What is happening in the heart
Each normal heartbeat starts in the sinus node at the top of the right atrium, spreads across both atria, and then has to pass through a single narrow gateway — the AV node — to reach the ventricles. From the AV node, the signal travels down a specialized wire called the His-Purkinje system that branches into the left and right bundles and delivers the impulse to every cell of the ventricles almost simultaneously.
“AV block” means that somewhere along this pathway, signals are not getting through reliably. The block can sit in the AV node itself or further downstream in the His-Purkinje system, and that location is one of the most important things we try to figure out — because it predicts how the block will behave.
The grades of AV block
First-degree AV block
The signal always makes it from the atria to the ventricles, but the trip takes longer than usual. On an ECG, this shows up as a prolonged PR interval. By itself, first-degree block is almost always benign — common with age, with certain medications, and in well-trained athletes. We watch it but do not treat it.
Second-degree AV block
Some atrial impulses get through, others don’t. The pattern of which ones drop tells us a great deal.
Mobitz I (Wenckebach). Each PR interval gets slightly longer, beat after beat, until one impulse fails to conduct entirely — then the cycle starts over. This pattern almost always means the block sits inside the AV node itself. It is usually benign, often appears during sleep in healthy people, and rarely progresses to anything worse. Treatment is reserved for patients with clear symptoms tied to the block.
Mobitz II. Some impulses suddenly fail to conduct without any warning lengthening of the PR interval. This pattern usually means the block sits below the AV node, in the His-Purkinje system — a structurally less forgiving location. Mobitz II is unstable and unpredictable, and can deteriorate suddenly into complete heart block with long pauses. A pacemaker is recommended for Mobitz II regardless of symptoms.
2:1 AV block. Every other beat conducts. Without other clues we can’t always tell whether this is a slow Mobitz I or a Mobitz II, and we sometimes need additional testing — or an electrophysiology study — to localize the level.
High-grade AV block. Two or more consecutive impulses fail to conduct. This is treated the same way as Mobitz II: a pacemaker is almost always indicated.
Third-degree (complete) AV block
No atrial impulses get through to the ventricles at all. The atria and ventricles beat completely independently — the atria still firing at the usual rate from the sinus node, and the ventricles relying on a slow “escape” rhythm from somewhere below the block. If the escape comes from just below the AV node, it is usually around 40–55 beats per minute and reasonably stable. If it comes from deep in the His-Purkinje system, it is often slower and far less reliable. Either way, complete AV block requires a pacemaker.
Why it matters
The risks tracked with AV block are two-fold:
- The heart rate is too slow for what the body needs. Patients feel fatigue, exertional shortness of breath, lightheadedness, exercise intolerance, and sometimes confusion. The lower and more unreliable the escape rhythm, the more profound the symptoms.
- Long pauses. When the escape rhythm fails to kick in promptly, a several-second pause can cause fainting and falls — sometimes with significant injury — and in rare cases sudden death.
Mobitz II and complete AV block are particularly dangerous because they sit below the AV node and the escape rhythms are unreliable. That’s why we pace them even when the patient hasn’t yet had severe symptoms.
How we diagnose it
- 12-lead ECG. Often diagnostic on its own, especially for higher grades of block.
- Holter or patch monitor. When block is intermittent. We look at how often non-conducted beats happen, whether they cluster at night versus during the day, and how the block behaves with exertion.
- Exercise treadmill test. A particularly useful test — block that worsens with exercise points strongly to a problem below the AV node and lowers our threshold to pace.
- Implantable loop recorder. For unexplained fainting where intermittent AV block is on the differential.
- Electrophysiology study. Used when we need to know exactly where the block sits.
We also look hard at reversible causes: medications (beta-blockers, calcium channel blockers, digoxin, certain antiarrhythmics), electrolyte abnormalities, ischemia, Lyme disease, sarcoidosis, and recent cardiac surgery. If a clear reversible cause is found and corrected, the block sometimes resolves entirely.
How we treat it
Observe
First-degree block and asymptomatic Mobitz I are observed. We address any contributing medications and follow over time.
Pacemaker
Recommended for:
- Symptomatic second-degree block of any type.
- Mobitz II AV block, even without symptoms.
- High-grade AV block.
- Third-degree (complete) AV block.
- Block that worsens with exercise.
In most patients with AV block we choose a dual-chamber pacemaker so we can pace the ventricle when the atrial impulse fails to make it through, while still tracking and preserving the heart’s own atrial rhythm. For some patients, particularly when extensive ventricular pacing is expected, we consider conduction-system pacing (His-bundle or left-bundle-branch-area pacing) to preserve a more natural pattern of ventricular activation.
What to expect at your visit
We’ll go over symptoms — fatigue, breathlessness, lightheadedness, any fainting — and any monitor or ECG findings that brought you in. We’ll examine you, review medications carefully, and decide whether more testing is needed or whether the picture already supports pacing. When a pacemaker is recommended, we’ll walk through what type, what the procedure involves, and what life looks like afterward.