Controlling the Ventricular Rate: Upper Rate Limit

Report:

Atrial fibrillation 5

Paced ventricular response 115 – 119/min 5

Comment:

The unit is pacing quite close to its upper rate limit of 120/min, obviously in response to atrial fibrillation (hence no atrial pacemaker activity – it remains suppressed by the f waves). Cardioversion by sotalol effected 100% paced rhythm.

Month later the patient returned to Casualty with the same problem (Fig 115a) and received the same treatment. This time the pacemaker response is more obviously irregular.

A mode switch to VVI would not have helped much: if the AF were transmitted with rapid ventricular response, the pacemaker would be completely suppressed; if the AF were blocked, the patient would be left with 100% ventricular pacing and no atrial contribution to the ventricular filling.

The main differential diagnosis here would be an endless loop tachycardia. If no atrial activity were observable and the ventricular rate were regular and at the upper rate limit of 120 this would be a tenable first diagnostic choice. But not in this patient, who could not have any visible or invisible retrograde P waves: he had a (successful) nodal ablation for AF only a few months previously!