Chapter 20 Bradycardias and Tachycardias Review and Differential Diagnosis
Bradycardias (Bradyarrhythmias)
BOX 20-1 Bradycardias: Simplified Classification
Sinus Bradycardia and Related Rhythms
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).
Wandering Atrial Pacemaker
Clinicians should be aware that WAP is quite distinct from multifocal atrial tachycardia (MAT), another arrhythmia with multiple different P waves. In WAP the rate is normal or slow. In MAT it is rapid. For rhythms that resemble MAT (see Chapter 14), but with rates between 60 to 100 beats/min, the more general term multifocal atrial rhythm can be used.
AV Heart Block (Second or Third Degree) or AV Dissociation
A slow ventricular rate of 60 beats/min or less (even as low as 20 beats/min) is the rule with complete heart block because of the slow intrinsic rate of the nodal (junctional) or idioventricular pacemaker (Fig. 20-4). In addition, patients with second-degree block (nodal or infranodal) often have a bradycardia because of the dropped (nonconducted) beats (see Chapter 17).
Atrial Fibrillation or Flutter with a Slow Ventricular Rate
Paroxysmal atrial fibrillation (AF), prior to treatment, is generally associated with a rapid ventricular rate. However, the rate may become excessively slow (less than 50 to 60 beats/min) because of drug effects or toxicity (e.g., beta blockers, calcium channel blockers, digoxin) or because of underlying disease of the AV junction (Fig. 20-5). In such cases the ECG shows the characteristic atrial fibrillatory (f) waves with a slow, sometimes regularized ventricular (QRS) rate. The f waves may be very fine and easily overlooked. A very slow, regularized ventricular response in AF suggests the presence of underlying complete AV heart block (see Chapters 15 and 17).
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)
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
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.