A 48-year-old man recovering in hospital after repair of a dissected thoraco-abdominal aortic aneurysm is having periods of marked bradycardia but no episodes of syncope:

• Marked sinus bradycardia (sinus rate 37/min)
• AV dissociation, with an escape junctional rhythm, one AV capture, ventricular rate 43/min
• Nonspecific T wave abnormality

There is marked sinus bradycardia (37/min) with and escape junctional pacemaker (ventricular rate 43/min). As the rate of the junctional pacemaker is faster than that of the sinus node, the QRS gradually “runs ahead of the P wave”. Eventually the stimulus from the atria  reaches the AV node when is no longer refractory and AV capture occurs.

Rule of thumb: In 3rd degree AV block usually the distal pacemaker is slower than the proximal pacemaker.

In AV dissociation the distal pacemaker is usually faster.

Remember: “rule of thumb” is a rapid approach, based on experience, useful in practice, but not accurate or reliable in every situation. Its application in a given case should be assessed in the light of the available clinical information.