AV Block

01Pathophysiology

The heart's conduction system transmits impulses from the sinoatrial (SA) node through the atria into the atrioventricular (AV) node, then down the Bundle of His, into the left and right bundle branches, and finally to the Purkinje fibers. AV block occurs when this transmission is delayed or completely interrupted at one or more of these levels.

The AV node itself is supplied by the AV nodal artery, which arises from the right coronary artery (RCA) in approximately 85% of patients. This is why inferior myocardial infarction (RCA territory) is strongly associated with AV nodal block, typically producing first-degree or second-degree Type I block. These blocks tend to be transient and reversible because the AV node has rich collateral blood supply and the block is located at a proximal level where escape rhythms are relatively reliable (junctional escape at 40 to 60 bpm with a narrow QRS).

In contrast, the infranodal conduction system (Bundle of His, bundle branches) is supplied by the left anterior descending artery (LAD). Damage here, as seen in anterior MI, produces second-degree Type II or third-degree block with wide QRS escape rhythms. These are far more dangerous because the ventricular escape pacemakers fire at only 20 to 40 bpm and are unreliable, placing the patient at high risk for hemodynamic collapse and asystole.

Beyond ischemia, degenerative fibrosis of the conduction system (Lenegre disease and Lev disease) is the most common cause of chronic AV block in elderly patients. Drugs that slow AV conduction (beta-blockers, non-dihydropyridine calcium channel blockers like verapamil and diltiazem, digoxin, and amiodarone) are a reversible and frequently tested cause. Lyme disease should always be considered in a young patient presenting with new-onset AV block, particularly in endemic areas, because AV nodal involvement is the most common cardiac manifestation of Lyme carditis. Infiltrative diseases such as cardiac sarcoidosis and amyloidosis can also destroy conduction tissue.

The symptoms of AV block are a direct consequence of the degree of conduction impairment. In first-degree block, every impulse still reaches the ventricles, so the patient is asymptomatic. In second-degree block, intermittent failure of conduction results in dropped beats, which may cause palpitations, lightheadedness, or exercise intolerance. In third-degree block, complete AV dissociation means the ventricles depend entirely on an escape rhythm, and the resulting bradycardia explains the syncope, heart failure, and risk of sudden death.

02Classification and Clinical Manifestation

Key point on the 2:1 AV block dilemma: When every other P wave is dropped in a fixed 2:1 pattern, you cannot determine whether the block is Type I or Type II by looking at PR prolongation alone (there is only one conducted PR interval to evaluate). In this situation, examine the QRS width: a narrow QRS suggests an AV nodal (Type I) mechanism, while a wide QRS suggests an infranodal (Type II) mechanism. This distinction is commonly tested.

03Diagnostic Workup

The best initial test is always the 12-lead ECG. This single study can classify the degree of AV block and often suggests the anatomic level of blockade based on the QRS morphology. The ECG should be interpreted systematically: measure the PR interval, assess whether every P wave is followed by a QRS, look for progressive PR prolongation or grouped beating, and determine whether P waves and QRS complexes are independent of each other.

When AV block is suspected but not captured on a resting ECG (for example, a patient with unexplained syncope), a Holter monitor (24 to 48 hours) or an event recorder (up to 30 days) is the next step. For very infrequent episodes, an implantable loop recorder can monitor continuously for up to 3 years.

The most accurate (confirmatory) test for determining the exact site of block is the electrophysiology (EP) study. The EP study directly measures the HV interval (the time from His bundle depolarization to the onset of ventricular depolarization). A normal HV interval is 35 to 55 ms. A prolonged HV interval (> 55 ms) confirms infranodal disease. However, EP studies are not routinely needed if the clinical picture and ECG are clear, such as an obvious third-degree block with wide QRS.

The atropine challenge is a practical bedside test: atropine blocks vagal input to the AV node. If the block is at the AV node, atropine will improve conduction (PR shortens, dropped beats resolve). If the block is infranodal, atropine increases the sinus rate without improving infranodal conduction, which can paradoxically worsen the block by sending more impulses into a diseased His-Purkinje system. This paradoxical worsening with atropine is a tested concept.

Always order basic labs including potassium (hyperkalemia causes AV block), magnesium, calcium, digoxin level (if applicable), and troponin (to rule out acute ischemia). In a young patient with new AV block, especially with a history of tick exposure or erythema migrans, order Lyme serology. In patients with unexplained high-degree AV block, consider cardiac sarcoidosis and order chest imaging, ACE level, and potentially cardiac MRI or PET scan.

04Management and Treatment

Acute management of any symptomatic bradycardia from AV block follows the ACLS bradycardia algorithm. The first pharmacologic intervention is atropine 0.5 mg IV, which can be repeated every 3 to 5 minutes up to a total dose of 3 mg. Remember that atropine works only on the AV node, so it is effective for first-degree and second-degree Type I block but is unreliable or harmful in Type II and third-degree block with infranodal escape.

If atropine fails or the block is infranodal, the next step is transcutaneous pacing as a temporizing measure, followed by transvenous pacing for sustained hemodynamic instability. Dopamine infusion (5 to 20 mcg/kg/min) or epinephrine infusion (2 to 10 mcg/min) can also be used as temporizing agents while awaiting pacing.

Permanent pacemaker placement is the definitive treatment for:

• Second-degree Type II block (regardless of symptoms)

• Third-degree block (regardless of symptoms)

• Symptomatic second-degree Type I block that does not resolve

• High-grade block (2:1 or greater)

• Any AV block associated with symptoms of hemodynamic compromise that is not due to a reversible cause

The critical exam concept here is recognizing reversible causes before committing to a permanent pacemaker. Always check for and correct drug effects, electrolyte abnormalities, ischemia, and infections (Lyme disease) first. A permanent pacemaker is not indicated if the cause is reversible. This is a common "Next Best Step" question: the answer is to remove the offending agent or treat the underlying cause before pacing.

For Lyme carditis, the treatment is antibiotics (IV ceftriaxone for high-degree block, oral doxycycline for first-degree block with PR < 300 ms). AV block from Lyme disease almost always resolves within 1 to 2 weeks of appropriate antibiotic therapy, and permanent pacemaker implantation is typically unnecessary.

For AV block in the setting of acute MI, the management depends on the territory. Inferior MI with Type I or transient AV block is managed conservatively with reperfusion and monitoring, as these blocks usually resolve. Anterior MI with Type II or third-degree block indicates extensive myocardial damage and almost always requires permanent pacing.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on AV block questions is by confusing second-degree Type I with Type II, especially when the vignette presents a 2:1 block. In a 2:1 pattern, there is no opportunity to observe progressive PR prolongation because only one PR interval is conducted before a beat is dropped. Students default to calling it "Type II" because there is no visible Wenckebach pattern. The correct approach is to look at the QRS width: narrow QRS favors a nodal (Type I) mechanism, while wide QRS with a bundle branch block pattern favors infranodal (Type II). If the question does not provide enough information to distinguish, focus on the clinical context (inferior MI favors Type I; anterior MI favors Type II).

Another common trap is failing to identify reversible causes before selecting permanent pacemaker as the answer. When a vignette describes a patient on a beta-blocker or calcium channel blocker who develops symptomatic AV block, the correct next step is to discontinue the offending medication, not to implant a pacemaker. Similarly, in a young patient with new AV block and a history of a camping trip or tick bite, the answer is Lyme serology and antibiotics, not permanent pacing.

Students also frequently miss the teaching point about atropine in infranodal block. A vignette may describe a patient with Type II block or third-degree block with a wide QRS escape who receives atropine and clinically worsens. The concept being tested is that atropine increases sinus rate, which sends more impulses into the already diseased infranodal tissue, paradoxically increasing the number of blocked beats. The correct intervention for infranodal block that does not respond to atropine is transcutaneous pacing, not more atropine.

Finally, remember the association between Lyme disease and AV block in any vignette featuring a young, otherwise healthy patient with new conduction disease. This is a classic "do not miss" diagnosis because the treatment (antibiotics) is curative and a permanent pacemaker is almost never needed, which is the opposite of the management for most other causes of high-degree AV block.