Physical Address

304 North Cardinal St.
Dorchester Center, MA 02124

Bradycardias and Tachycardias: Review and Differential Diagnosis


Preceding chapters have described the major arrhythmias and atrioventricular (AV) conduction disturbances. These abnormalities can be classified in multiple ways. This review/overview chapter categorizes arrhythmias into two major clinical groups: bradycardias and tachycardias. The tachycardia group is further subdivided into narrow and wide (broad) QRS complex variants. This differential diagnosis is a major focus of electrocardiogram (ECG) differential diagnosis in acute care medicine and in referrals to cardiologists.

Bradycardias (Bradyarrhythmias)

The term bradycardia (or bradyarrhythmia ) refers to arrhythmias and conduction abnormalities that produce a heart rate <50 to 60 beats/min. (Some authors and professional societies define bradycardia as <50 beats/min and others, as <60 beats/min). Fortunately, the differential diagnosis of these rhythms is usually straightforward in that only a few categories need to be considered. For most clinical purposes, we can classify bradyarrhythmias into five major groups ( Box 19.1 ), recognizing that sometimes more than one rhythm is present (e.g., sinus bradycardia with complete heart block and an idioventricular escape rhythm).

Box 19.1
Bradycardias: Simplified Classification

  • Sinus bradycardia, including sinoatrial (SA) block and wandering atrial pacemaker

  • Atrioventricular (AV) junctional (nodal) and ectopic atrial escape rhythms

  • AV heart block (second- or third-degree) or AV dissociation variants

  • Atrial fibrillation or flutter with a slow ventricular response

  • Idioventricular escape rhythm (exclude hyperkalemia!)

Sinus Bradycardia and Related Rhythms

Sinus bradycardia is simply sinus rhythm with a rate <50 to 60 beats/min ( Fig. 19.1 ). When 1:1 (normal) AV conduction is present, each QRS complex is preceded by a P wave that is positive in lead II and negative in lead aVR. Some individuals, especially trained athletes at rest and adults during deep sleep, may have sinus bradycardia with rates as low as 30 to 40 beats/min due to physiologic, not pathologic, mechanisms.

Fig. 19.1, Marked sinus bradycardia at about 40 beats/min. Sinus arrhythmia is also present. Sinus bradycardia (like sinus tachycardia) always needs to be interpreted in clinical context because it may be a normal variant (due to increased vagal tone in a resting athlete or in a healthy person during sleep) or due to drug effect/toxicity, or causes such as sinus node dysfunction, as discussed in Chapter 13 . The PR interval here is also slightly prolonged (240 msec), also consistent with increased vagal tone, intrinsic atrioventricular (AV) nodal conduction slowing, or certain drugs that depress activity in the sinoatrial (SA) and AV nodes (e.g., beta blockers).

Sinus bradycardia may be related to a decreased firing rate of the sinus node pacemaker cells (as with athletes who have high cardiac vagal tone at rest) or to actual sinoatrial (SA) block (see Chapter 13 ). Inappropriate sinus bradycardia may be seen with the sick sinus syndrome (discussed in the following sections). The most extreme example of SA dysfunction is SA node arrest (see 13, 21 ). As further described, sinus bradycardia may also be confused for or associated with wandering atrial pacemaker (WAP). In addition, sinus rhythm with atrial bigeminy—where each premature atrial complex (PAC) is blocked (nonconducted)—may mimic sinus bradycardia (see below).

Wandering Atrial Pacemaker

Wandering atrial (supraventricular) pacemaker (WAP) is an “electrophysiologic cousin” of sinus bradycardia. As shown in Fig. 19.2 , WAP is characterized by multiple P waves of varying configuration with a relatively normal or slow heart rate. The P wave variations reflect shifting of the intrinsic pacemaker between the sinus node (and likely regions within the SA node itself) and different atrial sites. WAP may be seen in a variety of settings. Often it appears in normal persons (particularly during sleep or states of high vagal tone) as a physiologic variant. It may also occur with certain drug toxicities, sick sinus syndrome, and different types of organic heart disease.

Fig. 19.2, The variability of the P wave configuration in this lead II rhythm strip is likely caused by shifting of the pacemaker site between the sinus node and ectopic atrial locations.

Clinicians should be aware that WAP is quite distinct from multifocal atrial tachycardia (MAT), a tachyarrhythmia with multiple different P waves. In WAP the rate is normal or slow. In MAT, the rate is rapid and most often is associated with a serious disease process, such as decompensated chronic obstructive pulmonary disease. For rhythms that resemble MAT but with rates between 60 and 100 beats/min the more general term multifocal atrial rhythm can be used. MAT is most likely to be mistaken for atrial fibrillation (AF) because both produce a rapid irregular rate; conversely, AF is sometimes misinterpreted as MAT.

Sinus Rhythm with Frequent Blocked PACs

Clinicians should also be aware that when sinus rhythm is present with frequent, blocked PACs ( Fig. 19.3 ), the rhythm will mimic sinus bradycardia. The early cycle PACs are not conducted because of refractoriness of the AV node from the previous sinus beat. The premature P wave may be partly or fully hidden in the T wave. The slow pulse (QRS) rate is due to the post-atrial ectopic pauses.

Fig. 19.3, Superficially, this rhythm looks like sinus bradycardia. However, careful inspection reveals subtle blocked premature atrial complexes (PACs) superimposed on the T waves of each beat ( arrow ). These ectopic P waves are so premature that they do not conduct to the ventricles because of refractoriness of the atrioventricular node. The effective pulse rate will be about 50/min. Shown are modified leads II and V 2 from a Holter recording.

AV Junctional (Nodal) and Related Rhythms

With a slow AV junctional escape rhythm ( Fig. 19.4 ), either the P waves (seen just or after the QRS complexes) are retrograde (inverted in lead II and upright in lead aVR) or they are not apparent (“hidden in the QRS”) if the atria and ventricles are stimulated simultaneously. Slow heart rates may also be associated with ectopic atrial rhythms, including WAP. One specific type of ectopic atrial rhythm—termed low atrial rhythm —is addressed in Chapter 13 .

Fig. 19.4, The heart rate is about 43 beats/min, consistent with an atrioventricular (AV) junctional escape rhythm. Note that the ECG baseline between the QRS complexes is perfectly flat, i.e., no P waves or other atrial activity is evident, This pattern results from simultaneous activation of the atria and ventricles by the junctional (nodal) pacemaker, such that the P waves are masked by the QRS complexes.

AV Heart Block (Second- or Third-Degree)/AV Dissociation

A slow, regular 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. 19.5 ). In addition, patients with second-degree block (nodal or infranodal) often have a bradycardia due to the nonconducted P waves (see Chapter 17 ). Isorhythmic AV dissociation and related arrhythmias, which may be confused with complete AV heart block, are also frequently associated with a heart rate of less than 60 beats/min (see Chapter 17 ). This rhythm must be distinguished from sinus rhythm with frequent blocked PACs in a bigeminal pattern (see Fig. 19.3 ), as described previously.

Fig. 19.5, The sinus (P wave) rate is about 80 beats/min. The ventricular (QRS complex) rate is about 43 beats/min. Because the atria and ventricles are beating independently, the PR intervals are variable. The QRS complex is wide because the ventricles are stimulated (paced) either by an intrinsic (escape) idioventricular pacemaker or by an infranodal pacemaker with a concomitant intraventricular conduction delay.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here

Related Posts