Disorders of Cardiac Rhythm and Conduction in Newborns



Disorders of Cardiac Rhythm and Conduction in Newborns


Bryan Cannon, Oleg Kovalenko and Christopher S. Snyder


Neonates are at risk for a variety of cardiac arrhythmias that vary from benign and asymptomatic to life threatening. This chapter discusses the normal and abnormal variations in cardiac rhythms that one frequently encounters in the newborn period.



Normal Sinus Rhythm and Sinus Node Dysfunction


Normal Sinus Rhythm and Its Variations


The initiation of electrical activity in the heart typically begins in the sinus node, which is located posteriorly at the junction of superior vena cava and right atrium.


The sinus node has inputs from both sympathetic and parasympathetic nervous systems. The balance of the two inputs determines the heart rate at any given moment. At rest and during sleep, vagal nerve stimulation is increased, causing slowing of the heart rate and potentially sinus bradycardia. During periods of stress, stimulation, or activity, sympathetic tone predominates, leading to an increase in heart rate and potentially sinus tachycardia.


It is important to remember that in children with a structurally normal heart, sinus bradycardia or sinus tachycardia are usually physiologic findings and are unlikely to be related to an underlying cardiac condition. As long as the impulse is generated from the sinus node (characterized by a positive P wave in leads I and aVF on the electrocardiogram or ECG), it is very rare to have an underlying cardiac problem.



Sinus Bradycardia


Sinus bradycardia is defined as heart rate below 60 beats per minute (bpm) in older children and adults. However, in newborns, the resting heart rate is typically faster. In newborns, a normal resting heart rate is between 90 and 160 bpm, but may intermittently drop as low as 70 bpm during sleep.


Sinus bradycardia usually is physiologic in children with a structurally normal heart and is secondary to increased vagal tone. This increase in vagal tone is almost always physiologic but can rarely be related to an underlying process, such as increased intracranial pressure, an acute abdominal process (e.g., necrotizing enterocolitis) or medications (e.g., sedatives, beta blockers).


Most cases of sinus bradycardia in neonates are episodic and related to an abrupt increase in vagal tone that may result in sinus pauses or drops in the heart rate to 40 to 50 beats per minute. Common causes in neonates include gastroesophageal reflux, apnea, and seizures. In addition, children who are intubated or have a tracheostomy may have episodic decreases in heart rate because of increased vagal tone from the endotracheal tube or suctioning. Episodic abrupt decreases in the heart rate with an otherwise normal rate are caused by a secondary process and almost never have a primary cardiac etiology.


The initial evaluation of bradycardia involves obtaining an electrocardiogram (ECG) to ensure that there is no evidence of atrioventricular (AV) block. The primary method used to assess the overall function of the sinus node in a neonate is a 24-hour cardioscan monitor (Holter monitor) that records every electrical cardiac impulse during the monitoring period. This monitor can be used to evaluate for heart rate variability as well as maximum and minimum heart rates. Even though rarely necessary in clinical practice, there are some ways to show 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. There is minimal or no response to atropine in those infants and children with primary sinus node dysfunction.


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, generally no intervention is necessary. A pacemaker may be placed if there is persistent hemodynamically significant sinus node dysfunction, but this is almost never necessary in the neonatal population.



Sinus Tachycardia


Sinus tachycardia is defined as increase in the sinus rate above 160 to 180 bpm in infants.


Sinus tachycardia is usually a normal physiologic response to anemia, fever, agitation, and infection, among many other 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 individual 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:



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 making the diagnosis in questionable cases.


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.



Sinus Arrhythmia


In children with sinus arrhythmia, sometimes called respiratory sinus arrhythmia, phasic variations in heart rate are seen with an increase in heart rate during inspiration and a decrease in heart rate with expiration (Figure 86-1).


image
Figure 86-1 Sinus arrhythmia in a healthy child. Note that all P waves look exactly the same although there is marked variation in their rate.

Sinus arrhythmia is caused by the so-called Bainbridge reflex (baroreceptor reflex). During inspiration, the intrathoracic pressure decreases and triggers increased venous blood return to the right atrium. The increased volume in the right atrium is registered by stretch receptors, which causes increase in heart rate. The opposite situation occurs during expiration, and heart rate decreases.


This variation is sometimes thought to be a pathologic arrhythmia because the heart rate variability may be very pronounced during auscultation in young children and infants. Although in most situations, the diagnosis can be made by noting heart rate variation that correlates with the respiratory cycle. ECG interpretation can be helpful if the diagnosis is in question. On an ECG, there is variation in the rate, but no change in the appearance of the P-wave morphology or axis, thus confirming the diagnosis.


This condition is physiologic and does not require any treatment or follow-up.



Tachyarrhythmias


Atrial Tachycardia


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 often has a different morphology than the P wave that results from the normal 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 can be refractory, the PACs will be not be conducted; this is referred to as a “blocked” PAC. In this case, the premature P wave is seen without a following QRS complex. 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. Often, conducted, nonconducted, and aberrantly conducted PACs may be seen in the same patient.


Premature atrial contractions are frequently seen in healthy newborns and may occur relatively frequently in an individual patient. Sometimes two PACs in a row occur, which is called an atrial couplet. If every other beat is a PAC, then this pattern is referred to as atrial bigeminy. Atrial couplets and bigeminy are generally benign and make no difference in the individual’s prognosis when compared with isolated PACs if no 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 patients with bradycardia to ensure that this pattern is not present. 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 any distress, and it is worthwhile to explain these circumstances to other care providers before any rush decisions are made concerning early delivery 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. A follow-up visit with a pediatric cardiologist may be recommended at 4 to 6 months to ensure resolution.



Atrial Ectopic Tachycardia


When three or more consecutive PACs occur in a neonate, 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 can also be incessant. The majority of neonates with AET also have frequent isolated PACs. The abnormal focus can be located almost anywhere in the left or right 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 can be localized close to the sinus node area, producing P-wave morphology similar to the one in sinus rhythm.


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 or incessant AET can affect ventricular function. Thus, 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 rare cause of AET, 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 case, the tachycardia 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 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 wavefront to propagate.


The typical rate of neonatal atrial flutter is between 300 and 600 beats per minute. However, the ventricular rate is much less, due to 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 (Figure 86-2) and may vary among these conduction rates, resulting in an irregular rhythm. Occasionally, the AV conduction can be very rapid, resulting in ventricular rates greater than 200 bpm, which may be poorly tolerated by the fetus and neonate alike.


image
Figure 86-2 Atrial flutter with 4 : 1 AV conduction in a 1-day-old newborn.

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, which blocks the AV node and allows for identification of the arrhythmia in patients when the P waves are masked by QRS complexes or T waves.


The initial therapy to treat atrial flutter typically involves synchronized electrical cardioversion with 0.5 to 2 Joules per kilogram.4,10 The best position for the paddles is front to back on 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 may be difficult to achieve with the rapid atrial rate seen in neonatal flutter and has a lower overall success rate.12,19 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.


Atrial flutter in an otherwise healthy newborn tends not to return once the patient is successfully treated unless there is underlying congenital heart disease or another arrhythmia such as atrial tachycardia. Therefore, no routine antiarrhythmic treatment is necessary for either short- or long-term treatment of standard atrial flutter.4,10,12,19 If there is underlying atrial dilation, recurrent atrial flutter, or structural heart disease, treatment with antiarrhythmic medications such as 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 structural congenital heart disease (such as Ebstein anomaly of the tricuspid valve) or in conjunction with an accessory pathway (Wolff-Parkinson-White).24


It is important to distinguish atrial fibrillation from chaotic atrial tachycardia because both may appear as an irregularly 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. On the other hand, chaotic atrial tachycardia tends to occur in short bursts with sinus beats interspersed with runs of tachycardia with 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. Chaotic atrial tachycardia requires medical treatment with antiarrhythmic medications such as beta blockers, flecainide, sotalol, or amiodarone, alone or in combinations, with a goal of rate control.



Reentrant Supraventricular Tachycardia


Supraventricular tachycardia (SVT) caused by a reentrant mechanism is the most common form of SVT in this population. In a reentrant tachycardia, there are two distinct conducting pathways linked around an area of nonconducting tissue. Failure to conduct in one of these pathways (block) causes the impulse to turn around in the other pathway, creating an electrical loop that causes tachycardia (Figure 86-3).


image
Figure 86-3 Reentrant SVT: A, Normal conduction down both sides of a reentrant circuit. The electrical impulses collide and there is no reentry. B, Block in one pathway and conduction in the other. C, The impulse from the conducting pathway reaches the other pathway. D, This pathway now conducts setting up an electrical circuit of reentrant tachycardia.

There are two common reentrant mechanisms of supraventricular tachycardia seen in the pediatric population. The first, and by far the most common in newborns and infants, is caused by an accessory pathway.2 The second form is due to reentry around the atrioventricular node, also known as atrioventricular nodal reentry tachycardia or AVNRT.



Supraventricular Tachycardia Caused by a Manifest Accessory Pathway


In healthy individuals, only one connection exists linking atrial impulses to the ventricles, which is the bundle of His. The bundle of His penetrates the fibrous atrioventricular ring and normally is the only structure capable of conducting electrical impulses from the atria to the ventricles. During normal sinus rhythm, the activation wavefront travels from the sinus node across the atrial myocardium, then to atrioventricular node, bundle of His to the right and left bundle branches, ultimately exiting to the ventricular myocardium to cause a depolarization.


If an accessory pathway is present, it serves as an additional conduction pathway for electrical impulses to travel between the atria to the ventricles. Accessory pathways are able to conduct an impulse in both directions (antegrade from atria to ventricles and retrograde from ventricles to atria), or only in one direction—either exclusively from the atria to the ventricles or from the ventricles to the atria. If an accessory pathway is capable of conducting the impulse only from the ventricles to the atria (retrograde), it is referred to as a concealed accessory pathway, and the baseline ECG will appear normal. If an accessory pathway is capable of conducting an impulse from the atria to the ventricles, it is referred to as a manifest accessory pathway,13 resulting in what is known as preexcitation on the baseline ECG. Patients with preexcitation (also called Wolff-Parkinson-White [WPW]) have an area of early ventricular activation on their ECG. This early ventricular activation on the ECG is called a delta wave, and results in a short PR interval and widened QRS complex (Figure 86-4). Preexcitation is present because the electrical impulse travels from the atria to the ventricles over two pathways—through the AV node/bundle of His, which has decremental properties, and down the accessory pathway. When activation travels down the conduction system (AV node), a normal physiologic delay of conduction occurs. This delay is necessary to make sure that atrial systole will occur before ventricular systole. Although there is a delay in conduction through the AV node, no such delay exists when a manifest accessory pathway is present. Therefore, the atrial impulse travels quickly from the atria to the ventricles across the accessory pathway and activates the ventricular myocardium quickly. This manifests as a delta wave on the ECG.


image
Figure 86-4 Electrocardiogram of a patient with Wolff-Parkinson-White syndrome shows a short PR interval, widened QRS, and slurring of the upstroke of the QRS.

About 20% of patients with Wolff-Parkinson-White have underlying structural heart disease (most commonly Ebstein anomaly); therefore, an echocardiogram should be performed when the diagnosis is made. Wolff-Parkinson-White syndrome can be hereditary, but most times is sporadic, occurring in about 1 to 3 per 1000 individuals.21


Patients with WPW, a manifest accessory pathway, have an increased risk of atrial fibrillation for reasons that are not well understood. If atrial fibrillation occurs in a patient with WPW, an irregularly irregular wide complex tachycardia ensues (Figure 86-5). In infants and neonates, there are almost no other causes of an irregularly wide complex tachycardia other than atrial fibrillation in the presence of an accessory pathway, although this occurrence is extremely rare in infants and neonates. Patients with WPW and atrial fibrillation also have the risk of sudden cardiac death.16 In atrial fibrillation, these accessory pathways may rapidly conduct the atrial impulses to the ventricles, resulting in rapid ventricular activation that can degenerate into ventricular fibrillation and results in cardiac arrest. For this reason, adenosine, which blocks AV nodal conduction, should only be used in a very controlled environment.


image
Figure 86-5 Preexcited atrial fibrillation in a patient with Wolff-Parkinson-White syndrome. Note the irregularly irregular wide complex rhythm.


Supraventricular Tachycardia Caused by a Concealed Accessory Pathway


Patients with a concealed accessory pathway are at risk of developing supraventricular tachycardia secondary to a reentrant mechanism.9,31 Supraventricular tachycardia is usually initiated by a premature atrial or ventricular contraction. The premature beat will block in one of these two conduction pathways—the accessory pathway or the AV node. If the premature beat blocks in the accessory pathway, the activation wavefront travels down the normal conduction system (AV node and bundle of His) to the ventricles. The conduction wavefront then passes through the ventricular myocardium. The wavefront then reaches the accessory pathway, which has had time to recover and is able to conduct electrical impulses. The electrical impulse travels retrograde, from the ventricle to the atria, then back down the AV node to perpetuate the tachycardia. This type of tachycardia, known as orthodromic tachycardia, is generally a narrow complex in nature as it travels down the normal conduction system to get to the ventricles. Each of the components of this circuit is integral to allow for propagation of the tachycardia, and disruption of the circuit in any of the limbs will terminate the tachycardia (Figure 86-6).


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Figure 86-6 Baseline electrocardiogram (first 10 beats) shows preexcitation. The QRS complex #11 represents premature atrial contraction followed by a reentrant narrow complex tachycardia (orthodromic supraventricular tachycardia).


Supraventricular Tachycardia Caused By Atrioventricular Nodal Reentry Tachycardia


Another form of reentrant tachycardia seen in neonates is atrioventricular nodal reentry tachycardia (AVNRT). In this form of SVT, the entire circuit is contained within the proximal portion of the AV node and no true accessory pathway is present. Its reentry circuit is located between the slow and fast pathway limbs of the atrioventricular node.3,11 In most individuals, the AV node has both of these inputs, which coalesce to form the compact AV node, and which then continues to the bundle of His. Because slow and fast pathways have different conduction properties, a reentry tachycardia between these inputs can and often does occur. The resulting tachycardia is referred to as atrioventricular nodal reentry. Because AVNRT has a reentry mechanism, it is usually started by a premature atrial or ventricular beat. On ECG, it usually presents as a narrow complex tachycardia that is indistinguishable from an accessory pathway-mediated tachycardia. Atrioventricular nodal reentry tachycardia has a developmental component and is quite rare in neonates and infants, but is relatively common as a mechanism of tachycardia in older teenagers and adults.2

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Jun 6, 2017 | Posted by in PEDIATRICS | Comments Off on Disorders of Cardiac Rhythm and Conduction in Newborns

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