IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS
Overview
The development of an implantable, battery-powered device able
to deliver sufficient energy to terminate ventricular tachycardia (VT) or fibrillation
(VF) has represented a major medical breakthrough for patients with a history of
ventricular tachydysrhythmias. These devices reduce deaths in the setting of malignant
ventricular tachydysrhythmias,[69]
[70]
and they clearly remain superior to antiarrhythmic drug therapy.[71]
Initially approved by the U.S. FDA in 1985, industry sources report that more than
290,000 patients have these devices today. Further, results from the Multicenter
Automatic Defibrillator Implantation Trial-II (MADIT-II)—suggesting that prophylactic
placement of an ICD is warranted in any patient with an ischemic cardiomyopathy and
ejection fraction less than 0.30 without evidence
of arrhythmic inducibility)—have significantly increased the number of patients
for whom ICD therapy is indicated.[72]
Currently,
the Centers for Medicare and Medicaid Services (CMS) recognize some MADIT-II criteria
for indication of an ICD.
A significant number of technologic advances have been applied
since the first ICD was placed, including considerable miniaturization (pectoral
pocket placement with transvenous leads is the norm) as well as battery improvements
that now permit permanent pacing with these devices. An examiner could easily confuse
a pectoral ICD with a pacemaker.
Like pacemakers, ICDs have a four-place generic code (NBD
*
) to indicate lead placement and function, which is shown
in Table 35-7
.[73]
The most robust form of identification, called the "label form," expands the fourth
character into its component generic pacemaker code (NBG).
Newer ICDs (since 1993) have many programmable features, but essentially
they measure each cardiac R-R interval and categorize the rate as normal, too fast
(short R-R interval), or too slow (long R-R interval). When the device detects a
sufficient number of short R-R intervals within a period of time (all programmable),
it will begin an antitachycardia event. The internal computer will
*The
NBD code is a joint project from NASPE (the "N") and BPEG (the "B"). The "D" stands
for defibrillator.
TABLE 35-7 -- NASPE/BPEG generic NBD defibrillator code
*
|
Position I |
Position II |
Position III |
Position IV
†
|
|
(Shock Chamber) |
(Antitachycardia Pacing Chamber) |
(Tachycardia Detection) |
(Antibradycardia Pacing Chamber) |
|
O = none |
O = none |
E = electrogram |
O = none |
|
A = atrium |
A = atrium |
H = hemodynamic |
A = atrium |
|
V = ventricle |
V = ventricle |
|
V = ventricle |
|
D = dual (A + V) |
D = dual (A + V) |
|
D = dual (A + V) |
|
NBD: North American Society of Pacing and Electrophysiology
(N) and British Pacing and Electrophysiology Group (B) generic defibrillator (D)
code. |
*The
generic defibrillator code[73]
is a joint project
of the North American Society of Pacing and Electrophysiology (NASPE) and the British
Pacing and Electrophysiology Group (BPEG).
†For
robust identification, position IV is expanded into its complete NBG code. For example,
a biventricular pacing-defibrillator with ventricular shock and antitachycardia pacing
functionality would be identified as VVE-DDDRV, assuming that the pacing section
was programmed DDDRV. No hemodynamic sensors have been approved for tachycardia
detection (position III).
choose antitachycardia pacing (less energy use, better tolerated by patient) or shock,
depending upon the presentation and device programming. If shock is chosen, an internal
capacitor is charged. Charging time is dependent upon the desired output, and it
can be 6 to 15 seconds for a maximum shock. Charging time is lengthened by lower
battery voltage, time from last charge,
*
and lower temperature.
Most ICDs can be programmed to "reconfirm" VT or VF after charging
in order to prevent inappropriate shock therapy. Typically, ICDs deliver 6–18
shocks per event. Once a shock is delivered, no further antitachycardia pacing can
take place. Despite considerable improvement in detection of ventricular dysrhythmias,
more than 10% of shocks are for rhythm other than VT or VF.[74]
[75]
Supraventricular tachycardia remains the most
common etiology of inappropriate shock therapy,[76]
[77]
and causes of inappropriate shock have been
reviewed elsewhere.[78]
Programmable features in
current ICDs to differentiate VT from a tachycardia of supraventricular origin (SVT)
include the following[79]
:
- Onset criteria—in general, onset of VT is abrupt, whereas onset of
SVT has sequentially shortening R-R intervals;
- Stability criteria—in general, the R-R interval of VT is relatively
constant, whereas the R-R interval of atrial fibrillation with rapid ventricular
response is quite variable;
- QRS width criteria—in general, the QRS width in SVT is narrow (<110
msec), whereas the QRS width in VT is wide (>120 msec);
- "Intelligence" in dual chamber devices attempting to associate atrial activity
to ventricular activity; and
- Morphology waveform analysis with comparison to stored historical templates.
After the R-R interval becomes sufficiently short for VF detection,
the ICD begins a shock sequence. After an ICD delivers any shock therapy, no further
antitachycardia pacing will take place.
An ICD with antibradycardia pacing capability will begin pacing
when the R-R interval is too long. In July, 1997, the U.S. FDA approved devices
with sophisticated dual chamber pacing modes and rate responsive behavior for ICD
patients who need permanent pacing (about 20% of ICD patients). Recently, though,
a report has suggested that placement of a dual chamber device into a patient without
a clear need for dual chamber pacing decreased survival when compared to single chamber
device placement.[80]
As a result of this study,
many electrophysiologists are programming long (>250 msec) AV delays in these
patients to limit ventricular pacing. As noted above, though (see Fig.
35-6A
), long AV delays can lead to R-on-T
pacing.
