Bradycardia, or a slow heart rate, is < 60 beats per minute (bpm). Inadequate tissue perfusion leading to hypoxia is a leading cause of symptomatic bradycardia. The symptoms arise from the progression of hypoxemia, shock, or respiratory failure.

Children with symptomatic bradycardia are at high risk for cardiac arrest. The team must adequately ventilate and provide oxygen supplementation. If these interventions do not alleviate the symptoms, the team should begin cardiopulmonary resuscitation (CPR) immediately.

When diagnosing bradycardia, the provider must determine the patient’s typical heart rate and rhythm by performing a quick patient history. Abnormalities in rate and rhythm are to be expected in children with congenital heart disease. If a child has poor baseline cardiac function, then the chances of encountering symptomatic bradycardia are high.

There are two types of bradycardia, primary and secondary. Primary bradycardia can be caused by a congenital abnormality, cardiomyopathy, myocarditis, or be iatrogenic.

Noncardiac etiologies that cause slowing of the sinus nodal pacemakers or the conduction architecture of the AV junction cause secondary bradycardia. These noncardiac causes can be due to hypoxia, acidosis, hypotension, hypothermia, or drug effects.

Signs and Symptoms of Bradycardia

Cardiac output is the product of cardiac rate and stroke volume. If the cardiac rate decreases, then cardiac output becomes dependent on stroke volume. Pediatric patients are vulnerable to symptomatic bradycardia because they are unable to produce a significant compensatory increase in stroke volume to maintain the cardiac output. 

Pediatric patients with bradycardia show signs and symptoms, including: 

• Hypotension
• A decreased level of consciousness
• Shock with signs of poor end-organ perfusion with or without hypotension
• Respiratory distress and failure
• Chest pain
• Sudden syncope
• ECG findings include:  
  • A prolonged R-R interval 
  • Presence or absence of P waves 
  • Narrow or wide QRS complexes
  • Prolonged or normal PR interval

The most common types of bradyarrhythmias are sinus bradycardia and AV block.

Sinus Bradycardia

The origin of impulses for sinus bradycardia originates from the sinus node. The ECG of the child in sinus bradycardia will have P waves at regular intervals. Sinus bradycardia is often a benign condition and is frequently present in pediatric patients at rest when the metabolic demands of the body are very minimal. 

Primary bradycardia is relatively rare in pediatric patients in the absence of congenital heart disease. Primary bradycardia occurs when something has damaged the sinus node, such as might occur inadvertently during a surgical procedure. Secondary bradycardia is much more common in children as a response to significant hypoxia, hypotension, and acidosis.

Related Video – ECG Rhythm Review – Sinus Bradycardia

Atrioventricular Block

AV blocks are conduction defects that produce an electrical disturbance through the AV node. Congenital heart disease, iatrogenic disorders, medications, electrolyte imbalances, hypoxia, hypotension, and shock may cause AV blocks. 

First-degree AV block is often benign, and its ECG findings show an increased PR interval > 0.20 seconds.

Related Video – One Quick Question: What are the First-Degree Heart Block Criteria?

Second-degree AV block, Mobitz type I (Wenckebach), occurs at the AV node. The child’s ECG will show a characteristic progressive prolongation of the PR interval until an atrial impulse (represented by a P wave) is entirely blocked from entering the ventricles. A QRS complex will not follow that P wave. This cycle often repeats.

Related Video – One Quick Question: What are the Second Degree Heart Block Criteria?

Second-degree AV block, Mobitz type II, blocks impulses below the AV node within the His-Purkinje system and is often the type of AV block that causes symptoms. In this rhythm, the provider will note that the PR interval is constant with occasional P waves that do not conduct through the AV node, which results in missing QRS complexes. It is commonly associated with acute myocardial infarction and may progress into third-degree AV block.

Related Video – ECG Rhythm Review – Second-Degree Heart Block (Type 2)

Third-degree AV block is a type of complete heart block. The conduction block occurs at the AV node, and no supraventricular impulses are conducted into the ventricles. The atrial rate is usually around 100 bpm, while the ventricular rate is approximately 40 bpm. Third-degree AV block may be permanent or transient.

Related Video – One Quick Question: What are the Second Degree Heart Block Criteria?