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Tachycardia in the presence of a pacemaker

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Q1: What is the electrocardiographic diagnosis?

Atrial fibrillation in the presence of dual chamber pacemaker.

Q2: Is the pacemaker malfunctioning?


Q3: How do you explain the tachycardia with pacing spikes?

The heart’s native atrial activity in patients with atrial flutter/fibrillation causes atrial sensing, which triggers the ventricular pacing at the pacemaker’s maximal preprogrammed rate.

Q4: What is the differential diagnosis for tachycardia with pacing spikes?

The differential diagnosis of wide complex tachycardia with pacing spikes includes pacemaker mediated tachycardia and runaway pacemaker syndrome.

Q5: What is the management?

Deactivation of atrial lead.


The main purpose of a dual chamber pacemaker is to achieve atrioventricular synchrony. This is achieved by the atrial lead sensing atrial depolarisation and triggering the ventricular lead to depolarise the ventricle after some atrioventricular delay. In patients with no native atrioventricular conduction as in complete heart block, if the native atrial rate increases, the ventricular lead follows suit and discharges at the corresponding rate to maintain atrioventricular synchrony. However, the maximum rate at which the ventricular lead can respond is usually programmed at 120–130 beats/min to prevent very rapid ventricular rates.

Pacemaker mediated tachycardia in those with a dual chamber pacemaker can be due to one of two mechanisms. Firstly, if the patient with a dual chamber pacemaker in the setting of third degree atrioventricular nodal block (complete heart block) develops any atrial tachydysrhythmia like fibrillation,1 flutter or atrial tachycardia, the atrial lead will sense the atrial activity and trigger ventricular pacing at the pacemaker’s maximal preprogrammed rate. The standard 12 lead electrocardiogram (ECG) will reveal a rapid ventricular paced rhythm. A magnet placed directly on the pacemaker switches off the sensing action and induces asynchronous ventricular pacing at the preset demand rate, which is usually 70 beats/min. However, when the magnet is removed and sensing restored, the problem will recur. The pacemaker interrogator/programmer, which is portable and compact, provides data about the pacemaker’s intrinsic functionality and its current functioning. It can generate an intracardiac ECG and atrial and ventricular depolarisation can be easily identified.

Learning points

  • The differential diagnosis for broad complex tachycardia with pacing spikes are pacemaker mediated tachycardia or runaway pacemaker syndrome.

  • Development of atrial fibrillation or flutter in patients with a dual chamber pacemaker (in the setting of complete heart block) leads to pacemaker mediated tachycardia. Atrial rhythm is sensed by the atrial lead, which triggers ventricular pacing at the maximal preprogrammed rate.

  • Deactivation of the atrial lead is often the preferred option to control this tachycardia.

Rate controlling drugs will not have any impact on this tachycardia. Pharmacological or electrical cardioversion may be useful. If cardioversion is considered inappropriate, the atrial lead can be deactivated using the interrogator. Anticoagulation should be started. If the underlying rhythm is atrial flutter, it is possible to overdrive the atrium and cardiovert it.2

The second mechanism by which pacemaker mediated tachycardia can occur is when premature ventricular contractions (occurring after the atrial refractory period) are conducted retrogradely via the atrioventricular node resulting in atrial depolarisation. The pacemaker senses this atrial activity and initiates ventricular pacing, which continues as an endless loop tachycardia or pacemaker mediated tachycardia. Adenosine and Valsalva manoeuvre may or may not terminate this tachycardia.3,4

Both the above problems should be differentiated from runaway pacemaker. Runaway pacemaker occurs when the pacemaker’s pulse generator discharges at a rate above its preset upper limit.5 The malfunction lies entirely within the pulse generator. It should be suspected if pacemaker dysrhythmias occur at rates greater than 130 beats/min or the upper rate limit if this is known. Magnet application or pharmacological treatment will have little or no effect on the runaway pacemaker but magnet application may be tried. It can be potentially life threatening. Fortunately it is extremely rare now because of improved design. In the setting of haemodynamic compromise, the definitive treatment is emergent removal of pulse generator.

Final diagnosis

Pacemaker mediated tachycardia due to tracking of atrial fibrillation by a dual chamber pacemaker.


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