decreases in heart rate over several beats rather than initiation or termination in a single beat. Most AETs are paroxysmal in nature, resulting in little if any harm to the patient, but they may be incessant. The majority of neonates with AET also have frequent isolated PACs. The abnormal focus can be located almost anywhere in the atrium, with common foci being along the crista terminalis in the right atrium or the pulmonary veins. In AET, the activation of atria is different from that of normal sinus rhythm, resulting in a different P-wave morphology on the ECG. Occasionally, the focus may occur near the sinus node (i.e., right upper pulmonary vein or right atrial appendage), producing P-wave morphology similar to the one in sinus rhythm, so careful inspection of the P-wave morphology using a 12- or 15-lead ECG is warranted to look for subtle changes in P-wave morphology.

Although an abnormal focus of atrial tissue is the usual cause of atrial tachycardia, mechanical stimulation of the atria can also cause an atrial tachycardia. This is frequently seen in neonates who have an intravenous catheter with the tip located in the atrium. This catheter then creates an atrial tachycardia by directly stimulating the atria. One differential clue to the mechanism of this unique form of SVT is a very fast and irregular atrial tachycardia. A chest radiograph to visualize line position should be performed to ensure that the cause of the tachycardia is not line related before initiation of treatment.

Evaluation of an atrial tachycardia involves an ECG and Holter monitoring. Rapid, incessant AET can negatively affect ventricular function because of the heart's inability to “rest” or slow down. Therefore, an echocardiogram is usually part of the initial evaluation in the newborn period to evaluate function and rule out congenital heart disease with a dilated atrium as the cause of the atrial tachycardia. Electrolyte disturbances are a very rare cause of AET in neonates but should be corrected if present.

Treatment of atrial ectopic tachycardia may be attempted with medications such as beta blockers (propranolol), sodium channel blockers (flecainide), or class III antiarrhythmic medications (sotalol, amiodarone). The majority of neonatal AETs resolve spontaneously in the first 6 months of life, and long-term therapy is rarely necessary. Cardiac catheterization and radiofrequency ablation (RFA) are very high-risk procedures in neonates and are typically not performed except in extreme cases of medically refractory atrial tachycardia with depressed function. In very rare cases, mechanical support using extracorporeal membrane oxygenation (ECMO) is required to support the circulation until the tachycardia can be controlled.

Atrial Flutter

Atrial flutter is a classic example of atrial reentry tachycardia. In this type of tachycardia, the circuit or substrate for reentry is the atrial myocardium located around the tricuspid valve. The area of relatively slow conduction in this type of arrhythmia is generally located between the inferior vena cava (IVC) and the tricuspid valve annulus in an area referred to as the isthmus . The velocity of the impulse propagation is slow through the isthmus, allowing recovery of the atrial myocardium ahead of it and, therefore, allowing the atrial wave front to propagate.

The typical atrial rate of neonatal atrial flutter is between 300 and 600 beats per minute and is much faster than the flutter seen in older children and adults. However, the ventricular rate is much less because of the decremental properties of the AV node not allowing for such rapid conduction. The conduction to the ventricles is generally 2 : 1, 3 : 1, or 4 : 1 ( Fig. 77.2 ) and may vary among these conduction rates, resulting in an irregular rhythm. Occasionally, the AV conduction can be rapid, resulting in ventricular rates greater than 200 bpm, which can be poorly tolerated by the fetus and neonate alike.

In most cases, the diagnosis can be made with a simple surface ECG, noting the continuous sawtooth pattern of atrial activity. However, it may be necessary to administer adenosine to the patient while recording an ECG, which blocks atrial conduction down the AV node and allows for rapid, easy identification of the sawtooth pattern classically described for this arrhythmia in patients when the P waves are masked by QRS complexes or T waves.

The initial treatment for atrial flutter is synchronized electrical cardioversion with 0.5-2 Joules per kilogram, but higher doses may be required because of the relatively small surface area of the cardioversion paddles/patches in neonates. The best position for the paddles or patches is front to back, slightly to the left side of the chest, and it is often necessary to turn the infant on its side to get the paddles into position. Rapid atrial pacing (overdrive pacing) may also terminate the tachycardia but is difficult to achieve with the rapid atrial rate seen in neonatal flutter and has a lower overall success rate. In addition to these two methods, some physicians use antiarrhythmic medications and observe for up to 48 hours to see if the tachycardia terminates spontaneously, but this method has lower success rates when compared to synchronized cardioversion.

Atrial flutter in an otherwise healthy newborn generally does not return once the patient is successfully treated unless there is another arrhythmia such as atrial tachycardia. Therefore no routine antiarrhythmic treatment is necessary for either short- or long-term treatment of standard atrial flutter once the patient is cardioverted. If there is underlying atrial dilation, recurrent atrial flutter, or structural heart disease, treatment with antiarrhythmic medications such as propranolol or digoxin may be warranted.

Atrial Fibrillation

Neonatal atrial fibrillation (AFib) is an exceptionally rare dysrhythmia in this patient population and is typically seen only in patients with severe structural congenital heart disease (such as Ebstein anomaly of the tricuspid valve) or in conjunction with an accessory pathway (Wolff-Parkinson-White) or those with cardiac channelopathies.

It is important to distinguish atrial fibrillation from a rare form of AET called chaotic atrial tachycardia (or multifocal atrial tachycardia) because both result in an irregular tachycardia with disorganized atrial activity on the ECG. Chaotic atrial tachycardia (also referred to as multifocal atrial tachycardia) is a form of atrial tachycardia in which there are rapid bursts of tachycardia from multiple areas of the atria. Atrial fibrillation is an incessant arrhythmia that typically will respond to cardioversion. However, chaotic atrial tachycardia tends to occur in short bursts with sinus beats interspersed with runs of tachycardia and a varying P-wave axis. This type of tachycardia either does not respond to cardioversion or quickly returns after cardioversion.

Treatment of AFib in neonates involves synchronized cardioversion. Pediatric cardiology consultation should be obtained because AFib is extremely rare in neonates and generally occurs in only the sickest neonates with other cardiac disorders. Chaotic atrial tachycardia requires medical treatment with antiarrhythmic medications such as beta blockers, flecainide, sotalol, or amiodarone, alone or in combinations, with a goal simply to control the ventricular rate, not to cure the atrial tachycardia.