(A very slow, regularized ventricular response in AF suggests the presence of underlying complete AV heart block see Chapter 17 .)įIGURE 20-4 Regularization and excessive slowing of the ventricular rate with atrial fibrillation are usually a sign of drug toxicity (especially digitalis) or intrinsic AV node disease (see Chapter 18 ). The f waves may be very fine and easily overlooked. In such cases, the ECG shows the characteristic atrial fibrillatory (f) waves with a slow ventricular (QRS) rate. The rate may become excessively slow (less than 50 to 60 beats/min), however, because of drug effects or toxicity (beta blockers, calcium channel blockers, digitalis) or because of underlying disease of the AV junction ( Fig. Paroxysmal atrial fibrillation (AF), prior to treatment, is generally associated with a rapid ventricular rate. Isorhythmic AV dissociation, which may be confused with complete heart block, is also frequently associated with a heart rate of less than 60 beats/min (see Chapter 17 ).ĪTRIAL FIBRILLATION OR FLUTTER WITH A SLOW VENTRICULAR RATE The QRS complex is wide because the ventricles are being paced by an idioventricular pacemaker. Because the atria and ventricles are beating independently, the PR intervals are variable. The ventricular (QRS) rate is about 43 beats/min. 17-3 and Table 17-1).įIGURE 20-3 The atrial (P wave) rate is about 80 beats/min. In addition, patients with second-degree AV block (either Mobitz type I or Mobitz type II) sometimes have a bradycardia because of the dropped beats (see Fig. The baseline between the QRS complexes is perfectly flat (i.e., no P waves are evident).ĪV HEART BLOCK (SECOND OR THIRD DEGREE) OR AV DISSOCIATIONĪ 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 junctional or idioventricular pacemaker ( Fig. 20-2 ), either the P waves (seen immediately before or just after the QRS complexes) are retrograde (inverted in lead II and upright in lead aV R), or no P waves are apparent if the atria and ventricles are stimulated simultaneously.įIGURE 20-2 The heart rate is about 43 beats/min. With a slow AV junctional escape rhythm ( Fig. † SA block refers to situations in which impulses formed in the sinus node fail to depolarize the atria. * Some cardiologists reserve the term sinus bradycardia for rates less than 50 beats/min. The most extreme example is SA node arrest (see Chapter 13 ).
Sinus bradycardia may be related to a decreased firing rate of the sinus node pacemaker cells or to actual SA block. Some patients may have a sinus bradycardia of 40 beats/min or less.įIGURE 20-1 Sinus bradycardia with a slight sinus arrhythmia. Each QRS complex is preceded by a P wave the P wave is negative in lead aV R and positive in lead II, indicating that the sinoatrial (SA) node is the pacemaker. Sinus bradycardia is sinus rhythm with a rate less than 60 beats/min ( Fig.
Sinus bradycardia, including sinoatrial blockĪtrioventricular (AV) junctional (nodal) escape rhythmĪV heart block (second or third degree) or AV dissociationĪtrial fibrillation or flutter with a slow ventricular response Bradyarrhythmias fall into five general classes ( Box 20-2 ). Fortunately, the differential diagnosis of a slow pulse is relatively simple in that only a few causes must be considered. The term bradycardia (or bradyarrhythmia) refers to arrhythmias and conduction abnormalities that produce a heart rate of less than 60 beats per minute. (For further details of this advanced topic, see selected references cited in the Bibliography .)Ī number of arrhythmias and conduction disturbances associated with a slow heart rate have been described. Other nonparoxysmal supraventricular tachycardias may also occur, including types of so-called incessant atrial, junctional, and bypass-tract tachycardias. The three most common types of PSVTs are AV nodal reentrant tachycardias (AVNRT), atrioventricular reentrant tachycardia (AVRT) involving a bypass tract, and atrial tachycardia (AT) including unifocal and multifocal atrial tachycardia, as discussed in Chapter 14 . Supraventricular tachycardia with aberration caused by a bundle branch block– type or Wolff-Parkinson-White patternĪtrial fibrillation preexcitation syndrome Paroxysmal supraventricular tachycardias (PSVTs) Major Tachyarrhythmias: Simplified Classification This “review and overview” chapter simply divides them into two major clinical groups-bradycardias and tachycardias-and discusses the differential diagnosis of each group ( Box 20-1 ).
These abnormalities can be classified in numerous ways. The preceding chapters described the major arrhythmias and atrioventricular (AV) conduction disturbances.
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