6.3 Third-degree (complete) AV block & AV dissociation

Third-Degree (Complete) AV Block

Third-degree AV block, also called complete heart block, is the total failure of conduction between the atria and ventricles: no atrial impulse reaches the ventricles. The atria and ventricles are driven by two entirely separate pacemakers that beat independently of one another. This is the most severe AV block and a frequent high-yield item on the CRAT exam.

The Diagnostic Criteria

On the rhythm strip you will see:

• Regular P-P intervals: the SA node continues to fire at its own steady rate (often 60-100 bpm).
• Regular R-R intervals: an escape pacemaker below the block fires at its own steady rate.
• No relationship between P waves and QRS complexes: the PR intervals are completely variable and random. P waves march through the strip, landing before, on, and after QRS complexes with no consistency.
• The atrial rate is faster than the ventricular rate (more P waves than QRS complexes).

The phrase to memorize is P's and QRS's each regular but marching to their own drummer. Both rhythms are regular in isolation, but they are dissociated from each other.

The Escape Rhythm Sets the Ventricular Rate

Because the ventricles are cut off from the atria, survival depends on an escape pacemaker arising below the block. Where that focus sits determines the rate and the QRS width, a distinction the exam tests directly:

A junctional escape (narrow QRS, 40-60) implies the block is at the AV node, and the patient is often more stable. A ventricular escape (wide QRS, 20-40) implies the block is low in the His-Purkinje system; the rate is dangerously slow and the escape focus is unreliable, making this pattern far more ominous.

Why Complete Heart Block Is an Emergency

The escape pacemaker can fail entirely, producing ventricular standstill (asystole). Even when it holds, the slow ventricular rate frequently causes hypotension, syncope, and hemodynamic collapse. Complete heart block often requires transcutaneous pacing followed by a permanent pacemaker. On the exam, treat third-degree block as a medical emergency regardless of the escape rate.

Common Causes

Complete heart block most often arises from acute myocardial infarction: an inferior wall MI tends to cause a usually transient nodal block with a narrow-complex junctional escape, while an anterior wall MI causes an ominous infranodal block with a wide-complex ventricular escape. Other causes include degenerative fibrosis of the conduction system in older patients (Lenegre and Lev disease), digoxin or beta-blocker toxicity, hyperkalemia, cardiac surgery, and infiltrative or infectious disease such as Lyme carditis. Recognizing the clinical setting helps predict the escape site and stability: an inferior MI or drug effect tends to produce a more reliable narrow-complex junctional escape, whereas anterior infarction or advanced conduction disease produces the slow, unreliable ventricular escape that most urgently needs pacing.

AV Dissociation

AV dissociation is the broader concept that the atria and ventricles are being controlled by independent pacemakers. Complete heart block is one cause of AV dissociation, but the two terms are not synonymous:

• In complete heart block, dissociation exists because conduction is blocked: the ventricular (escape) rate is slower than the atrial rate, and no P wave can ever capture the ventricle.
• In other forms of AV dissociation, such as an accelerated junctional or ventricular rhythm that simply outpaces a slowed sinus node, the two pacemakers coincidentally run at similar rates and conduction may not be blocked at all. Here the ventricular rate may equal or exceed the atrial rate.

The practical takeaway: all complete heart block is AV dissociation, but not all AV dissociation is complete heart block. Look at the rates; if the atrial rate clearly exceeds a slow ventricular rate and no P wave conducts, complete block is the reason, and the appropriate response is to prepare for pacing rather than to give a drug aimed at the atrial rhythm.

Distinguishing Complete Block from High-Grade (Advanced) Block

A common exam trap is confusing third-degree block with high-grade (advanced) second-degree AV block, in which two or more consecutive P waves fail to conduct (for example, 3:1 or 4:1). The critical difference is whether any P wave still captures the ventricle:

• In high-grade block, some P waves do conduct: you will find at least one P wave followed by a QRS at a consistent, appropriate PR interval, proving conduction still occurs occasionally.
• In complete block, no P wave conducts: there is never a fixed, reproducible PR relationship anywhere on the strip.

Checklist for third-degree block:

1. Are the P-P intervals regular? (Yes.)
2. Are the R-R intervals regular? (Yes.)
3. Is the atrial rate faster than the ventricular rate? (Yes.)
4. Is there any consistent PR interval? (No; if you find one, it is not complete block.)

Exam tip: The presence of even one conducted beat with a consistent PR interval rules out third-degree block. Complete heart block requires complete AV dissociation with the atrial rate exceeding a regular, slower escape rhythm.