Third-Degree (Complete) AV Block — "If P's and QRS Don't Agree, It's a Third Degree"

Recognize complete heart block: the atria and ventricles beat independently (AV dissociation) — P waves and QRS complexes march to their own separate, unrelated rhythms, and no atrial impulse ever reaches the ventricles.

P — P waves (independent atrial rhythm)

The atria beat at their own rate (SA node, ~60–100 bpm) — the husband on his own schedule

The atria continue to be driven by the SA node at a normal or near-normal rate (usually 60–100 bpm), producing regular P waves. However, none of these impulses conduct through the AV node to the ventricles — the connection is completely severed.

Physiology. In complete heart block there is total failure of AV conduction. The SA node keeps firing normally, so P waves are regular — but they are physiologically ignored by the ventricles, which must generate their own rhythm.

Safety. Regular P waves with no conduction tell you the atria are fine and the failure is in the AV conduction system — count the P rate separately from the QRS rate.

QRS — QRS complexes (slow ventricular escape rhythm)

The ventricles beat at their own slow escape rate (~20–60 bpm) — the wife sets a slow alarm

When no supraventricular impulses arrive, a lower pacemaker takes over. A junctional escape rhythm produces ~40–60 bpm with a narrow QRS; a ventricular escape rhythm produces ~20–40 bpm with a wide QRS. The ventricular rate is therefore slow and often inadequate for perfusion.

Physiology. Escape rhythms are a safety net — the His bundle, bundle branches, and Purkinje fibers can all depolarize spontaneously. But the lower the pacemaker, the slower and less reliable the rate, which is why complete block causes hypotension, syncope, and shock.

Safety. The ventricular (QRS) rate is what perfuses the body — a slow escape rhythm is a pre-arrest state; prepare for immediate pacing.

Disagree — AV dissociation (no relationship between P and QRS)

P waves and QRS complexes are regular but independent — a total "divorce"

AV dissociation is the hallmark of complete heart block: P waves and QRS complexes are regular but independent, with the atrial rate faster than the ventricular rate. The PR interval appears to vary continuously because the P waves "walk through" the QRS complexes.

Physiology. Because no impulse crosses the AV junction, the timing of each QRS is set entirely by the ventricular escape focus, independent of the P waves. This produces the classic "P's and QRS don't agree" appearance.

Safety. Complete heart block is a medical emergency — it can cause syncope (Stokes-Adams attacks), hypotension, and cardiac arrest; treat with transcutaneous pacing immediately, then transvenous or permanent pacing.

Memory aids

• P = P waves (independent atrial rhythm)
• QRS = QRS complexes (slow ventricular escape rhythm)
• Disagree = AV dissociation (no relationship between P and QRS)

Quick review

• P = P waves (independent atrial rhythm) — The atria beat at their own rate (SA node, ~60–100 bpm) — the husband on his own schedule
• QRS = QRS complexes (slow ventricular escape rhythm) — The ventricles beat at their own slow escape rate (~20–60 bpm) — the wife sets a slow alarm
• Disagree = AV dissociation (no relationship between P and QRS) — P waves and QRS complexes are regular but independent — a total "divorce"