- In complete heart block, there is complete absence of AV conduction – none of the supraventricular impulses are conducted to the ventricles.
- Perfusing rhythm is maintained by a junctional or ventricular escape rhythm. Alternatively, the patient may suffer ventricular standstill leading to syncope (if self-terminating) or sudden cardiac death (if prolonged).
- Typically the patient will have severe bradycardia with independent atrial and ventricular rates, i.e. AV dissociation.
Example of complete heart block
- The atrial rate is approximately 100 bpm.
- The ventricular rate is approximately 40 bpm.
- The two rates are independent; there is no evidence that any of the atrial impulses are conducted to the ventricles.
- Complete heart block is essentially the end point of either Mobitz I or Mobitz II AV block.
- It may be due to progressive fatigue of AV nodal cells as per Mobitz I (e.g. secondary to increased vagal tone in the acute phase of an inferior MI).
- Alternatively, it may be due to sudden onset of complete conduction failure throughout the His-Purkinje system, as per Mobitz II (e.g. secondary to septal infarction in acute anterior MI).
- The former is more likely to respond to atropine and has a better overall prognosis.
Causes of complete heart block
- Patients with third degree heart block are at high risk of ventricular standstill and sudden cardiac death.
- They require urgent admission for cardiac monitoring, backup temporary pacing and usually insertion of a permanent pacemaker.
Complete heart block should not be confused with:
- High grade AV block: A type of severe second degree heart block with a very slow ventricular rate but still some evidence of occasional AV conduction.
- AV dissociation: This term indicates only the occurrence of independent atrial and ventricular contractions and may be caused by entities other than complete heart block (e.g. “interference-dissociation” due to the presence of a ventricular rhythm such as AIVR or VT).
Complete Heart Block:
Complete Heart Block:
- Atrial rate is ~ 60 bpm.
- Ventricular rate is ~ 27 bpm.
- None of the atrial impulses appear to be conducted to the ventricles.
- There is a slow ventricular escape rhythm.
- Atrial rate 100 bpm
- Ventricular rate only 15 bpm!
- This patient needs urgent treatment with atropine / isoprenaline and pacing!
- Atrial rate ~ 85 bpm
- Ventricular rate ~ 42bpm
- There is a junctional escape rhythm.
- As the ventricular rate is approximately half the atrial rate, this rhythm at first glance appears to be second-degree AV block with 2:1 conduction.
- However on closer inspection the PR interval varies, with some of the P waves superimposed on the QRS complexes. The ventricular rate remains regular.
- This confirms that the atrial impulses are not being conducted to the ventricles.
- The apparent relationship between the P waves and QRS complexes occurs merely by chance (= isorhythmic AV dissociation).
- Hampton, JR. The ECG In Practice, 6e
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Wagner, GS. Marriott’s Practical Electrocardiography 12e
- Chan, TC. ECG in Emergency Medicine and Acute Care
- Mattu, A. ECG’s for the Emergency Physician
LITFL Further Reading
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.