POTENTIALLY lethal arrhythmias have long been recognized as a rare complication of cardiac pacing. 1,2The incidence of this catastrophic event is thought to be low even in cases of asynchronous ventricular pacing, presumably because the electrical stimuli provided by the pulse stimulator usually remain below the fibrillation threshold. 3Furthermore, the possibility of serious pacemaker-induced arrhythmias has been greatly reduced by the introduction of advanced demand modes of pacing into clinical practice. Nevertheless, this problem can still be encountered when pacemaker pulse generator is switched to asynchronous mode. We report a case in which a recently introduced and commonly used epicardial temporary pacemaker (Model 5388, Medtronic Inc., Minneapolis, MN) was accidentally switched several times to asynchronous mode, causing ventricular fibrillation in a patient after cardiac surgery.

Case Report 

A 49-yr-old man with a history of aortic insufficiency of rheumatic origin and hypertension, treated with calcium channel blockers and angiotensin-converting enzyme inhibitors, was admitted to the hospital for elective aortic valve replacement. The preoperative coronary angiogram showed no signs of coronary artery disease. The electrocardiogram (ECG) demonstrated signs of left ventricular hypertrophy with strain pattern. Holter monitoring showed sinus rhythm with occasional premature atrial contractions and premature ventricular contractions (couplets and triplets).

The patient underwent aortic valve replacement with a 27-mm stentless porcine valve. No intraoperative problems were encountered. In the immediate postoperative hours, several episodes of premature ventricular contractions were noticed, which resolved after normalization of potassium balance (fig. 1A). Two hours later his heart rate slowed down. Second-degree atrioventricular block with a ventricular rate of 50–54 beats/min was apparent on the ECG trace. Dual chamber paced, dual chamber sensing, and dual response (DDD) pacing was started (Medtronic 5388 dual chamber temporary pacemaker) with an atrial sensitivity of 0.5 mV and output of 10 mA, ventricular sensitivity of 2 mV and output of 10 mA, and an atrioventricular interval of 170 ms. This resulted in ventricular rate of 100–102 beats/min (atrial sensing and ventricular pacing;fig. 1B). The patient was uneventfully extubated 6 h after surgery. Occasional premature ventricular contractions, present during the first night after surgery, resolved after removal of the pulmonary artery catheter on the morning of the first postoperative day. The parameters of the pacemaker were checked. The underlying rhythm at this time was third-degree atrioventricular block with a rate of ventricular escape of 50–60 beats/min. DDD pacing with the previous parameters was commenced, and the patient was transferred to the surgical ward on a telemetry bed. Before the transfer, the pacemaker was checked again, and settings were locked. Serum magnesium and potassium levels were normal.

Approximately 1 h after the transfer, asynchronous ventricular pacing at a rate of 80 beats/min was noticed on ECG telemetric monitoring. Several minutes later, the patient suddenly lost consciousness. A decrease of pacing stimuli on the T wave with resultant ventricular fibrillation was observed on the ECG tracing (fig. 2). Cardiopulmonary resuscitation was initiated. Over an 8-min period, the patient received three external defibrillatory shocks of 300 J. After the second shock, slow ventricular rhythm of 50 beats/min was observed with rapid degeneration to polymorphic ventricular tachycardia. After the administration of 80 mg lidocaine, 2 g magnesium sulfate, and a third shock, stable rhythm with dual-chamber pacing of 100 beats/min was achieved. The patient regained consciousness and was transferred back to the intensive care unit. On arrival, the pacemaker was found to be in asynchronous mode with maximal atrial and ventricular output (20 and 25 mA, respectively). The underlying rhythm at this time was complete heart block with ventricular escape of 60–65 beats/min. The impulse generator was reprogrammed to DDD mode with the parameters described previously and was locked. The patient was stable and breathing spontaneously with a face mask. No neurologic deficit was found. After 24 h in the intensive care unit, the patient continued with DDD pacing with atrial tracking and a ventricular rate of 100–104 paced pulses/min. The underlying rhythm was complete heart block. At this point, the pacemaker was checked again because a short run of ventricular tachycardia was observed on the ECG monitor. The pacemaker was found to be in asynchronous mode again (dual chamber paced, zero sensing, zero inhibition [DDO]: dual-chamber asynchronous pacing at 80 paced pulses/min, atrial output of 20 mA, and ventricular output of 25 mA). The impulse generator was replaced and checked in detail. On close examination it was found that this particular model of temporary pulse generator was very easily switched from DDD mode (atrial-triggered, ventricular-inhibited pacing) to dual-chamber asynchronous (DOO) pacing (the factory programmed default mode) by light pressure (single-finger touch) on the emergency button situated on the front panel of the device (fig. 3). This emergency DOO default mode overrides any preset (locked) pacing parameters and can be switched on even when the pacemaker is off.

The patient experienced no further dysrhythmias after changing to another atrioventricular sequential demand pulse generator (Model 5330, Medtronic Inc.). He remained in complete atrioventricular block and eventually required implantation of a permanent pacemaker.


Reported cases of ventricular tachyarrhythmias caused by a decrease in pacemaker stimulus during the “vulnerable period” of cardiac cycle have occurred in settings of acute myocardial ischemia, electrolyte disturbances, and drug effects, when the ventricular vulnerability to fibrillation is lower. 4Our patient did not have demonstrable coronary artery disease. Potassium and magnesium concentrations before arrhythmias were normal. However, the potential for altered ventricular excitation threshold must have existed. Evidence for this was atrial and ventricular dysrhythmias on the patient's preoperative Holter recording. The probable cause for this was cardiomyopathy secondary to long-standing ventricular volume overload.

This case report should alert clinicians and nursing staff to the possible danger of accidental asynchronous pacing with this model of temporary pulse generator. This could be atrial asynchronous (AOO), ventricular asynchronous (VOO), or DOO depending on whether the device was programmed AAI (atrial inhibited or demand), VVI (ventricular inhibited or demand) or DDD (also DDI; dual-chamber inhibited), respectively. Even light pressure on the emergency button, located on the front panel of the device, may trigger default mode of asynchronous pacing. This easily could happen if the patient or care provider mistakenly mishandled or touched the emergency button on the pulse generator (e.g. , by picking up the pacemaker). Possible means to reduce this risk include eliminating this feature or requiring more than one depression or some other method to activate default-mode asynchronous pacing.


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