Perinatal Arrhythmias

Sinus Node

The sinus node resides in the area between the vena cava and the right atrium and is predominantly located near the superior vena cava to the right-atrial junction. However, there is no anatomic feature that is visible on gross inspection. Functional electrophysiology testing studies and animal studies have demonstrated that the nodal tissue can span the lateral wall of the right atrium, extending from the superior vena cava to the inferior vena cava.

The sinoatrial nodal tissue is histologically distinct from the atrial myocardium and has no contractile components. Electrical impulses are generated by automatic action potential behavior. Activation of the adjacent atrial myocardium by the sinus nodal tissue results in a wave of depolarization that propagates by cell-to-cell activation in a superior-to-inferior and a right-toward-left (in the structurally normal heart) fashion.

Atrioventricular Node

The atrioventricular (AV) node is a more organized area of conduction in the right atrium near the crux of the heart. More well-defined than the sinus node, the compact AV node is located in the posterior portion of the interatrial septum just anterior to the tricuspid valve and (in most cases) provides a singular conduction channel into the bundle of His and the distal conduction system.

The majority of the atrial propagation wave depolarizes the mass of the atria. However, impulses that reach the area of the AV node enter the transition zone of conduction “input” into the AV node. Overall conduction velocity is slowed through the compact AV node but then exits the node into the specialized conduction fibers of the His–Purkinje system.

His–Purkinje System

In the normal heart, the atrial and ventricular myocardium are electrically isolated from each other by the AV rings (tricuspid and mitral annuli). The penetrating bundle of His is usually the only conductive tissue that traverses the AV rings into the ventricles. The bundle of His is insulated from the ventricular myocardium until it gives off branches. The first branch from the His bundle enables activation of the septum. Next, the bundle bifurcates into the right- and left-bundle branches. The left bundle divides into anterior and posterior fascicles. At the terminus of each of these bundle branches, the specialized conduction tracts fan into an intricate network of short Purkinje fibers that insert into numerous sites on the ventricular myocardium. Electrical impulses that are conducted through the His–Purkinje system result in activation of the ventricular myocardium in a highly organized and synchronized fashion, translating into a mechanically efficient contraction of both left and right ventricles almost simultaneously.

First-Degree Atrioventricular Block

Any conduction delay that prolongs the PR interval beyond the normal range for age is considered a first-degree AV block. By definition, impulses must still be conducted such that a 1:1 AV relationship persists. In the neonate, the normal PR interval is generally between 80 and 120 ms, up to 140 ms in the first few months of life, and up to 160 ms in the first 6 months of life. In general, first-degree AV block is benign and does not require any special treatment. It can be a normal variant or may be the result of influences that prolong the overall conduction time through the AV node and His–Purkinje system. Increased vagal tone is a common cause of PR prolongation. In the newborn, this can be a manifestation of vagal stimulation caused by a nasogastric or orogastric tube stimulating the oropharynx. More pathologic causes of PR prolongation can include neonatal lupus syndrome or myocarditis. Although there is evidence that significant PR prolongation has a negative impact on cardiac output in the adult heart, there is no indication that this is true in the neonate with a structurally and functionally normal heart. Even when the PR interval is extremely prolonged, there is usually no hemodynamic impact, and the cardiac examination remains essentially unchanged.

Second-Degree Atrioventricular Block

Second-degree AV block is ascribed when there is incomplete conduction from the atrium to the ventricle; that is to say, not every P wave is conducted to a QRS complex. This usually manifests as a skipped beat on physical examination or on cardiac monitoring. Mobitz type I conduction block, also known as the Wenckebach pattern , describes a pattern wherein the AV conduction becomes progressively prolonged with each successive beat. The PR interval becomes gradually longer, and after a number of beats (usually two or three, although this can certainly be longer), there is failure to conduct for a single beat. The subsequent P wave is then conducted with a normalized PR interval, and the cycle begins anew. Mobitz type II conduction block is present when there is a cyclical lack of conduction; however, the progressive PR prolongation seen in the Wenckebach pattern is lacking. Mobitz type I conduction can be seen in conditions of increased vagal tone (as described previously), whereas Mobitz type II is practically unheard of in the neonate in the absence of any other heart disease.

Third-Degree Atrioventricular Block

Third-degree, or complete AV block, occurs when there is a complete lack of conduction between the atria and the ventricles. In most situations of complete AV block, there is an escape depolarization mechanism, either junctional or ventricular in origin, which ensures that cardiac output is maintained. Complete AV block can be congenital or acquired. Congenital complete AV block is a particularly challenging entity and is described in a subsequent section. The most common causes of acquired complete heart block are infections or neonatal myocarditis. In addition, acquired complete heart block can occur as a complication of neonatal cardiac surgery in 1% to 3% of cases that involve surgical intervention near the interventricular septum.

Ventricular Preexcitation

Ventricular preexcitation occurs when the ventricular myocardium is activated abnormally, usually by an accessory bypass tract. The ventricular depolarization pattern is thus a combination (fusion) of early activation of a portion of the ventricles through an accessory pathway and the remainder of activation occurring via the normal His–Purkinje system. In general, these bypass tracts bridge the AV groove that typically separates the atria from the ventricles and result in an anomalous electrical connection to the ventricles.

There are three electrocardiographic features that define this “Wolff–Parkinson–White” pattern: (1) short PR interval, (2) a slurred “delta wave,” and (3) widened QRS duration. How evident this pattern is on electrocardiogram (ECG) can vary depending on the location of the accessory pathway (and thus how early the preexcited portion is activated) and how quickly the ventricles are also activated by the His–Purkinje system. In the newborn heart, the rapid transit time through the AV node and His–Purkinje system can result in a very minimal amount of preexcitation being evident, and appreciation of the presence of the Wolff–Parkinson–White pattern can be delayed, sometimes for years. Usually, this diagnosis is made only when the newborn patient experiences a tachyarrhythmia (as discussed later).

Ectopic Beats