- A junctional rhythm with a rate of 40-60 bpm.
- QRS complexes are typically narrow (< 120 ms).
- No relationship between the QRS complexes and any preceding atrial activity (e.g. P-waves, flutter waves, fibrillatory waves).
Pacemaker cells are found at various sites throughout the conducting system, with each site capable of independently sustaining the heart rhythm. The rate of spontaneous depolarisation of pacemaker cells decreases down the conducting system:
- SA node (60-100 bpm)
- Atria (< 60 bpm)
- AV node (40-60 bpm)
- Ventricles (20-40 bpm)
Under normal conditions, subsidiary pacemakers are suppressed by the more rapid impulses from above (i.e. sinus rhythm). Junctional and ventricular escape rhythms arise when the rate of supraventricular impulses arriving at the AV node or ventricle is less than the intrinsic rate of the ectopic pacemaker.
Conditions leading to the emergence of a junctional or ventricular escape rhythm include:
Atrial fibrillation with 3rd degree AV block and a junctional escape rhythm (“regularised AF”)
The ECG shows:
- Coarse atrial fibrillation (irregular baseline with atrial complexes at rate > 400 bpm).
- Regular narrow complex rhythm at 60 bpm.
- The combination of atrial fibrillation with a regular rhythm (“regularised AF”) indicates that none of the atrial impulses are conducted to the ventricles, i.e. complete heart block is present.
- The narrow complex rhythm is therefore a junctional escape rhythm.
- Regularised AF is characteristically seen as a consequence of digoxin toxicity.
Terminology of junctional rhythms
- Junctional bradycardia = junctional rhythm at a rate of < 40 bpm.
- Junctional escape rhythm = junctional rhythm at a rate of 40-60 bpm.
- Accelerated junctional rhythm = junctional rhythm at 60-100 bpm.
- Junctional tachycardia = junctional rhythm at > 100 bpm.
- Chan TC, Brady WJ, Harrigan RA, Ornato JP and Rosen PR. ECG in Emergency Medicine and Acute Care. Elsevier 2005.
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.