at rest or with exercise The relative heart rate labeling someone as bradycardic is age dependent For example, a resting heart rate of 45 beats/min may be a normal heart rate in a fit adolescent, but in a neonate would invoke significant concerns Symptomatic bradycardia is defined as a documented bradyarrhythmia that is directly responsible for the clinical manifestations of syncope or near syncope, transient dizziness, light-headedness, or periods of confusion resulting from cerebral hypoperfusion Fatigue, exercise intolerance, and heart failure may also result from bradycardia Although a resting electrocardiogram (ECG) will give a snapshot assessment of the sinus node, 24-hour ambulatory monitors are better measures of the sinus node and provide low, average, and high heart rates Guidelines for the Holter diagnosis of sinus bradycardia have been proposed; for neonates and infants, they are less than 60 beats/min sleeping and less than 80 beats/min while awake However, there is little distinction between the low heart rate at rest and the low heart rate while awake The relevant rate for children aged 1 to 6 years is less than 60 beats/min; for those aged 7 to 11 years, less than 45 beats/min; for adolescents and young adults, less than 40 beats/min; and in highly trained athletes, less than 30 beats/min.1 None of these numbers are based on children and adolescents with congenital heart disease Correlation of symptoms with periods of bradycardia should be sought before a patient receives a permanent pacemaker Likewise, elite athletes may have low resting heart rates, and this level of physiologic sinus bradycardia should not be confused with a pathologic mechanism Certain channelopathies (i.e., long-QT syndrome [LQTS] may manifest as bradycardia; see later) Patients with congenital heart disease may have significant hemodynamic perturbations due to the loss of AV synchrony even with acceptable normal junctional rates and may benefit from atrial pacing This has been well demonstrated in single-ventricle patients after the Fontan procedure and in adults who underwent an atrial baffle procedure for transposition of the great arteries.2–4 Rychik showed that in patients with a univentricular circulation following a superior cavopulmonary connection, junctional rhythm results in early systolic flow reversal in the pulmonary veins and reduces effective single-ventricular cardiac output.5 Bradycardia in the Neonate In the healthy newborn the heart rate ranges from 120 to 160 beats/min, but the range exists from 90 to 230 beats/min Transient bradycardia is relatively common in newborns in the form of sinus bradycardia, sinus arrest, and junctional escape rhythm; it may relate to stressful labor or delivery and typically resolves within 48 to 72 hours Potential causes of transient bradycardia in the neonate relate to sedation given to the mother that crosses the placenta to the baby as well as jaundice, hypothyroidism, increased intracranial pressure, apnea, and increased vagal tone Pauses of greater than 1 second are not infrequently observed in neonates and generally do not require anything beyond observation Pauses greater than 2 seconds are less common and should be considered abnormal Bradycardia in the preterm and term neonate can also be a result of blocked premature atrial contractions (PACs) PACs are relatively common (0.7% to 14%) in healthy newborns As in older children, PACs may be conducted, blocked, or conducted with aberrancy In an otherwise well-appearing acyanotic neonate with PACs, there is near-zero morbidity, and they will persist in fewer than 5% of those beyond 12 weeks of age.6 Premature atrial beats may appear in patterns of bigeminy, trigeminy, or quadrigeminy but still have a relatively benign prognosis It is important that nonconducted PACs are not confused with complete AV block Careful observation of P-wave morphology and the P-wave axis may be helpful in confirming atrial and AV nodal escape beats in neonates.7 Episodes of apnea in term and preterm infants may precede transient bradycardia Longitudinal polysomnographic studies have reported these transient episodes of bradycardia to be a normal neonatal reflex.8 Premature ventricular beats have been reported in 0.2% of healthy newborns; however, an echocardiogram should likely be performed to rule out structural heart defects or an associated cardiomyopathy Rare, isolated premature ventricular contractions in an otherwise healthy newborn with a structurally normal heart are generally considered to have a similarly benign natural history to their premature atrial counterparts Increasing the sinus rate typically abolishes both the atrial and ventricular ectopy Atrioventricular Block First-degree AV block is PR prolongation beyond the upper limit of normal for age Normative pediatric ECG data have been published.9 First-degree AV block may be observed in patients with dilated atria, where the impulse takes longer to propagate through the atrial tissue This is frequently seen in patients with a functionally univentricular heart following an atriopulmonary Fontan procedure First-degree AV block is one of the minor criteria used in the affirmation of rheumatic heart disease Second-degree AV block can be subdivided into Mobitz type I (Wenckebach) and Mobitz II second-degree block Mobitz I AV block is typically associated with a narrow QRS complex and is typically considered benign In contradistinction, Mobitz II AV block is a rare pattern with less reliable escape rhythms and is always infranodal Mobitz type I AV block (Fig 22.1) is characterized by progressive PR prolongation before a single nonconducted P wave and is typically associated with a narrow QRS complex unless the patient has an underlying bundle-branch block Clues to the diagnosis include the observation that the postblock PR interval is shorter than the preblock PR interval as well as the recognition of what has been described as grouped beating Mobitz I AV block is frequently observed in very healthy adolescents with high vagal tone in the early morning hours of sleep In contrast, type II second-degree AV block is characterized by fixed PR intervals before and after blocked beats and is usually associated with a wide QRS Caution should be exercised before labeling someone as having type II second-degree AV (Fig 22.2) block, as the natural history and management of Mobitz type I (Fig 22.3) and Mobitz type II AV block are different It is often helpful to obtain a long rhythm strip to accurately measure the PR intervals over a period of time Generally it would be uncommon to see both Mobitz I and Mobitz II in the same patient, as one relates to block above the node and one below the node It is important to pay close attention to a 2 : 1 pattern of AV block whereby neither Mobitz I nor Mobitz II may be affirmed, although a wide QRS in the setting of 2 : 1 AV block suggests Mobitz II AV block FIG 22.1 Rhythm strip demonstrating Mobitz I atrioventricular block (Wenckebach pattern); PR prolongation with a dropped ventricular beat The PP interval is constant ... Atrioventricular Block First-degree AV block is PR prolongation beyond the upper limit of normal for age Normative pediatric ECG data have been published.9 First-degree AV block may be observed in patients with dilated atria, where the impulse takes longer to