Heart Block: First, Second, and Third Degree — What You Need to Understand

The Scene

The following scene is drawn from the composite of patients I have cared for in clinic. All identifying details are changed.

Thomas is 68 years old, a retired teacher in suburban St. Louis. He came in for an echocardiogram to follow up on mild aortic valve disease his cardiologist diagnosed three years ago. The echocardiogram was unchanged. Routine, until the technician performing the study notices something on the cardiac monitor during the procedure. She calls the cardiologist in.

On the monitor screen, a rhythm strip shows a pattern that takes a second to parse: P-waves marching at a normal rate, QRS complexes marching at a completely different, slower rate, the two entirely divorced from one another. The P-waves are not causing the QRS complexes. Something below the AV node is generating escape beats independently of the atria.

Thomas is in complete heart block. His ventricular rate is 38 bpm. His blood pressure is 88/60. He feels “a little woozy.” A temporary pacing wire is placed in the cath lab within two hours. He receives a permanent pacemaker three days later.

He had felt mildly fatigued for the past three months but had not attributed it to his heart.

Heart block is a spectrum. At one end is a benign prolongation of the PR interval that needs only monitoring. At the other is complete dissociation between the atria and ventricles, requiring emergency pacing. Understanding the anatomy of the conduction system and the clinical meaning of each degree of block is the core of this article.

What It Is

Heart block refers to a delay or failure of electrical conduction through the atrioventricular (AV) node and/or the His-Purkinje system. Under normal circumstances, every sinus impulse that reaches the AV node is conducted to the ventricles after a brief physiological delay (the PR interval on ECG, typically 120-200 ms). Heart block disrupts this 1:1 relationship to varying degrees.

First-degree AV block: Every P-wave is conducted to the ventricles, but the PR interval is prolonged beyond 200 ms (0.20 seconds). It is not a “block” in the sense of preventing conduction; it is a delay. By itself, first-degree block has minimal clinical significance in most patients.

Second-degree AV block: Some P-waves fail to conduct to the ventricles. This creates dropped beats on the ECG: P-waves that are not followed by a QRS complex. There are two fundamentally different types with different clinical implications:

• Mobitz Type I (Wenckebach): The PR interval progressively prolongs from beat to beat until a P-wave is not conducted (a “dropped” beat), after which the cycle resets. The ECG pattern: PR gradually gets longer, QRS drops, PR shortens again. The block is within the AV node itself.
• Mobitz Type II: The PR interval is fixed (constant) from beat to beat, and then without warning, a P-wave fails to conduct to the ventricles. The block is below the AV node, in the bundle of His or the bundle branches. This distinction is critical: Mobitz II is far more dangerous than Wenckebach.

Third-degree (complete) AV block: No P-waves conduct to the ventricles at all. The atria and ventricles operate entirely independently. The atria activate under sinus node control; the ventricles are paced by a subsidiary escape rhythm from the AV junction (at 40-60 bpm) or from a ventricular focus (at 20-40 bpm). Complete heart block is a medical emergency unless it has been chronic and stable for years with an adequate junctional escape rate.

High-degree AV block: Sometimes used to describe AV block greater than 2:1 (only every third or fourth P-wave conducts) that does not fit cleanly into the Mobitz I/II classification.

The Mechanism

AV Node and His-Purkinje Anatomy

The AV node is a compact cluster of specialized conducting tissue at the junction of the atria and ventricles, positioned at the inferior interatrial septum near the coronary sinus ostium. Its essential function is dual: it provides the only normal electrical pathway between atria and ventricles, and it introduces a physiological delay that allows atrial contraction to complete before ventricular activation begins.

Below the AV node, the His bundle provides a single electrical highway descending through the fibrous skeleton of the heart. It then divides into the right bundle branch (thin, conducting rapidly to the right ventricle) and the left bundle branch (which further divides into anterior and posterior fascicles before distributing to the left ventricular myocardium). This bundle branch system constitutes the His-Purkinje network.

The anatomical level of the block determines both its ECG appearance and its clinical significance:

• AV nodal block: typically Wenckebach (Mobitz I) or first-degree. The AV node has relatively rich blood supply (from the AV nodal artery, a branch of the right coronary or left circumflex artery in most people) and autonomic innervation. Blocks at this level often respond to atropine (vagolytic) or are associated with reversible conditions.
• Infra-Hisian block (below the His bundle, in the bundle branches): typically Mobitz II or complete heart block at the bundle branch level. This is a disease of the specialized conduction tissue itself. It does not respond to atropine. It requires pacing.

Wenckebach: Why the PR Gets Longer

The progressive PR prolongation in Wenckebach is explained by the AV node’s decremental conduction: each conducted impulse leaves the AV node slightly less recovered than before, requiring a longer time to conduct the next impulse. The increment decreases with each beat until the AV node is completely refractory at the time the next P-wave arrives, producing the dropped beat. The AV node then recovers fully during the pause, and the cycle resets.

Wenckebach is a hallmark of high vagal tone (common in trained athletes, during sleep, in the setting of inferior MI) and AV nodal disease. It is generally considered benign unless it is symptomatic, because the AV node’s decremental property protects against very rapid conduction of atrial arrhythmias.

Why Mobitz II Is Dangerous

Mobitz II block, located in or below the His bundle, occurs because of structural disease in the bundle branches: fibrosis, ischemia, or infiltration. The block is fixed rather than decremental: the conduction either succeeds or fails, without progressive PR prolongation. The danger: Mobitz II can progress unpredictably to complete heart block. There is no warning. The patient can be in normal conduction one moment and in complete heart block with a ventricular escape rate of 30 bpm the next. In the absence of a pacemaker, this transition can be fatal.

This is why the Mobitz I / Mobitz II distinction is one of the most clinically important pattern recognitions in ECG interpretation.

How We Diagnose

ECG Diagnosis

The 12-lead ECG and rhythm strip are the primary tools:

First-degree block: PR interval greater than 200 ms on every complex. Every P-wave followed by a QRS.

Mobitz I (Wenckebach): Progressive PR prolongation ending in a dropped beat (P-wave with no QRS), then PR shortens and the cycle repeats. The QRS complexes are usually narrow (the block is at the AV node, which is above the bundle branches).

Mobitz II: Fixed PR interval from beat to beat, then a P-wave without a QRS. Critically: the QRS complexes in Mobitz II are often wide (bundle branch block pattern), because the block is at or below the bundle of His. Wide QRS with Mobitz II pattern is a high-risk finding.

2:1 AV block: Every other P-wave fails to conduct. Because the ratio is fixed, progressive PR prolongation cannot be observed, making it impossible to distinguish Mobitz I from Mobitz II on the ECG alone. Clinical context (history, QRS width, response to atropine) guides the assessment. A His bundle electrogram (obtained at EP study) definitively localizes the block.

Complete heart block: P-waves and QRS complexes march at different rates, entirely independently. The QRS rate determines the escape focus: narrow escape at 40-60 bpm suggests junctional (AV node or His bundle) origin; wide escape at 20-40 bpm suggests ventricular origin, which is less reliable and more dangerous.

Identifying Reversible Causes

Before committing to a permanent pacemaker, reversible causes of AV block must be considered:

• Inferior MI: The right coronary artery supplies the AV node in approximately 80-90% of people. Inferior MI (right coronary occlusion) causes transient AV block in approximately 15% of cases. This block is usually at the AV node level (Wenckebach), responds to atropine, and resolves within days to weeks as the ischemia resolves. Temporary pacing may be required during the acute phase, but permanent pacing is usually not needed.
• Lyme disease: Lyme carditis (Borrelia burgdorferi infection) produces AV block in approximately 1% of Lyme patients, most commonly in endemic areas of the northeastern United States. The block can be complete but is typically reversible with antibiotic treatment. An appropriate travel and exposure history should be obtained in any young patient with new complete heart block.
• Medication-induced: As with bradycardia, any rate-slowing medication can contribute to or worsen AV block in a susceptible patient. Review includes beta-blockers, calcium channel blockers (diltiazem, verapamil), digoxin, and amiodarone.
• Sarcoidosis: Cardiac sarcoidosis produces granulomatous infiltration of the conduction system, causing complete heart block in approximately 25-30% of cardiac sarcoidosis cases and VT in others. Sarcoidosis should be considered in any young or middle-aged patient with complete heart block and no other explanation.
• Surgical trauma: AV block complicating cardiac surgery (particularly TAVR, septal myectomy, mitral valve surgery, or ventricular septal defect repair) can be due to edema and may resolve, or may be permanent from direct conduction tissue injury.

The Evidence

ACC/AHA 2018 Pacing Guidelines for AV Block

The 2018 ACC/AHA guidelines provide the framework 5 / Solid :

Class I (pacemaker indicated):

• Symptomatic second-degree AV block (Mobitz I or II) with documented correlation between symptoms and block
• Asymptomatic Mobitz II second-degree AV block (because of the risk of unpredictable progression to complete heart block)
• High-degree AV block (2:1 or greater) that is persistent
• Complete heart block (third degree), symptomatic or asymptomatic, if the escape rate is below 40 bpm or the escape rhythm is ventricular (wide complex, unreliable)

Class I without requiring symptoms (the critical exception to the “symptoms required” principle):

• Asymptomatic Mobitz II and complete heart block are Class I pacing indications regardless of symptoms, because the risk of unpredictable hemodynamic collapse without a pacemaker exceeds the procedural risk of pacing.

This distinguishes AV block from sinus node dysfunction: sinus node dysfunction requires symptoms to indicate pacing; significant AV block (Mobitz II, complete) requires pacing even without symptoms.

Class III (not indicated):

• Asymptomatic first-degree AV block
• Asymptomatic Wenckebach (Mobitz I) in patients without structural heart disease or underlying bundle branch block

Temporary Pacing for Acute Heart Block

In the acute setting (inferior MI, Lyme carditis, post-cardiac surgery), temporary pacing provides a bridge while the reversible cause is treated. Options include:

• Transcutaneous pacing (external): Immediately available via defibrillator pads. Painful and not reliable for extended periods; appropriate for emergency stabilization only.
• Temporary transvenous pacing: A balloon-flotation catheter or manual placement catheter advanced via the internal jugular, subclavian, or femoral vein to the right ventricular apex. Provides reliable pacing for days to weeks. The standard bridge for acute heart block requiring more than transcutaneous pacing.

What Happens Without Pacing in Complete Heart Block

In the absence of pacing, patients in complete heart block depend entirely on the escape rhythm. A junctional escape at 50 bpm may provide marginal hemodynamic stability; a ventricular escape at 30 bpm typically does not, particularly under any physiological stress (infection, bleeding, exertion). Patients in complete heart block without adequate escape rates can die suddenly from hemodynamic collapse or VFib triggered by the extreme bradycardia.

The Patient Experience

What Heart Block Feels Like

First-degree block: typically asymptomatic. Patients do not feel a prolonged PR interval.

Wenckebach: often asymptomatic. Patients with symptomatic Wenckebach may feel an occasional “missed beat” (the dropped QRS) or fatigue with exertion if the ratio of block produces a slow ventricular rate.

Mobitz II: The symptomatic experience varies. Some patients feel the dropped beats as pauses. If Mobitz II progresses to complete heart block abruptly, the transition can cause sudden presyncope or syncope as the ventricular rate falls before the escape rhythm establishes itself.

Complete heart block: Symptoms range from profound fatigue and exercise intolerance to hemodynamic collapse and syncope. The severity depends on the escape rate. Thomas in the opening scene had an escape rate of 38 bpm and a systolic blood pressure of 88 mmHg: he was mildly symptomatic but hemodynamically compromised without knowing it.

What Your Doctor Will Not Have Time to Explain

1. Mobitz II is not the same as Wenckebach. If you have been told you have “second-degree heart block” without specification of type, ask which type. Wenckebach (Mobitz I) is typically managed conservatively in the absence of symptoms. Mobitz II requires pacemaker implantation even if you feel fine today.

2. Complete heart block requires a pacemaker even without symptoms. The rule that “symptoms drive pacing decisions” applies to sinus node dysfunction and first-degree block, not to Mobitz II or complete heart block. The risk of sudden decompensation without pacing in these patients is too high to defer.

3. If you are in an area with Lyme disease and you develop new AV block, mention tick exposure. Lyme carditis is a reversible cause of AV block that is treatable with antibiotics, potentially avoiding a permanent pacemaker. It is most common in the northeastern United States and parts of the upper Midwest, though it can occur anywhere Ixodes tick habitats are present.

4. TAVR and cardiac surgery carry a risk of needing a pacemaker. The AV bundle of His runs just beneath the aortic valve. TAVR, particularly with certain prosthesis types and implant depths, injures the His bundle in approximately 5-25% of patients. If you are having TAVR, ask your team about the specific pacemaker risk for the device being used and the implant technique.

Decisions and Trade-Offs

When to Implant vs. When to Watch

The decision algorithm for AV block follows the guidelines clearly:

Pacemaker Type for AV Block

For patients with AV block and intact sinus node function (PR block causing slow ventricular rate, but atria contracting normally), a dual-chamber (DDD) pacemaker is preferred: it tracks the sinus P-wave and delivers a ventricular pacing impulse after an appropriate AV delay, preserving AV synchrony. This maintains normal hemodynamic efficiency.

For patients in persistent AF with AV block (a common scenario in elderly patients on rate-controlling medications), single-chamber ventricular pacing (VVI) is appropriate, because there is no organized atrial activity to track.

For patients requiring only ventricular pacing with preserved LV function, minimizing RV apical pacing is a priority: chronic right ventricular apical pacing induces ventricular dyssynchrony and increases the risk of developing cardiomyopathy 5 / Solid . His bundle pacing or left bundle branch area pacing provides more physiological ventricular activation and is now available at specialized EP centers.

Temporary Pacing as a Bridge

When a patient is in acute complete heart block from a reversible cause (inferior MI, Lyme, medication), the temporary pacing bridge strategy avoids committing the patient to a permanent device before determining whether the block will resolve. The timing depends on the cause: inferior MI-related AV block typically resolves within 5-7 days; Lyme carditis within 1-3 weeks of starting antibiotics; surgical trauma may take up to 6 weeks. The standard approach at most centers is to observe for 5-7 days before permanent pacing if the reversible cause is being treated.

Three Questions to Ask Your Cardiologist

1. “Is my heart block Mobitz I or Mobitz II, and does that change whether I need a pacemaker even if I feel fine now?”
2. “Has a reversible cause been excluded (Lyme disease, medication, inferior MI, sarcoidosis) before we commit to a permanent device?”
3. “If I do need a pacemaker, will the device use conventional RV apical pacing or a physiological pacing approach (His bundle or left bundle branch area), and what are the availability and outcomes data for physiological pacing at your center?”

The SDE Synthesis

Heart block is the final electrical manifestation of structural disease in the conduction system. The fibrosis of the AV node and His-Purkinje system that produces complete heart block at 68 has been accumulating for decades, driven by the same hypertensive and metabolic processes that cause coronary artery disease and atrial fibrillation. The pacemaker that Thomas received solved the immediate problem. It did not address the question of what drove the conduction system disease.

The SDE Audit, for a patient like Thomas, provides the complete upstream picture: blood pressure trajectory, metabolic risk, calcium scoring for atherosclerotic burden. A patient who required a pacemaker at 68 for presumed idiopathic AV block has a 40-50% probability of harboring additional untreated cardiovascular risk that the pacemaker implant alone does not address.

Cross-links within the SDE system: The Foundations article on Sinus Bradycardia (SDE-F-RHTM-007) covers the SA node dysfunction that often coexists with AV conduction disease. The Foundations article on Dual Chamber Pacemakers (SDE-F-DEVI-009) provides the device evidence in full. The Foundations article on ICD and Pacemaker Implantation (SDE-F-PROC-016) covers the implant procedure.