Third Degree AV Block (Complete Heart Block) — ECG Rhythm | Telemetric Pro

Complete failure of AV conduction — no atrial impulses reach the ventricles. The atria and ventricles beat independently (AV dissociation), and the ventricles are sustained only by an escape rhythm.

Third Degree AV Block (Complete Heart Block) EKG rhythm strip showing Complete failure of AV conduction — no atrial impulses reach the ventricles. The atria and ventricles beat independently (AV dissociation), and the ventricles are sustained only by an escape rhythm.

At a Glance
Rate	Ventricular 20–60 bpm (escape dependent)
Rhythm	Regular (P and QRS each regular, independent)
P Waves	Present, march through independently of QRS
PR Interval	Variable (no relationship to QRS)
QRS Duration	Narrow or wide (depends on escape level)

Third Degree AV Block — Complete Heart Block — is the end of the AV block spectrum. In first degree, all beats conduct slowly. In second degree, some beats are dropped. In third degree, nothing gets through. The AV node (or infranodal conduction system) has completely failed, and the atria and ventricles beat as two entirely independent units.

For monitor technicians, complete heart block is one of the most recognizable critical rhythms. You see two independent rhythms running simultaneously: P waves marching at one rate and QRS complexes marching at a slower rate, with no relationship between them. The patient depends entirely on the escape rhythm — which is slow and potentially unreliable.

What Changed from Normal Sinus Rhythm

Complete heart block changes everything from NSR: the pacemaker relationship is broken (atria and ventricles are independent), the ventricular rate is slow (escape rhythm), the PR interval is meaningless (varies randomly), and the QRS may be wide (if ventricular escape). The P waves themselves are still normal — the SA node is working fine. The problem is that its impulses cannot reach the ventricles.

Five Criteria: Complete Heart Block vs NSR

Rate: Slow (depends on escape focus): Atrial rate is normal (60-100 BPM). Ventricular rate depends on the escape: junctional escape (40-60) or ventricular escape (20-40). The atrial and ventricular rates are always different.
Regularity: Regular (both independently): Both P-P intervals and R-R intervals are individually regular. But they are at different rates. This "two independent regular rhythms" is the signature of complete heart block.
P Waves: Present, normal — but independent of QRS: P waves march at their own rate, completely unrelated to the QRS complexes. P waves may appear before, during, or after QRS complexes — their position shifts from beat to beat as the two rhythms drift in and out of alignment.
PR Interval: Variable (no relationship): The PR interval changes randomly from beat to beat because there is no conduction between atria and ventricles. The varying PR is diagnostic — in second degree blocks, conducted beats have a consistent PR.
QRS Complex: Depends on escape focus: Narrow QRS = junctional escape (block at AV node, escape from junction, 40-60 BPM, better prognosis). Wide QRS = ventricular escape (block infranodal, escape from Purkinje, 20-40 BPM, worse prognosis).

What Complete Heart Block Looks Like on the Strip

On the strip, complete heart block is unmistakable once you see it: P waves marching at one rate and QRS complexes marching at a different, slower rate. The P waves appear to "walk through" the QRS complexes — sometimes falling before, sometimes during, sometimes after the QRS. Use calipers: set them to the P-P interval and confirm the P waves are regular. Then set them to the R-R interval and confirm the QRS complexes are regular. If both are regular but at different rates, you have complete heart block.

The Complete AV Block Spectrum

Third degree block is the end of a progression: **First Degree** — All beats conduct, but slowly (PR > 200ms). A delay. **Second Degree Type I** — PR progressively lengthens until a beat drops. A pattern. **Second Degree Type II** — Beats drop suddenly without PR change. A warning. **Second Degree 2:1** — Every other beat drops. Ambiguous type. **Third Degree** — No beats conduct at all. Complete dissociation. Understanding this progression helps you recognize when a patient is worsening. If second degree block progresses to third degree, the clinical urgency escalates dramatically.

Going Deeper: AV Dissociation Is Not Always AV Block

Clinical Pearl: **AV dissociation** (independent P waves and QRS complexes) is a finding, not a diagnosis. It has five possible causes: 1. **AV block** (third degree) — the conduction system has failed 2. **Increased subsidiary pacemaker automaticity** — a junctional or ventricular focus speeds up and "outruns" the sinus node 3. **Decreased SA node automaticity** — the sinus node slows enough that a lower pacemaker takes over 4. **Combination** of the above 5. **Reentrant tachyarrhythmias** (VT) — retrograde conduction blocks the AV node Only cause #1 is true complete heart block. The others produce AV dissociation without any conduction system disease. When atrial and ventricular rates are similar (within ~5 BPM), the dissociation may be "isorhythmic" — a benign finding where two pacemakers happen to fire at nearly the same rate. The key: true complete block is confirmed only when a P wave falls far enough from the QRS (outside the ventricular refractory period) and still fails to conduct.

Locating the Block: The 4-Step Algorithm

Clinical Pearl: **Marriott's 4-Step AV Block Location Algorithm:** **Step 1 — QRS duration prolonged?** No → Block is in the AV node (narrow QRS = intact bundle branches = nodal block). Yes → Continue to Step 2. **Step 2 — Evidence of impulse conduction?** No → Location cannot be determined from this recording. Yes → Continue to Step 3. **Step 3 — Variation in conduction time (PR interval changes)?** Yes → Block is in the AV node (Type I). The AV node can vary its conduction speed. No → Continue to Step 4. **Step 4 — Variation in RP interval with constant PR?** Yes → Block is in the bundle branches (Type II). PR stays fixed despite changing RP. No → Location cannot be determined. This algorithm applies to first- and second-degree blocks where some conduction occurs. In complete (third-degree) block with wide escape QRS, location cannot be definitively determined from the surface ECG alone.

Clinical Context for Monitor Technicians

Complete heart block is most commonly seen with acute MI (inferior MI usually produces AV nodal block that may resolve; anterior MI produces infranodal block that often requires permanent pacing), degenerative conduction disease, drug toxicity (beta-blockers, calcium channel blockers, digoxin), post-cardiac surgery, myocarditis (including Lyme disease), and infiltrative diseases.

When to Escalate

**Notify immediately:** - Any complete heart block — this always requires urgent evaluation - Wide QRS escape rhythm (ventricular escape at 20-40 BPM — less reliable) - Patient is symptomatic — syncope, altered consciousness, hypotension - Escape rate is dropping or becoming irregular (escape failure imminent) - New complete heart block in an acute MI patient Complete heart block is never a "document and monitor" rhythm. It always requires immediate notification.

Transcutaneous Pacing on the Monitor

When transcutaneous pacing is initiated for complete heart block, the monitor shows tall, wide pacing spikes followed by wide QRS complexes — this confirms mechanical capture. The spikes are much larger than permanent pacemaker spikes because the energy is delivered through skin electrodes on the chest wall rather than through a wire directly on the heart muscle. Spikes without QRS = no capture. Spikes with wide QRS = capture. If you see intermittent capture (some spikes produce QRS complexes, others do not), report it — the output may need adjustment.