Hardware for the Heart: The Increasing Impact of Pacemakers, ICDs, and LVADs

Indications

Implantable cardioverter defibrillators are generally indicated for the primary and secondary prevention of sudden cardiac death.⁸ The commonly accepted indications for ICD use are summarized here:

Primary Prevention

• Patients with previous MI and LV ejection fraction (LVEF) < 30%
• Patients with cardiomyopathy, New York Heart Association functional class III or IV and LVEF < 35%.

Secondary Prevention

• I Patients with an episode of sustained or unstable VT/VF with no reversible cause.
• I Patients with nonprovoked VT/VF with concomitant structural heart disease (valvular, ischemic, hypertrophic, infiltrative, dilated, channelopathies).

ICD Design

Current ICDs are third-generation device, only slightly larger than pacemakers.  ICDs are small (25-45 mm), reliable, and contain sophisticated electrophysiologic analysis algorithms. They can store and report a large number of variables, such as ECGs, defibrillation logs, various energies, lead impedance, as well as battery charge.^3,9 Stevenson et al¹ describe four major functions of the ICD: sensing of electrical activity from the heart, detection of appropriate therapy, provision of therapy to terminate VT/VF, and pacing for bradycardia and/or CRT.

Components

The components of an ICD can be organized in the following manner:

I Pacing/sensing electrodes. Contemporary units complete these functions through use of two electrodes; one at the distal tip of the lead and one several millimeters back (bipolar leads).¹

I Defibrillation electrodes/coils. The defibrillation electrode is a small coil of wire that has a relatively large surface area and extends along the distal aspect of the ventricular lead, positioned at the apex. This lead delivers current directly to the myocardium.^11,12 Both the sensing and defibrillation electrodes are often housed in the same, single wire.

I Pulse generator. The pulse generator contains a microprocessor with sensing circuitry as well as high voltage capacitors, a battery, and memory storage component. Modern battery life is typically 5 to 7 years (frequency of shocks will lead to early termination of the battery life).^2,11 Some ICDs have automatic self-checks of battery life and will emit a tone when the battery is low or near failure; these patients should be promptly evaluated and referred to the electrophysiologist as indicated.

Functions 

The original concept of the ICD was to sense a potentially lethal dysrhythmia and to provide an appropriate therapy. As ICD technology has evolved, the number and variety of available programming and therapies have dramatically increased. Detection of the cardiac rhythm was designed initially to only detect ventricular fibrillation. With current generation models, the ventricular sensing lead filters the incoming signal to eliminate unwanted low frequency components (eg, T-waveand baseline drift) and high frequency components (eg, skeletal muscle electrical activity).^3,13 Newer ICDs have the capability for remote monitoring and communication via telephone line or the Internet.

During implantation, the device is programmed with analysis criteria. Criteria for therapy are largely based on the rate, duration, polarity, and waveform of the signal sensed. When the device detects a signal fulfilling the preprogrammed criteria for VT/VF, it selects the appropriate tier of treatment as follows:

I Antitachycardia pacing (ATP). Ventricular tachycardia, particularly reentrant VT associated with scar formation from a prior MI, can sometimes be terminated by pacing the ventricle at a rate slightly faster than the tachycardia. This form of therapy involves the delivery of short bursts (eg, eight beats) of rapid ventricular pacing to terminate VT.^14,15 This therapy is low voltage and usually not felt by patients. Antitachycardia pacing successfully terminates VT in over 80% of those with sustained dysrhythmia.¹⁶ In the Pain-FREE Rx II trial, data indicate ATP could successfully treat not only standard but rapid VT as well; outcomes revealed a 70% reduction in shocks without adverse effects.^5,16

I Synchronized cardioversion. Typically, VT is an organized rhythm. Synchronization of the shock (delivered on R wave peak) and conversion can often be accomplished with low voltage. This helps to minimize discomfort and avoids defibrillation, which potentially could lead to degeneration of VT to VF.

I Defibrillation. This is the delivery of an unsynchronized shock during the cardiac cycle. This can be accomplished through a range of energies. Initial shocks are often programmed for lower energies to reduce capacitor charge time and expedite therapy. Typically, shocks are set to 5 to 10 joules above the defibrillatory threshold (determined at time of implantation).^9,16

I Cardiac pacing. All models now have pacing modes similar to single- or dual-chamber pacers.

Implantation

Original ICDs were placed into the intraabdominal cavity through a large thoracotomy. With current-generation ICDs, leads are typically placed transvenously (subclavian, axillary, or cephalic vein), which has led to fewer perioperative complications, including shorter procedure time, shorter hospital stay, and lower costs as compared to abdominal implantation.^5,17