Bradycardias and Tachycardias: Review and Differential Diagnosis

Chapter 20 Bradycardias and Tachycardias Review and Differential Diagnosis

The preceding chapters have described the major arrhythmias and atrioventricular (AV) conduction disturbances. These abnormalities can be classified in multiple ways. This chapter first divides them into two major clinical groups—bradycardias and tachycardias—and discusses the differential diagnosis of each group.

The tachycardias are then subdivided into narrow and wide complex variants, a major focus of ECG differential diagnosis in acute care medicine.

Bradycardias (Bradyarrhythmias)

A number of arrhythmias and conduction disturbances associated with a slow heart rate have been described. The term bradycardia (or bradyarrhythmia) refers to arrhythmias and conduction abnormalities that produce a heart rate of less than 60 beats/min. Fortunately, the differential diagnosis of a slow pulse is relatively simple in that only a few causes must be considered. Bradyarrhythmias fall into five general classes (Box 20-1).

Sinus Bradycardia and Related Rhythms

Sinus bradycardia is sinus rhythm with a rate less than 60 beats/min (Fig. 20-1). When 1:1 AV conduction is present, each QRS complex is preceded by a P wave; the P wave is negative in lead aVR and positive in lead II, indicating that the sinoatrial (SA) node is the pacemaker. Some individuals may have sinus bradycardia of 30 to 40 beats/min or less.

Sinus bradycardia may be related to a decreased firing rate of the sinus node pacemaker cells or to actual SA block (see Chapter 13). The most extreme example of sinus node dysfunction is SA node arrest (see Chapters 13 and 19). Sinus bradycardia may also be associated with wandering atrial pacemaker (WAP).

AV Junctional (Nodal) and Ectopic Atrial Rhythms

With a slow AV junctional escape rhythm (Fig. 20-3) either the P waves (seen immediately before or just after the QRS complexes) are retrograde (inverted in lead II and upright in lead aVR), or no P waves are apparent if the atria and ventricles are stimulated simultaneously. Slow heart rates may also be associated with ectopic atrial rhythms, including WAP (see previous discussion).

Idioventricular Escape Rhythm

When the SA nodal and AV junctional escape pacemakers fail to function, a very slow pacemaker in the ventricular conduction (His-Purkinje) system may take over. This rhythm is referred to as an idioventricular escape rhythm (see Fig. 13-10). The rate is usually very slow (often less than 45 beats/min), and the QRS complexes are wide without any preceding P waves. In such cases of “pure” idioventricular rhythm, hyperkalemia should always be excluded. In some cases of complete heart block, you may see sinus rhythm with an idioventricular escape rhythm, as described here. Idioventricular rhythm may be a terminal finding in irreversible cardiac arrest (also see Chapter 19).

Tachycardias (Tachyarrhythmias)

At the opposite end of the spectrum from bradyarrhythmias are the tachycardias. These rhythm disturbances produce a heart rate faster than 100 beats/min.

From a clinician’s perspective, the tachyarrhythmias can be most usefully divided into two general groups: those with a “narrow” (normal) QRS duration and those with a “wide” QRS duration (Table 20-1).

TABLE 20-1 Major Tachyarrhythmias: Simplified Classification

Narrow QRS Complexes Wide QRS Complexes
Sinus tachycardia Ventricular tachycardia
Paroxysmal supraventricular tachycardias (PSVTs) Supraventricular tachycardia with aberration caused by a bundle branch block or Wolff-Parkinson-White preexcitation with (antegrade) conduction down the bypass tract
Atrial flutter  
Atrial fibrillation  

The three most common types of PSVTs are AV nodal reentrant tachycardia (AVNRT), atrioventricular reentrant tachycardia (AVRT) involving a bypass tract, and atrial tachycardia (AT) including unifocal and multifocal atrial tachycardia, as discussed in Chapter 14. Other nonparoxysmal supraventricular tachycardias also may occur, including types of so-called incessant atrial, junctional, and bypass tract tachycardias. (For further details of this advanced topic, see selected references cited in the Bibliography.)

Narrow complex tachycardias are almost invariably supraventricular (i.e., the focus of stimulation is within or above the AV junction). Wide complex tachycardias, by contrast, are either ventricular or supraventricular with aberrant ventricular conduction (i.e., supraventricular tachycardia [SVT] with aberrancy).

The four major classes of supraventricular tachyarrhythmia are sinus tachycardia, paroxysmal supraventricular tachycardia (PSVT), atrial flutter, and AF. With each class, cardiac activation occurs at one or more sites in the atria or AV junction (node), above the ventricles (hence supraventricular). This activation sequence is in contrast to ventricular tachycardia (VT) in which the depolarization impulses originate in the ventricles. VT is simply a run of three or more consecutive premature ventricular depolarizations (see Chapter 16). The QRS complexes are always wide because the ventricles are not being stimulated simultaneously. The rate of VT is usually between 100 and 200 beats/min. By contrast, with supraventricular arrhythmias the ventricles are stimulated normally (simultaneously), and the QRS complexes are therefore narrow (unless a bundle branch block is also present).

Differential Diagnosis of Narrow Complex Tachyarrhythmias

The characteristics of sinus tachycardia, PSVTs, AF, and atrial flutter have been described in previous chapters. Sinus tachycardia in adults generally produces a heart rate between 100 and 180 beats/min, with the higher rates (150 to 180 beats/min) generally occurring in association with exercise. If you find a narrow (normal QRS duration) complex tachycardia with a rate of 150 beats/min or more in a resting adult, especially an elderly one, you are most likely dealing with one of the other three types of (nonsinus) arrhythmias mentioned previously.

PSVT and AF can generally be distinguished on the basis of their regularity. PSVT resulting from AV nodal reentry or a concealed bypass tract is usually an almost perfectly regular tachycardia with a ventricular rate between 140 and 250 beats/min (see Chapter 14). AF, on the other hand, is distinguished by its irregularity. Remember that with rapid AF (Fig. 20-6) the f waves may not be clearly visible, but the diagnosis can be made in almost every case by noting the absence of true P waves and the haphazardly irregular QRS complexes.

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Jun 11, 2016 | Posted by in CARDIOLOGY | Comments Off on Bradycardias and Tachycardias: Review and Differential Diagnosis

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