A third-degree atrioventricular (AV) block is a complete electrical disconnect between the upper and lower chambers of the heart. In the PALS (Pediatric Advanced Life Support) framework, it’s one of the most serious causes of bradycardia in children because the ventricles can no longer receive signals from the heart’s natural pacemaker. Instead, the ventricles beat on their own at a much slower backup rate, which may not pump enough blood to sustain the child’s organs.
How Third-Degree Block Appears on ECG
The hallmark of third-degree AV block is complete atrioventricular dissociation. On the monitor, the P waves (representing atrial activity) and QRS complexes (representing ventricular activity) march along at their own independent rates with no relationship to each other. The atrial rate will be faster than the ventricular rate because the SA node fires normally while the ventricles rely on a slower escape rhythm to keep beating.
The shape of the QRS complex tells you where the backup pacemaker is located. A narrow QRS suggests the escape rhythm originates from the junction just below the block, which tends to produce a somewhat faster and more stable rate. A wide QRS means the escape rhythm comes from the ventricles themselves, which is slower and less reliable. In a pediatric emergency, a wide-complex escape rhythm is more concerning because the heart rate it generates is often too low to maintain adequate perfusion.
What Causes It in Children
Pediatric third-degree block falls into two broad categories: congenital and acquired.
Congenital complete heart block is most often caused by an autoimmune process. Antibodies from the mother (anti-Ro/SSA or anti-La/SSB) cross the placenta during pregnancy and damage the developing conduction system. In roughly one-third of these infants, the mother has a diagnosed autoimmune condition such as lupus. Some mothers carry these antibodies without knowing it, so congenital heart block can be the first clue to maternal autoimmune disease.
Acquired complete heart block in children most commonly follows congenital heart disease surgery. In a large review of over 103,000 pediatric cardiac surgeries across 45 U.S. centers, complete heart block requiring a pacemaker occurred in about 1.2% of cases. The risk was highest after mitral valve repair (3.7%), aortic valve repair (2.7%), atrioventricular canal surgery (1.9%), and ventricular septal defect repair (1.8%). Cardiac catheterization can also cause heart block, though at a lower rate of about 2.2%, and recovery is common. Other acquired causes include myocarditis, infections, metabolic abnormalities, and hypothyroidism.
Signs of Poor Perfusion in Children
Third-degree block matters in PALS because of what it does to cardiac output. A child’s heart compensates for low stroke volume primarily by increasing heart rate. When the ventricular rate drops significantly, the body can’t keep up. The three key signs that a bradycardic child is in trouble are hypotension, acutely altered mental status, and signs of shock (cold extremities, weak pulses, delayed capillary refill, mottled skin).
Hypotension thresholds in pediatrics vary by age. For newborns up to 28 days, a systolic blood pressure below 60 mmHg is considered hypotensive. For infants aged 1 to 12 months, that threshold drops to 70 mmHg. For children aged 1 to 10, the formula is 70 plus twice the age in years. Children older than 10 use the adult threshold of 90 mmHg. Any child with a heart rate below 60 beats per minute and signs of poor perfusion needs immediate intervention.
The PALS Bradycardia Algorithm
PALS treats third-degree block through its bradycardia with a pulse algorithm. The steps are sequential, and speed matters.
The first priority is always airway and breathing. Open the airway, provide positive-pressure ventilation with supplemental oxygen as needed, and place the child on a cardiac monitor with pulse oximetry and blood pressure monitoring. Many causes of pediatric bradycardia are respiratory in origin, and some children will improve with ventilation and oxygenation alone. Third-degree block, however, is a primary cardiac problem, so these children typically won’t recover with airway management alone.
If the heart rate remains below 60 bpm with signs of poor perfusion despite adequate oxygenation and ventilation, start chest compressions. This is a critical threshold in PALS: compressions begin before the child becomes pulseless. The goal is to maintain perfusion while you move to the next steps.
Once IV or intraosseous access is established, the algorithm calls for epinephrine and atropine. Epinephrine is given at 0.01 mg/kg with a maximum single dose of 1 mg. Atropine is dosed at 0.02 mg/kg, with a minimum dose of 0.1 mg and a maximum single dose of 0.5 mg. One repeat dose of atropine is allowed. Atropine works by blocking vagal tone on the AV node, so it is specifically indicated when increased vagal tone or a primary AV block is the cause of the bradycardia. In complete heart block, atropine may speed up the atrial rate without improving ventricular conduction, so its effectiveness can be limited.
When Pacing Is Considered
Emergency transcutaneous pacing occupies a specific place in the PALS algorithm. It is considered for children with complete heart block or sinus node dysfunction who have not responded to oxygenation, ventilation, medications, or CPR. The 2025 AHA/AAP guidelines note that data on transcutaneous pacing for refractory bradycardia in children remain limited, but it may be a reasonable option when nothing else has restored an adequate heart rate.
In practice, transcutaneous pacing involves placing pads on the chest and delivering small electrical impulses that stimulate the ventricles to contract at a set rate. It’s a bridge to more definitive treatment, such as transvenous pacing or permanent pacemaker implantation. For children with congenital complete heart block or post-surgical block that doesn’t resolve, a permanent pacemaker is often the long-term solution.
Why Third-Degree Block Is Different From Other Bradycardias
Most pediatric bradycardias are caused by hypoxia, and they respond to fixing the respiratory problem. Third-degree AV block is structural or electrical. The conduction pathway is physically or functionally interrupted, which means the usual chain of “oxygenate, ventilate, then reassess” often won’t resolve it. This distinction is important because it changes how aggressively you move through the algorithm. A child in third-degree block with signs of poor perfusion will almost certainly need medications and potentially pacing, so establishing vascular access early is essential.
On the monitor, distinguishing third-degree block from other bradyarrhythmias comes down to the relationship between P waves and QRS complexes. In sinus bradycardia, every P wave is followed by a QRS at a consistent interval. In second-degree block, some P waves conduct and some don’t, creating a pattern. In third-degree block, the P waves and QRS complexes have no relationship at all. The atria and ventricles are electrically independent, each beating at their own rate. Recognizing this dissociation on the rhythm strip is the key to identifying the block and anticipating the need for escalated treatment.