In the PALS (Pediatric Advanced Life Support) framework, bradycardia is defined as a heart rate below 60 beats per minute in infants and children. The electrocardiographic characteristics that matter most are the heart rate itself, the relationship between P waves and QRS complexes, the PR interval, and QRS width. These features tell you whether the slow rate is coming from a simple sinus mechanism, a conduction block, or a more dangerous rhythm that needs immediate intervention.
Sinus Bradycardia on ECG
Sinus bradycardia is the most straightforward pattern. Each QRS complex is preceded by a single upright P wave in lead II, the PR interval is consistent from beat to beat, and the QRS complex is narrow. The only abnormality is that the rate falls below 60 bpm. In children, this is often caused by increased vagal tone, hypoxia, or hypothermia rather than intrinsic heart disease.
The key distinction on the ECG is that sinus bradycardia maintains a normal, predictable one-to-one relationship between the P wave and the QRS. The rhythm is regular, and the morphology of each complex looks normal for the child’s age.
PR Interval Changes in AV Blocks
When bradycardia results from a problem in the conduction system between the atria and ventricles, the PR interval is where you’ll see it first.
In a first-degree AV block, every P wave is followed by a QRS complex, but the PR interval is prolonged beyond normal for the child’s age. The rhythm is regular and the rate may or may not be slow. On its own, first-degree block is usually benign, but in the setting of a drug ingestion or poisoning, it can be an early warning sign of worsening conduction problems.
Second-degree AV block comes in two forms. In Mobitz Type I (also called Wenckebach), the PR interval gets progressively longer with each beat until one P wave fails to conduct entirely, producing a “dropped” QRS. This creates a characteristic grouped beating pattern on the rhythm strip. In Mobitz Type II, the PR interval stays constant, but some P waves simply fail to conduct without any preceding lengthening. Mobitz Type II is more concerning because it suggests structural damage in the conduction pathway and is more likely to progress to complete heart block.
Complete Heart Block and AV Dissociation
Third-degree (complete) heart block produces the most distinctive ECG pattern in pediatric bradycardia. The atria and ventricles beat completely independently of each other. On the ECG, you’ll see P waves “marching through” the QRS complexes at their own regular rate, while the QRS complexes fire at a separate, slower, regular rate. The PP intervals stay constant, the RR intervals stay constant, but the PR interval varies randomly because there is no actual relationship between the two.
There are no capture beats or fusion beats in complete AV dissociation. The atria never manage to trigger a ventricular contraction. In incomplete AV dissociation, by contrast, you’ll occasionally see an atrial impulse “capture” the ventricle when the timing is right, creating an irregular RR interval interrupted by a beat that looks different from the escape rhythm. Fusion beats can also appear when a ventricular impulse and a conducted atrial impulse activate the ventricles simultaneously, producing a QRS with a blended, unusual shape.
Complete heart block in children can be congenital or result from myocarditis, conduction system damage, or certain medications.
Narrow Versus Wide QRS Complexes
QRS width helps determine where the slow rhythm is originating and how stable it is. Normal QRS duration in children is shorter than in adults and varies by age:
- Neonates: up to 70 milliseconds
- Infants to age 5: up to 75 milliseconds
- Ages 6 to 12: up to 85 milliseconds
- Over age 12: up to 100 milliseconds
A narrow QRS during bradycardia suggests the escape rhythm is coming from high in the conduction system, near the AV node. This type of escape rhythm tends to be more reliable and produces a faster rate, typically 40 to 60 bpm. A wide QRS suggests the escape pacemaker is lower in the ventricles, which produces a slower, less stable rhythm and carries a higher risk of deterioration. Wide QRS bradycardia can also result from bundle branch blocks, electrolyte abnormalities (particularly high potassium), or drug toxicity.
With elevated potassium levels, the ECG changes follow a recognizable progression: tall, peaked T waves appear first, then the QRS and PR intervals widen, P waves disappear, and eventually the QRS takes on a wide, sinusoidal “sine wave” appearance before deteriorating to asystole.
When Bradycardia Triggers CPR in PALS
Not every slow heart rate on the monitor requires aggressive intervention. The 2025 American Heart Association guidelines specify that bradycardia below 60 bpm should prompt an evaluation for cardiopulmonary compromise. The three signs that define compromise are acutely altered mental status, hypotension, and other signs of shock such as poor perfusion, weak pulses, or delayed capillary refill.
If a child has a heart rate below 60 bpm with any of these signs, chest compressions should be started even though a pulse is present. This is a critical threshold in the PALS algorithm because bradycardia with compromise is considered a warning sign of impending pulseless cardiac arrest.
The ECG characteristics guide what happens next. If the rhythm shows increased vagal tone or an AV conduction block, atropine can increase the heart rate by blocking the vagus nerve’s slowing effect on the heart. However, atropine is not effective when the bradycardia is caused by hypoxia, so oxygenation and ventilation always come first. For children with complete heart block or sinus node dysfunction who do not respond to medications, ventilation, or CPR, emergency transcutaneous pacing (using external pads to electrically stimulate the heart) may be considered.
Putting the ECG Findings Together
When you’re reading a pediatric bradycardia strip in a PALS context, the systematic approach is to assess four things in order. First, confirm the rate is below 60 bpm. Second, look at the P waves: are they present, and is there one before every QRS? Third, examine the PR interval: is it consistent, progressively lengthening, or completely variable? Fourth, measure the QRS width and compare it to age-based norms.
A regular rhythm with one P wave per QRS and a normal PR interval points to sinus bradycardia. A lengthening PR with dropped beats suggests Mobitz Type I. A fixed PR with unexpected dropped beats suggests Mobitz Type II. P waves and QRS complexes firing at independent rates with no consistent PR relationship means complete heart block. A wide QRS in any of these patterns raises additional concern about the stability of the escape rhythm and possible toxic or metabolic causes.