Complete heart block ECG interpretation becomes much easier when you use one rule: prove that the P waves and QRS complexes have no fixed relationship. Complete heart block, also called third-degree atrioventricular (AV) block, means atrial impulses are not conducted to the ventricles. The atria continue under their own pacemaker while a slower escape rhythm drives the ventricles.
This visual guide uses a real source-labelled 12-lead ECG from the SlideBites ECG library (PTB-XL record 00959, PhysioNet, CC BY 4.0). Use the trace to practise the sequence below rather than memorising a single picture.
What does complete heart block mean?
Normally an impulse starts in the sinus node, crosses the atria, passes through the AV node and His-Purkinje system, and activates the ventricles. In complete heart block, conduction between atria and ventricles fails completely.
That produces two independent rhythms:
- P waves represent atrial activity.
- QRS complexes represent an escape rhythm controlling the ventricles.
- The atrial rate is usually faster than the ventricular rate.
- There is no consistent PR interval because the P waves and QRS complexes are not linked.
The key concept is AV dissociation. A P wave may appear before a QRS by chance, but the next P wave will not keep the same relationship.
How to recognise complete heart block on an ECG
Use the same order every time.
1. Check the ECG calibration
Before interpreting the rhythm, confirm the paper speed and gain shown on the ECG. Standard recordings are commonly displayed at 25 mm/s and 10 mm/mV, but always read the calibration printed on the actual trace rather than assuming it.
2. Find the P waves
Look across several leads and identify repeated atrial activity. Do not judge the diagnosis from one complex.
Ask:
- Are P waves present?
- Are the P waves regular?
- What is the approximate atrial rate?
3. Find the QRS complexes
Now ignore the P waves temporarily and follow the QRS complexes.
Ask:
- Is the ventricular rhythm regular?
- What is the ventricular rate?
- Is the QRS narrow or broad?
A slower ventricular escape rhythm is typical. The exact rate and QRS width depend on where the escape rhythm originates, so neither feature alone diagnoses complete heart block.
4. Compare P waves with QRS complexes
This is the decisive step.
Choose several consecutive P waves and look at their position relative to the nearest QRS. In complete heart block, the PR relationship continually changes because the atria and ventricles are running independently.
Memory rule: two clocks, one ECG. The P waves have their clock. The QRS complexes have another. If the two clocks never lock together, think AV dissociation.
The SlideBites ECG example
The SlideBites ECG library contains a source-labelled PTB-XL example of complete heart block. It is a real 12-lead waveform rather than an AI-generated diagnostic trace.
When studying the visual, do not start by reading the diagnosis. Cover the label and work through this sequence:
- Mark several P waves.
- Mark several QRS complexes.
- Compare the atrial and ventricular rates.
- Follow the PR relationship across the strip.
- Decide whether any stable P-to-QRS conduction exists.
- Only then reveal the diagnosis.
This turns the image into an interpretation exercise instead of a recognition flashcard. You can also explore the wider SlideBites library and use interactive teaching sessions to practise rhythm recognition.
Complete heart block versus other AV blocks
The common trap is confusing complete heart block with second-degree AV block.
In first-degree AV block, every P wave conducts, but the PR interval is prolonged. In Mobitz I, the PR interval progressively lengthens before a QRS is dropped. In Mobitz II, some P waves fail to conduct, but conducted beats retain a relationship between P waves and QRS complexes.
In complete heart block, there is no reliable atrioventricular relationship at all.
Another trap is mistaking AV dissociation for complete heart block when another mechanism is present. Always interpret the whole ECG and clinical context rather than applying the label from one feature.
How can complete heart block present clinically?
Presentation depends on the ventricular rate, escape rhythm, underlying disease and haemodynamic effect. Patients may have:
- dizziness or presyncope
- syncope or collapse
- fatigue or reduced exercise tolerance
- dyspnoea
- chest discomfort
- confusion
- hypotension or shock
- acute heart failure
- cardiac arrest in severe deterioration
A patient can also look relatively well despite a striking ECG. Clinical stability at one moment does not make the rhythm unimportant.
Causes to think about
Complete heart block is a rhythm diagnosis, not an explanation for why it happened. Causes and associations include degenerative conduction-system disease, myocardial ischaemia or infarction, drugs that impair AV conduction, electrolyte or metabolic disturbance, inflammatory or infiltrative disease, infection, cardiac procedures or surgery, and congenital disease.
The useful clinical question is therefore not only “Is this complete heart block?” but also “Why does this patient have it?”
Emergency management: what matters first?
Treat the patient, not just the ECG. Perform an ABCDE assessment, obtain IV access, monitor ECG, blood pressure and oxygen saturation, record a 12-lead ECG, and identify reversible causes.
The Resuscitation Council UK 2025 adult bradyarrhythmia guidance identifies shock, syncope, myocardial ischaemia, severe heart failure and the immediate post-ROSC state as life-threatening features of bradycardia.
For bradycardia with adverse features, current RCUK guidance uses atropine 500 micrograms IV, repeated if necessary every 3–5 minutes to a maximum total of 3 mg. If the response is unsatisfactory, interim options include isoprenaline, adrenaline and/or transcutaneous pacing while expert help is sought. RCUK specifically advises against atropine in high-degree AV block with a broad QRS because it is ineffective and may worsen the block.
Complete heart block with a broad QRS is also listed by RCUK as a risk factor for asystole. Unstable symptomatic bradycardia refractory to drug therapy should prompt pacing, with early transvenous pacing considered and transcutaneous pacing used as a bridge when appropriate.
Longer-term treatment depends on the cause and clinical situation. NICE describes pacemakers as established treatment for symptomatic bradycardia associated with AV block.
A 20-second interpretation checklist
When complete heart block is possible, say the ECG out loud in this order:
- Rate: atrial rate, then ventricular rate.
- Rhythm: are P waves regular? Are QRS complexes regular?
- P waves: identify them clearly.
- PR relationship: fixed, changing, or completely independent?
- QRS: narrow or broad?
- Conclusion: is there AV dissociation consistent with complete heart block?
- Patient: stable or showing life-threatening features?
The diagnosis should come from the relationship between atrial and ventricular activity, not simply from seeing a slow heart rate.
The one thing to remember
Complete heart block = P waves and QRS complexes marching independently.
If you can demonstrate that independence systematically, the ECG becomes much less intimidating. Then move immediately from rhythm recognition to the clinical question: is the patient compromised, is there a reversible cause, and is pacing likely to be required?
For more rhythm practice, use the ECG learning resources or build a teaching session in SlideBites around the trace.