Bradycardia is a frequent finding in clinical practice. Simply put, bradycardia refers to any heart rate that is slower than the normal or predicted heart rate for a given patient. For adult patients, the term bradycardia typically refers to heart rates slower than 60 bpm (beats per minute). In children, the normal heart rate varies depending on the age of the child.

The correct diagnosis of a bradycardic rhythm is crucial to understanding the clinical significance of that rhythm and choosing the appropriate management strategy (Boxes 8.1 and 8.2). The causes of bradycardia are classified into intrinsic and extrinsic entities (Table 8.1). Intrinsic causes focus on disorders based within the conduction system itself; extrinsic causes result from issues external to the conduction system, most commonly coronary ischemic, respiratory, metabolic, and toxic syndromes. In this chapter, we will discuss the ECG characteristics of the following bradycardic rhythms: sinus bradycardia, sinus arrhythmia, sinoatrial (SA) block, sinus pause/arrest, junctional rhythms, idioventricular rhythms, sinoventricular rhythm, and slow atrial fibrillation. Atrioventricular (AV) blocks, which also often occur at rates less than 60 bpm, are discussed in Chapter 9.

Sinus Bradycardia

The term sinus bradycardia simply refers to a regular rhythm of sinus origin that presents with a rate less than 60 bpm in the adult or less than the expected lower limit of normal for the pediatric patient. Sinus bradycardia may be further classified into regular and irregular variations.

Regular sinus bradycardia is defined as sinus rhythm with a rate less than 60 bpm (Figure 8.1). This rhythm is generated by the SA node and proceeds through the normal conduction pathways. In essence, sinus rhythm with a rate less than 60 bpm in the adult and less than the lower limit of normal for the pediatric patient is sinus bradycardia. By definition, sinus bradycardia has a P wave of normal morphology and a normal axis (or unchanged from the patient’s previous baseline axis). The PR interval (PRI) is consistent and is less than 0.2 s, and the QRS complex is narrow (<0.12 s). There is a P wave before every QRS complex and a QRS complex following every P wave. Note that the QRS duration may be widened when intraventricular conduction is impaired and that AV conduction blocks may also coexist with sinus bradycardia. The rhythm is regular with consistent P–P and R–R intervals and these are equal.

Irregular sinus bradycardias include sinus arrhythmia, SA block, and sinus arrest. Sinus arrhythmia (Figure 8.2) may occur at rates less than 60 bpm and thus may be classified as a form of sinus bradycardia. Sinus arrhythmia is generated by the SA node and conducts through the normal cardiac conduction pathway. It has the same features of sinus bradycardia with the exception of irregularity; in fact, the rhythm has all of the hallmarks of a regular sinus bradycardia except that it is somewhat (and in some cases markedly) irregular owing to varying P–P and R–R intervals. This irregularity is often due to differences in intrathoracic pressure during the respiratory cycle or from intermittent stimulation of the vagus nerve, both of which lead to beat‐to‐beat variability in the heart rate. Bradycardic sinus arrythmia is frequently found in healthy young patients and does not necessarily indicate pathology – unless the clinical presentation suggests that decompensation is present.