Advances in device therapy for cardiac arrhythmias

Cardiology Review® Online, March 2004, Volume 21, Issue 3

From the Cardiology Division, State University of New York, Stony Brook

Advances in technology have greatly enhanced the ability to diagnose and treat cardiac arrhythmias. This review focuses on the major device innovations achieved in the past decade and highlights their importance for the clinician. Implantable loop recorder Without a confirmed diagnosis, physicians cannot prescribe appropriate treatment for cardiac arrhythmias. Dizziness or syncope may be related to another cause. The Holter monitor was a significant diagnostic advance because it documented the cardiac rhythm for a 24-hour period, and its results could then be compared with the diary of symptoms that the patient recorded. In many cases, however, symptoms recurred after the patient had returned the Holter monitor for analysis. With an external loop recorder, the patient has a 30-day period to record symptoms. Although electrophysiology study documents conduction disturbances (atrial and ventricular arrhythmias), it may on occasion be nondiagnostic if the abnormalities are intermittent of if the arrhythmia is noninducible at the time of study. If the cause for a patient’s symptoms is elusive, an implantable loop recorder can be helpful. Inserted under the skin, it can record five auto-

matic events as well as three patient-triggered events. The automatic events are programmed: for example, asystole greater than 3 seconds, and specified rates of tachycardia and of bradycardia.

If symptoms (dizziness, syncope, and palpitations) are reported by the patient, the implantable loop recorder is interrogated. If sinus rhythm is documented, then the cause is not arrhythmia. The implantable loop recorder accurately documents every known rhythm disturbance, including heart block, supraventricular and ventricular tachycardia, asystole, and even myocardial ischemia.1 This device has a battery life of 14 months.

Pacemakers

Pacemaker technology has resulted in dramatic improvements in programmability and diagnostic features. Physicians know that available programmable features include rate, amplitude, time intervals, and rate response (the ability of the pacemaker to increase rate based on skeletal muscle activity). Today, however, pacemakers have many additional features that may be underutilized.

One such important feature is stored telemetry. The ability to record the heart rate from both the

atrial and ventricular electrodes allows documentation of cardiac arrhythmias. The rate histogram in-dicates the time that the patient spends at a particular heart rate, allowing the clinician to determine whether the heart rate is appropriate for the patient. If a high rate of atrial or ventricular activity is observed, the patient has an arrhythmia that requires attention. This is especially true for patients with silent atrial

fibrillation.

It is well known that atrial fibrillation is common after the age of 65 years. If a patient does not have symptoms, the arrhythmia may go unnoticed, and thrombus may occur and result in stroke. Anticoagulation can then be recommended (if otherwise appropriate) for pacemaker patients in whom atrial fibrillation is documented.2

Automatic threshold measurement is available on many contemporary pacemakers, allowing the pacemaker to perform a test during which it measures the minimal amplitude necessary to evoke a potential. If loss of capture is noted, the pacemaker will automatically increase the output, ensuring a safety margin.

In addition to documenting atrial arrhythmias, certain pacemakers can respond by attempting to pace the rhythm to normal.3 Antitachycardia pacing algorithms are used to interrupt a reentrant tachycardia that may lead to patient symptoms. This application may be useful for patients in whom radiofrequency catheter ablation has failed to eliminate the atrial arrhythmia. Antitachycardia pacing may also be used as the first therapy for ventricular tachycardia in patients who have implantable cardioverter defibrillators (ICDs).

ICDs

The superiority of ICD therapy over antiarrhythmic drug therapy is well established. For patients with life-threatening ventricular arrhythmias, the ICD is able to recognize the problem and deliver therapy within seconds. Physicians are able to program either antitachycardia pacing algorithms or shock therapy depending on the arrhythmia and the known response of the patient to pacing. Ventricular tachycardia is recognized in 3 to 4 seconds; the device charges in 3 to 4 seconds; and the arrhythmia is over in 6 to 8 seconds, which is faster than one can telephone for help.

The Multicenter Automatic Defibrillator Implantation Trial II (MADIT II) study showed an improved outcome using ICD therapy for postinfarction patients with an ejection fraction of 0.30 or less and a QRS duration of 130 milliseconds or more.4 Insurance reimbursement for this indication is controversial. Although Medicare has recommended payment, reimbursement is not in effect in all areas of the country.

Innovations in ICD therapy include increased energy delivery and smaller battery and pulse generator size. The pectoral implant is easy to accomplish and is tolerated well by most patients. ICD pulse generators last approximately 5 to 6 years. ICDs also have enhanced telemetry data storage, enabling documentation of the arrhythmia. In a dual-chamber device, the stored electrograms from both the atrial and ventricular electrodes permit distinction between tachycardias from each chamber. Advances in electrode technology are designed to provide better sensing, energy delivery, and longevity.

It is relatively straightforward to replace the pulse generator. The electrode is more problematic because it is an indwelling structure that can adhere to veins and the myocardium and thus may be difficult to extract.

Health care providers should know that all ICDs have the ability to pace. Whether this function is enabled or is used as backup depends on the individual needs of the patient. Some patients require pacing because of conduction disturbances or to suppress arrhythmias. In other patients, constant ventricular pacing from the right ventricular apex may alter the geometry of contraction and thus be detrimental. In these patients, pacing may be programmed only for backup.5 For those who have heart failure related to pacing-induced bundle branch block, an upgrade to cardiac resynchronization therapy may be an option.

Remote monitoring has recently become available with some ICDs. This ability benefits patients who require monitoring of the device but live far away. Presently, remote programming is not possible and is a limitation of this method of follow-up.

Cardiac resynchronization therapy

Biventricular pacing for pacemaker and defibrillator patients has provided a major benefit for patients with congestive heart failure.6-8 Dyssynchrony between the right and left ventricles resulted in a reduction in cardiac output and a change in the geometry of ventricular contraction. As a result, the left ventricle dilates, the left ventricular ejection fraction decreases, in some cases the mitral annulus may enlarge, and mitral regurgitation occurs or increases.

Restoring synchrony between

the left and right ventricle has been demonstrated to improve both morbidity and mortality. Most patients show an improvement in the following: New York Heart Associa-tion (NYHA) functional class, the ability to participate in a 6-minute walk test, myocardial oxygen consumption (decreases), left ventricular ejection fraction, and the amount of mitral regurgitation (decreases).

Current indications for cardiac resynchronization therapy include heart failure patients with bundle branch block, QRS duration greater than 130 milliseconds, NYHA functional class III or IV heart failure on maximal tolerated medical therapy, and a left ventricular ejection fraction of 0.35 or more. Resynchronization is accomplished with an additional electrode to pace the left ventricle simultaneously with the right ventricle. To accomplish this, an electrode is inserted into a coronary sinus and advanced to a position in a branch vein, optimally a posterolateral branch. Alternate approaches are being studied.

Extraction of implanted

electrodes

As more patients receive implanted devices, management of electrodes becomes an issue. Electrodes usually have longevity of more than 10 to 15 years before they deteriorate and cannot be used. Insulation and conduction failures, sometimes from a subclavian crush injury, result in the premature need for a new electrode.

Every intravascular electrode limits the space and alters the anatomy for subsequent leads. In short, it gets “crowded” in the intravascular space and sometimes the vein becomes thrombosed. Removal of indwelling electrodes is difficult, particularly after the first 6 to 12 months, because they adhere to tissue. Pulling can tear the lining of the vein or remove a piece of myocardium, both of which can result in catastrophic bleeding or tamponade. Sometimes it is difficult to completely remove the electrode from the vein.

An extraction system was developed in which a sheath was placed over the electrode and advanced down to the tip, breaking adhesions along the way. When the sheath reached the tip, it was braced against the myocardial wall, providing countertraction as the electrode was pulled back, avoiding eventration. The most recent innovation uses a laser extraction sheath that lyses the adhesions as the sheath is advanced and results in easier removal.9

Some physicians favor the removal of all old hardware. The procedure has risks, however, and is currently reserved for those situations in which removal of the electrode is absolutely necessary, such as in cases of infection. Another situation in which extraction is helpful is when the vein is thrombosed. The old electrode can be removed through the laser-guided sheath and the new electrode introduced through the same sheath.

Automatic external

defibrillators

Sudden cardiac arrest is treated most successfully the faster the arrhythmia is terminated. The recent development and widespread availability of automatic external defibrillators is expected to improve outcome. The devices are small, lightweight, and easy to use. Although they were once limited to paramedics and fire and police departments, they are becoming readily available in public facilities, including schools, offices, shopping malls, and theaters. Health care providers have a responsibility to educate the public about their benefit.

Recommendations

Clinical electrophysiology continues to evolve with better understanding of arrhythmias and novel ways to prevent or eliminate them. Health care providers should be aware of the indications for pacing and ICD therapy. These recommendations are available from the NASPE-Heart Rhythm Society, the American College of Cardiology, and the American Heart Association.10 n