Conduction Disturbances and Cardiac Pacemakers

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80 Conduction Disturbances and Cardiac Pacemakers

image Conduction Disturbances

Bradyarrhythmias and conduction blocks are common in the ICU. A broad range of clinical presentations and pathologic findings occurs in this group of arrhythmias. Some bradyarrhythmias are benign and asymptomatic and do not require treatment. Other atrioventricular (AV) blocks and arrhythmias are life threatening and warrant immediate intervention.

Normal Cardiac Conduction

Normal depolarization and impulse conduction are central to maintaining cardiac output. Two types of cells are found in the heart: (1) cells responsible for impulse generation and conduction, and (2) cells responsible for contraction. Depolarization of the myocardium begins in the sinoatrial (SA) node. The SA node is located in the posterior and superior portion of the right atrium and is innervated by the sympathetic and parasympathetic nervous systems.

The impulse is generated by a specialized group of cells with the ability to depolarize spontaneously. Initial depolarization of the SA node is not seen on the electrocardiogram (ECG). The P wave is generated when the impulse spreads throughout the atria. There is no specific conduction system in the atria to convey the SA node impulse to the AV node.1 The impulse is transmitted by depolarization of adjacent atrial myofibrils. Approximately halfway through the P wave, the impulse reaches the AV node. The second half of the P wave is due to left atrial depolarization.

In a normal heart, the atria and ventricles are electrically isolated from each other except at the AV node. The AV node is located in the atrial septum near the apex of the triangle of Koch. The AV node is innervated by the sympathetic and parasympathetic nervous systems. Conduction through the AV node accounts for the majority of the PR interval. After emerging from the AV node, the impulse is conducted through the bundle of His. From there, the impulse travels down the right and left bundle branches and their fascicles to the Purkinje network, which causes ventricular contraction.

Failure of Impulse Conduction

Failure of conduction can occur anywhere along the conduction pathway. AV node block is most often caused by medications, increased parasympathetic tone, or ischemia. AV node blocks are usually reversible, except when infarction permanently damages a portion of the conduction pathway. Infranodal blocks are rarely caused by physiologic abnormalities. Structural heart disease and anatomic disruption of the conduction system are the main causes of infranodal heart block. Rare causes of infranodal block include disruption of the bundle of His from aortic valve calcification, Lenègre’s disease (idiopathic degeneration of Purkinje fibers), and Chagas’ disease.2

Once AV block is identified, it is helpful to determine the site of conduction pathology. The anatomic site can be identified in most cases by synthesizing the type of AV block, the width of the QRS complex, and the QRS morphology. When the QRS complex is narrow (<0.12 seconds), the site of pathology is most likely supraventricular. When the QRS complex is wide, the most likely site of AV block is infranodal. Bundle branch and fascicular blocks produce various QRS morphologies that may aid in determining the specific anatomic location of pathology.

Clinical Presentation

Syncope and presyncope are the most dramatic symptoms of conduction disturbances; palpitations, dyspnea, angina, and fatigue are seen as well. Many patients are asymptomatic. A significant number of patients develop bradydysrhythmias after an acute myocardial infarction (AMI) (Table 80-1).3

TABLE 80-1 Incidence of Bradydysrhythmias in Acute Myocardial Infarction

Rhythm Incidence (%)
Any bradydysrhythmia 25-30
Sinus bradycardia 25
Junctional escape rhythm 20
Idioventricular escape rhythm 15
First-degree atrioventricular (AV) node block 15
Second-degree AV block type I 12
Second-degree AV block type II 4
Third-degree block 15
Right bundle branch block 7
Left bundle branch block 5
Left anterior fascicular block 8
Left posterior fascicular block 0.5

Sinus Node Abnormalities

Atrioventricular Node Dysfunction

There are many causes and several manifestations of AV node dysfunction. Box 80-1 lists the causes of AV node abnormalities.

First-Degree Atrioventricular Block

First-degree AV block is characterized by a prolonged PR interval greater than 0.20 second in adults and 0.18 second in children who are not taking medications that can prolong the PR interval (Figure 80-1). All the P waves are conducted to the ventricles, and the PR interval is typically fixed. Potential causes of first-degree AV block include delayed conduction through the atria from the SA node to the AV node, a delay in AV node conduction, or prolonged infranodal conduction.

Conduction delays from the SA node to the AV node are typically due to structural causes such as right atrial enlargement or an ostium primum atrial septal defect. A delay in AV node impulse conduction is the most common cause of first-degree AV block. Patients with delayed conduction in the AV node often have a PR interval greater than 0.30 second. Infranodal causes of first-degree AV block are rare and are typically associated with a wide QRS complex due to disease in the fascicles or the bundle of His. First-degree AV block can also occur when each of these conduction times is at the upper limit of normal and summate to produce an overall prolongation of the PR interval.7

First-degree AV block is typically benign and asymptomatic. It can be seen in 0.5% of young adults without heart disease. In older people, first-degree block is most often the result of idiopathic degenerative disease. A prolonged PR interval is often an incidental finding when an ECG is ordered for other reasons. It rarely warrants further workup or treatment.

Second-Degree Atrioventricular Block Type I

Second-degree AV block type I, or a Wenckebach (or Mobitz type I) rhythm, is defined by a progressive prolongation of the PR interval with each successive beat, with eventual failure of a P wave to conduct to the ventricles (Figure 80-2). This results in a dropped beat and failure of the ventricles to depolarize. The P waves occur at regular intervals. As the PR interval lengthens, the RR interval becomes shorter, which eventually results in decremental conduction. There is a reciprocal relationship between the RP interval and the PR interval.

The pathophysiology of second-degree AV block type I is similar to that of first-degree AV block, except that intraatrial block is usually not a cause. For all practical purposes, second-degree AV block type I is caused by a block in AV node conduction. The QRS complex is generally narrow.

QRS complexes are typically grouped in twos, threes, fours, and so on. Group beating is characteristic of Wenckebach rhythms. The rhythm is described by recording the number of P waves and QRS complexes involved in the pattern of block (e.g., 4 : 3 or 3 : 2). During a dropped beat, a P wave is observed with no corresponding QRS complex. Second-degree AV block type I is a stable rhythm and has a much better prognosis than does a Mobitz type II rhythm. If the Wenckebach rhythm is due to medication, resolution of the block can be monitored with an ECG. Once the medication is discontinued, a shortening of the PR interval and a lengthening of the RP interval, with a corresponding improvement in AV node conduction, may be observed.

Second-Degree Atrioventricular Block Type II

Second-degree AV block type II (or Mobitz type II block) is characterized by a sudden nonconducted P wave without a change in the PR interval. A P wave with no corresponding QRS complex is observed on the ECG (Figure 80-3). This is an inherently unstable rhythm, and serious pathology may be present. In contrast to the Mobitz type I rhythm, type II is described as a high degree of AV block, with P wave–to–QRS ratios of 3 : 1 and 4 : 1. A Mobitz type II rhythm is almost always due to an infranodal conduction disturbance. The conducted QRS complexes are often wide, and a bundle branch block pattern is often observed. Second-degree AV block can result from anterior wall MI. Type II second-degree AV block can progress to complete heart block.

2 : 1 Atrioventricular Block

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