Bradyarrhythmia is defined as any cardiac rhythm with a slow ventricular response. In the adult patient, a heart rate less than 60 bpm is considered “slow” and thus a bradyarrhythmia; infants and young children, of course, have age‐related rate definitions for bradycardia. Bradycardic rhythms include a broad range of diagnoses such as sinus bradycardia, junctional and idioventricular rhythms, advanced atrioventricular (AV) block with varying escape rhythms (i.e. second‐ and third‐degree AV blocks), atrial rhythms with heightened AV block (i.e. atrial fibrillation with slow ventricular response), and rhythms related to metabolic issues (i.e. hyperkalemia). In most instances, a slow heart rate is indicative of pathology; sinus bradycardia, however, can be encountered in high endurance athletes, among other patient types, representing a normal variant finding.

The differential diagnosis of bradycardia is listed in Table 24.1.

Sinus bradycardia is a rhythm with a heart rate less than 60 bpm; the impulse originates from the sinus node, demonstrating the characteristics of sinus rhythm, including upright P waves in leads I–III and a 1 : 1 P wave‐to‐QRS complex ratio. It is the most common form of bradycardia seen in clinical medicine and has a multitude of causes including acute coronary syndrome (ACS), chronic conduction system disease, medication effect, electrolyte disturbances, and extreme athletic conditioning (Figure 24.1a).

See Management Box 24.1.

Junctional escape rhythms have a cardiac impulse originating from the AV junction. This rhythm occurs when impulses from the sinoatrial (SA) node or other atrial sources are not present, are not functioning properly, or are not being sensed by the AV node; a junctional rhythm is thus an escape rhythm. The AV nodal tissue assumes the pacemaker function of the heart, creating a junctional rhythm, which is bradycardic. It can be identified as regular narrow complex bradycardia (Figure 24.1b) at a rate of 40–60 bpm with absent, misplaced, and/or inverted P waves. The misplaced and/or inverted P waves will be present if there is retrograde transmission of the impulse from the AV node to the atrium and are termed retrograde P waves. In the setting of third‐degree heart block, a junctional rhythm can be seen as the escape rhythm; in this case, there may also be normal appearing upright P waves occurring at regular intervals but at a rate and rhythm completely detached from the “junctional” QRS complexes. The upright P waves are caused by the SA node‐firing impulses that are not being sensed by the AV node and thus are not being transmitted to the ventricles. The junctional rhythm, when encountered, should be considered an escape rhythm, assuming the role of pacemaker if the SA node is non‐functional. The clinical causes are similar to those of sinus bradycardia (Clinical Presentation Box 24.1).

Idioventricular rhythm has a cardiac impulse originating from a focus in the ventricles. This rhythm occurs when dysfunction in the conducting system proximal to the ventricles forces a focus in the ventricular myocardium to assume the function as a cardiac pacemaker. This assumption of pace‐making function may occur when SA node and all other foci in the atria and AV node fail to either conduct or generate an impulse or when there is complete AV block. Similar to the junctional rhythm, the idioventricular rhythm should be considered an escape rhythm. An idioventricular rhythm (Figure 24.1c) can be identified as a regular, wide complex bradycardia with a rate of 20–40 bpm with absent P waves. Not unlike the situation with the junctional rhythm and complete heart block, an idioventricular rhythm can be the escape rhythm in patients with third‐degree AV block; in this situation, there will be regularly spaced, identical appearing upright P waves with no apparent association to the ventricular QRS complexes.