Troubleshooting Implantable Cardioverter-Defibrillators

Implantable cardioverter-defibrillators (ICDs) have emerged as the most effective therapy for the prevention of sudden cardiac death in high-risk patients. As the guidelines for ICDs have expanded, their use has increased significantly. It is very likely that physicians not trained in cardiac electrophysiology will encounter these patients in their practice and will be asked for help or advice about problems that might involve these devices. This brief review tries to provide a stepwise approach to some of the ICD problems with which patients present.

Device Function

Implantable cardioverter-defibrillators are programmed to detect arrhythmia on the basis of rate. All such devices use the signal rate recorded by the right ventricular lead as the 1st detection criterion. For an arrhythmia to be declared, a specified number or percentage of sensed events must occur at a rate higher than the programmed cutoff rate. Once this criterion is met, the device will apply certain discriminators to differentiate between supraventricular tachycardia (SVT) or ventricular tachycardia (VT) and ventricular fibrillation (VF). If the device confirms VT or VF, either antitachycardia pacing (ATP) or shock is delivered. Conversely, if SVT is diagnosed by the device, all therapies are withheld.

Presentation

Patients with an ICD generally present with problems that can be grouped into one of the following categories.

1. Shocks delivered by the device

2. Dizziness, lightheadedness, syncope

3. Palpitations

4. Alerts, audible beeps, or vibrations from the device

Shocks

When a patient presents with an ICD shock, a thorough but focused history and physical examination should be performed, along with device interrogation (Fig. 1 ¹). Typically, shocks (whether appropriate or inappropriate) cause symptoms of dizziness, lightheadedness, syncope, chest pain, or diaphoresis. In the event of inappropriate shocks, the patient might feel completely normal until the shock occurs.

Fig. 1 Stepwise algorithm for patients who present with implantable cardioverter-defibrillator (ICD) shock. (Adapted by permission from BMJ Publishing Group Limited. From: van Erven L, Schalij MJ. Arrhythmias: Troubleshooting implantable cardioverter-defibrillator related problems. Heart 2008;94(5):649–60.¹ Copyright 2008 by BMJ Publishing Group Limited.)

SVT = supraventricular tachycardia; VF = ventricular fibrillation; VT = ventricular tachycardia

Appropriate Shock

If the device interrogation reveals an episode of VT or VF that has resulted in appropriate therapy, the device function is within normal limits. In these cases, all efforts are directed towards treatment and towards prevention of more episodes of VT and VF. If a single episode of VT or VF has resulted in an ICD shock, no further treatment may be needed. In case of recurrent VT/VF or VT storm, treatment of underlying heart disease along with antiarrhythmic therapy to prevent more VT/VF will be required.

Inappropriate Shock

If device interrogation reveals intracardiac electrograms (EGMs) consistent with SVT, both device reprogramming to prevent recurrent shocks and treatment of SVT are needed. Atrial fibrillation is the most common cause of inappropriate shock, followed by sinus tachycardia, atrial flutter, and atrial tachycardia. Other types of SVT, like atrioventricular nodal reentry tachycardia, can also lead to inappropriate shocks, although less commonly. In patients who have received inappropriate ICD shocks for SVT, device programming can be very helpful. Turning SVT discriminators “on,” optimizing them to the individual patient's characteristics, and adjusting therapy zones and detection intervals to more aggressive levels can significantly reduce the risk of recurrent shocks.^2,3

In addition, the use of negative chronotropic agents with strong atrioventricular nodal blocking effect (for example, β-blockers or calcium channel blockers) and antiarrhythmic agents (for example, class I and class III agents) might help by preventing recurrences of SVT and slowing down the conducted ventricular rate.⁴

Oversensing

Some type of ventricular channel oversensing should be suspected in a patient who presents with ICD shock but with no tachycardia that can be documented by device interrogation.⁵ Oversensing of P wave or T wave—as well as double-counting of R wave on the ventricular channel—is easily diagnosed from analysis of the EGM and the marker channel. If high-frequency nonphysiologic noise is detected on intracardiac EGM during an episode, then either device-component failure or oversensing, from sources inside or outside the body, is possible. On the basis of measurements obtained through the pacing system, such as lead impedance and certain maneuvers performed to re-create the noise during device interrogation, the diagnosis can be narrowed further.

In patients with myopotential or diaphragmatic oversensing, intermittent noise can be re-created by upper-extremity isometric exercises or deep breathing, respectively. In these cases, the lead impedance and pacing-related measurements are usually within normal range. If lead impedance is high, conductor fracture should be suspected, while low lead impedance is associated with insulation problems. A set screw in poor contact with a lead header can produce intermittent oversensing, which might or might not be reproducible by means of shoulder exercise.

Oversensing that arises from a source outside the body—such as electromechanical interference—displays normal lead impedance; and shoulder or arm exercises do not re-create the noise.^6,7 Recalling the events that immediately preceded the ICD shock is helpful in narrowing down the offending source of the electromechanical interference and in avoiding recurrences.

Syncope

When a patient who has an ICD presents with syncope, it is important to consider all the various causes of syncope in the differential diagnosis (Fig. 2). Complete device interrogation can easily rule out arrhythmic causes and device malfunction. If no episodes of tachyarrhythmia or bradyarrhythmia are stored in device memory and if the patient is not pacemaker dependent, cardiac arrhythmia probably did not cause the syncopal episode. Attention should then be focused on other cardiac, and noncardiac, causes of syncope.

Fig. 2 Stepwise algorithm for patients with an implantable cardioverter-defibrillator (ICD) who present with syncope.

ATP = antitachycardia pacing; DFT = defibrillation threshold; SVT = supraventricular tachycardia; VF = ventricular fibrillation; VT = ventricular tachycardia

In pacemaker-dependent patients, special attention should be paid to the exclusion of oversensing (with pacing inhibition) as the cause of syncope. If no oversensing is manifest at rest or on the stored EGM, such physical maneuvers as deep breathing and upper-extremity isometric exercises should be tried in an effort to re-create oversensing, before ruling that out as the cause of syncope.

Palpitations

Palpitations are a very frequent complaint among patients who have ICDs. In these patients, much information can be gathered by analyzing the data stored in the device's memory. Most modern ICDs have the ability to store not only sustained episodes leading to device therapy, but also nonsustained episodes. These devices can also give a fairly accurate assessment of the burden imposed by premature ventricular contractions. If a careful analysis of the stored EGMs does not help to answer the question, some form of external continuous or intermittent electrocardiographic monitoring—like a 24-hour Holter monitor or an event monitor—has to be used.

Audible Beep or Alerts

Modern ICDs are programmed to emit an audible tone or alert when they detect a change in certain measurable values. These values can range from battery voltage or lead impedance that pertain to device integrity, to the presence of atrial fibrillation or other measures that pertain to arrhythmia burden.⁸ The best course of action in such situations is to perform device interrogation at the earliest possible time.

Conclusion

It is possible for a physician who is not an electrophysiologist to perform some basic ICD troubleshooting by using a stepwise approach, on the basis of data gathered from the patient's history and physical examination, and from device interrogation. Most of the problems can be resolved by means of a simple ICD programming change or a change in medical therapy.