(Circulation. 1996;93:753-762.)
© 1996 American Heart Association, Inc.
Articles |
Incidence and Clinical Significance of Multiple Consecutive, Appropriate, High-Energy Discharges in Patients With Implanted Cardioverter-Defibrillators
Presented in part at the Scientific Sessions of the American College of Cardiology, Anaheim, Calif, March 1993.
From the Department of Cardiology, Hospital General Gregorio Marañón, Facultad de Medicina de la Universidad Complutense, Madrid, Spain.
Correspondence to Jesús Almendral, MD, Laboratorio de Electrofisiología, Departamento de Cardiología, Hospital General Gregorio Marañón (Planta 5), Calle Dr Esquerdo 46, 28007 Madrid, Spain.
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Abstract |
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 Top Abstract IntroductionMethods Results Discussion References |
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Background Some patients with an automatic implantable cardioverter-defibrillator (ICD) suffer multiple appropriate, consecutive, high-energy discharges (MCDs) during follow-up. Such events might represent resistant ventricular arrhythmias and might have prognostic significance.
Methods and Results Eighty consecutive patients with an ICD were followed up for up to 82 months (mean, 21±19 months). Thirty-eight patients had survived an out-of-hospital cardiac arrest and 42 had recurrent ventricular tachycardia. During follow-up, 16 patients had MCD (group A), 26 patients had episodes of single appropriate discharges (group B), and 38 patients had no appropriate discharges (group C). Group A patients had worse functional status (P=.001), lower left ventricular ejection fractions (LVEFs) (P=.001), and lower survival rates (log rank, P=.003) than the remaining two groups of patients. Cox analysis showed LVEF (P=.001) to be an independent predictor of MCD. Independent predictors of death or heart transplant were MCD (P=.001), female sex (P=.001), age (P=.001), history of cardiac arrest (P=.003), and functional status (P=.003). The only independent predictor of total mortality was female sex (P=.002). Independent predictors of cardiac death were MCD (P=.007) and female sex (P=.018). Independent predictors of arrhythmic death were age (P=.001), female sex (P=.02), and MCD (P=.023).
Conclusions In patients with an ICD, the development of MCD is an independent predictor of cardiac and arrhythmic mortality. If this finding is confirmed in larger studies, it may help to identify patients in whom other therapeutic alternatives, ie, heart transplantation, should be considered during follow-up after ICD implantation.
Key Words: defibrillation • survival • prognosis • tachyarrhythmias
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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The ability of the ICD to terminate rapid ventricular tachyarrhythmias and to reduce the risk of sudden cardiac death is well established.1 2 3 4 5 6 7 8 9 However, the impact of ICD therapy on overall patient survival remains controversial given the lack of randomized and controlled trials.9 10 11 12 13 14
Several studies have analyzed the prognostic significance of ICD discharges. Some reports10 15 suggest that appropriate shock occurrence in ICD patients, although effective, may be a marker for a worse clinical outcome compared with patients without these events. Other studies16 17 18 19 20 21 failed to confirm this finding. However, to our knowledge no studies have addressed the clinical significance of multiple ICD discharges. If the first discharge fails because of refractory or immediately recurrent arrhythmia, all ICDs are designed to deliver three to six additional shocks (depending on the manufacturer). It could be hypothesized that the resistance to electric termination expressed by the need for multiple ICD discharges may represent a more severe electrical substrate and therefore carry prognostic significance. Alternatively, and given the statistical nature of the defibrillation threshold, such a phenomenon could occur just by chance and have no clinical implications.
The purpose of the present study was to analyze the incidence, causes, and prognostic significance of MCDs in patients with an ICD.
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Methods |
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TopAbstractIntroductionMethods Results Discussion References |
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Eighty consecutive patients who were discharged from the hospital after implantation of an ICD at our hospital between March 1986 and February 1994 were included in this study. During this time period, 3 additional patients in whom an ICD was implanted died in the hospital within 30 days after surgery, giving a surgical mortality of 3.6%. The first 42 patients were analyzed retrospectively, whereas the remaining 38 patients (since May 1992) were prospectively studied. Their clinical characteristics are summarized in Table 1
. All patients had preoperative coronary
angiography. LVEF was determined by the area-length method.
Classifications for congestive heart failure by NYHA criteria were
based on clinical history and physical findings before implantation of
the ICD device and during follow-up. Underlying cardiac disease
included previous myocardial infarction in 47 patients (59%),
nonischemic idiopathic dilated
cardiomyopathy in 10 (12%), valvular heart
disease in 8 (10%), hypertrophic cardiomyopathy in
3 (4%), restrictive cardiomyopathy in 1 (1%), and
arrhythmogenic right ventricular dysplasia in 3 patients
(4%). Eight patients did not show evidence of structural heart disease
(10%). Thirty-eight patients had survived an
out-of-hospital cardiac arrest, and 42 patients had recurrent
sustained VT unrelated to an acute myocardial infarction or a
reversible metabolic condition.
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An ICD was indicated in patients with sustained ventricular tachyarrhythmias that were poorly tolerated or resulted in cardiac arrest if an antiarrhythmic drug regimen that was well tolerated and prevented spontaneous episodes and induction of hemodynamically unstable ventricular tachyarrhythmias could not be identified. In patients with cardiac arrest, an ICD was also indicated if left ventricular function was severely depressed and/or sustained ventricular tachyarrhythmias could not be induced (particularly if revascularization was not feasible).
All but 15 patients had preoperative electrophysiological testing. The laboratory protocol included a standard stimulation protocol22 with up to three extrastimuli at two right ventricular sites. Induced arrhythmias were sustained VT in 49 patients (61%) and VF in 8 patients (10%). In 8 subjects (10%), no arrhythmias were induced during the electrophysiological study.
The devices implanted were Ventak or Ventak II (16 patients); Ventak
PRX (17 patients); Ventak P2 (3 patients) (Cardiac Pacemaker Inc); and
PCD (Medtronic Inc) models 7216A or 7217B in 44 patients. The electrode
lead system used included a patch-patch configuration in 26
patients and a nonthoracotomy implantation in 54 patients. With the
final lead configuration, at least two consecutive and efficacious
defibrillation shocks with an energy 
25 J were achieved in all
patients. In the 8 patients included in the PCD clinical
study,23 three consecutive efficacious shocks with an
energy 18 J were required. Defibrillation thresholds (the lowest
energy of defibrillation; see "Definitions") were tested before
the ICD generator was attached to the electrodes. Successful
recognition and termination of VT or fibrillation was also confirmed
with the implanted ICD generator unit. Patients routinely underwent
predischarge electrophysiological study. No
patient failed to defibrillate successfully with the first discharge
with the maximum output (30 to 34 J). The following guidelines for
programming of VF therapies were used: (1) Set the initial VF therapy
at 34 J; (2) Set VF detection interval 
30 ms shorter than the cycle
length of the fastest VT that should be treated with
antitachycardia pacing or low-energy cardioversion; and
(3) Do not use VF detection intervals shorter than 300 ms. VT therapies
were prescribed on a more individual basis. Preference to initial
cardioversion was given to patients with rapid, compromising VT.
Patients had concomitant surgical procedures if clinically indicated. Seven patients had coronary artery bypass surgery, four had valve replacement, and one had aneurysmectomy and subendocardial resection.
Patients were followed up for up to 82 months (mean, 21±19 months) after implantation at 2- to 3-month intervals. Details of subsequent deterioration, clinical symptoms, death, or cardiac transplantation were recorded. At each follow-up, the device was interrogated to obtain its present programmed status and data on its operation. Cardiac defibrillation thresholds were not systematically evaluated during follow-up unless antiarrhythmic drugs had to be used. After ICD implantation, efforts were made to stop antiarrhythmic therapy. If a patient had frequent and fast nonsustained or sustained VTs, an antiarrhythmic regimen was initiated to prevent excessive triggering of the ICD by such arrhythmias. If additional antiarrhythmic agents, other than AV nodal blocking drugs, were prescribed during follow-up, successful defibrillation with the first discharge at the maximum output (30 to 34 J) was confirmed. The end point of follow-up study was the occurrence of sudden cardiac death, arrhythmia-related non–sudden death, nonarrhythmic cardiac death, noncardiac death, or arrhythmia-free survival as of July 31, 1994.
Definitions
Appropriate ICD discharges: All ICD discharges
accompanied by
hypotensive symptoms (severe presyncope or syncope), recorded
sustained ventricular tachyarrhythmia (via
telemetry when feasible), or witnessed cardiac arrest were considered
appropriate. We also considered appropriate ICD discharges to be those
that occurred immediately before the patient's hospital admission, if
ventricular tachyarrhythmias were
documented in the emergency department.
Arrhythmic death: Sudden death or nonsudden cardiac death resulting from antecedent recurrent arrhythmias.
Inappropriate ICD discharges: Discharges not preceded by hypotensive symptoms or associated with ECG recordings that showed atrial arrhythmia, sinus tachycardia, or sinus rhythm.
Lowest energy of defibrillation: Lowest energy that defibrillated the heart during the testing, regardless of whether attempts were made to demonstrate a lower energy that failed.23
MCDs: Two or more discharges that were considered as part of a single arrhythmic episode by telemetry readings or that occurred only a few seconds apart according to anamnesis. Discharges that were not multiple consecutive were termed "single" discharges.
Sudden death: Instantaneous or unwitnessed death.
Total events: Death or cardiac transplant.
Statistical Analysis
Numerical data are reported as
mean±SD when they fit a normal
distribution and as median and 25% to 75% quantiles when they did
not. Univariate analysis included all the
variables listed in Table 1
. Comparisons between means were
performed by one-way ANOVA (Tukey test for differences among the
means) or nonparametric tests. Proportions were compared
with the Fisher's exact and 
2 tests by use of
Systat 5.2.1 statistical software. Survival curves were calculated
according to the Kaplan-Meier actuarial method and compared by the
Mantel-Haenszel log-rank test with use of a statistical package
(BMDP Statistical Software, Inc). Patients with missing data for any
variable were excluded from analyses of that variable.
Variables were entered into a Cox proportional hazards regression
model in order of univariate significance to identify those
variables that were predictive of outcome. The following events
were analyzed as dependent variables: MCD, total events,
death, cardiac death, arrhythmic death, and sudden death. All
probability values are two-tailed, and a value of P<.05
was considered significant.
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Results |
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Preliminary statistical analysis did not show significant differences between the clinical characteristics of the prospectively and retrospectively included groups of patients (Table 2
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Patients studied retrospectively used epicardial
patches more often than patients studied prospectively (67% versus
5%, P<.001), reflecting the increasing use of
nonthoracotomy lead systems in recent years. However, both the lowest
energy of defibrillation (see "Definitions") and the programmed
therapies were similar in each group (Table 2).
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Incidence of ICD Discharges During Follow-up
Twenty-four of
the 80 patients had MCDs during follow-up,
which were spurious in 8 patients (10%) and appropriate in 16 (20%).
These 16 patients constitute group A. Twenty-six patients (32.5%)
suffered appropriate single discharges (group B). Thirty-eight
patients (47.5%) did not suffer appropriate discharges during
follow-up (group C), although 4 of them had episodes of VT that
were successfully treated with the programmed
antitachycardia pacing therapies. The mean follow-up
was similar in the three groups: 20±5 months in group A patients,
25±4 months in group B, and 19±3 months in group C. The first
episodes of MCD occurred a mean of 10±10 months after implantation and
were presumably a consequence of VT in 10 patients and VF in 6
patients. The decision to consider episodes of MCD appropriate was
reinforced in all patients by additional findings. In 3 patients, a
sustained ventricular tachyarrhythmia (VT
in 2 patients and VF in 1) was documented as the first rhythm when the
patients arrived at the hospital after experiencing MCD. Three other
patients were admitted because of MCD, and during hospitalization, we
documented episodes of ventricular
tachyarrhythmias (VT in 2 patients and VF in 1) that
required more than one high-energy discharge for termination. One
patient suffered MCDs accompanied by syncope and had VT that caused new
ICD firing immediately after the patient's admission. One patient had
MCDs during mild exercise, preceded by symptoms that were reproduced
during an exercise test coincident with the development of VT. In 7
patients with symptoms that preceded ICD shocks, the telemetry data
were highly suggestive of ventricular arrhythmias:
VT in 4 patients (RR intervals were constant and similar to that of
previously documented sustained VTs) and VF in the remaining 3
patients. One patient had symptomatic MCDs coincident with
the subacute phase of a new myocardial infarction.
In the 8 patients who were considered to have multiple consecutive, inappropriate ICD discharges, probable causes for such inappropriate discharges were identified as follows: atrial fibrillation with rapid ventricular response in four cases, sinus tachycardia during exercise in one patient, and lead insulation break in the remaining three patients. Three of four patients with documented atrial fibrillation had epicardial patches. These patients developed chronic atrial fibrillation during follow-up, and the ventricular rate had to be controlled with digoxin and ß-blockers. One patient with nonthoracotomy leads suffered an episode of paroxysmal atrial fibrillation. The ICD rate cutoff was increased, and the patient had no new ICD discharges. The patient who had inappropriate discharges due to sinus tachycardia during exercise was treated with ß-blockers (exercise-test adjusted), and the three patients with lead insulation break were treated surgically. None of these patients had a recurrence of multiple ICD discharges.
Ten group A patients had ischemic heart disease. Six group A patients were taking antiarrhythmic drugs when the MCDs occurred. Three patients were taking digoxin because of previous episodes of heart failure. Two patients were taking amiodarone (one patient due to very frequent episodes of VT and the other to prevent episodes of paroxysmal atrial fibrillation). The remaining patient was taking procainamide because of frequent episodes of VT. There were no changes in antiarrhythmic regimens after ICD discharges in these patients. After admission, several factors were identified that could have facilitated the arrhythmia development in four patients with MCDs due to VT: hypokalemia, acute myocardial infarction, atrial tachycardia, and exercise-induced VT. In one patient, atrial tachycardia was the consequence of low-energy discharges (2 J) programmed to treat VT. This atrial tachycardia degenerated spontaneously into VT that was responsible for MCDs. In the remaining patients, no evidence of clinical, analytic, or electrocardiographic precipitating factors could be observed after their admission. After discharge, espironolactone was recommended to prevent hypokalemia in one patient, and the dose of digoxin was increased in the patient who had atrial tachycardia preceding low-energy ICD discharges (this patient did not tolerate minimal doses of ß-blockers). No other significant changes in medical treatment were made in these patients after MCD. Specifically, no other changes in their pharmacological antiarrhythmic regimen were performed.
All but 1 patient with MCD had previously suffered single appropriate ICD firing. The first episode of appropriate but single discharges occurred 11.9±10.6 months after the implantation in group B patients and 3.8±3.4 months in group A patients (P<.05). Interestingly, after their first episode of MCD, 14 group A patients had single appropriate and efficacious high-energy discharges.
Predictors of ICD Discharges
When the three groups of
patients were compared (Table 3), no differences were found in
age, sex, clinical or
induced arrhythmias, lowest energy of defibrillation, use of
probability density function, or programmed cutoff rate. The 8 patients
without apparent heart disease belonged to group C. No differences were
found among the three groups of patients either in the number of
subjects receiving antiarrhythmic drugs or in the class of such drugs.
No group C patients and only 2 group B patients were taking
amiodarone during follow-up. NYHA functional status was
worse (P<.001) and LVEF was significantly lower
(P<.001) in group A. In this group, defibrillators more
frequently used epicardial patches (P<.03) and less often
had programmed antitachycardia therapies
(P<.04). All other characteristics were not significantly
different among the three groups. The functional class worsened during
follow-up in 9 of 16 patients with MCD (56%), 4 of 26 patients
with single discharges (15%), and 3 of 38 patients without discharges
(8%) (P<.001).
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No statistically significant differences were found when clinical characteristics and survival curves of patients with epicardial and endocardial lead systems were compared. Both the lowest energy of defibrillation and the programmed therapies were similar in each group.
Multivariate stepwise regression analysis with
the Cox proportional hazards model (Table 4) showed left
ventricular function as the only independent factor that
predicted MCD (odds ratio 0.84; 95% CI, 0.77 to 0.92;
P=.0001), since the presence of programmed
antitachycardia therapy did not reach statistical
significance (odds ratio 0.35; 95% CI, 0.09 to 1.26;
P=.08).
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Predictors of Death
During follow-up, 7 group A patients
(44%), 2 group B
patients (8%), and 3 group C patients (8%) died (P=.003).
Table 5 includes detailed information about the 12
patients who died. Death occurred in 4 of the 47 patients with a
previous myocardial infarction and in 7 of the 25 patients with any
form of nonischemic cardiomyopathy
(P=.03). Death was sudden in 4 patients. One patient arrived
unconscious at the hospital, and we documented multiple episodes of VF
that finally could be cardioverted. This patient remained in a coma
until death 2 weeks later. Death was secondary to heart failure
in 4 patients. Noncardiac deaths were due to pulmonary
amiodarone toxicity in 1 patient, pulmonary embolism in
another, and a tumor of the brain in a third patient. Unfortunately, in
only one patient could the defibrillator be explanted and
analyzed after the patient died.
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Four patients underwent cardiac
transplantation because of severe
deterioration of their functional capacity to NYHA class IV (three
patients) or because MCD was secondary to VF (one patient) (Table
5).
This last patient had suffered two episodes of MCD. Conversion of VF
was documented during one of these episodes with the second internal
shock of 34 J.
Total cumulative survival free of death or heart transplantation in our series at 1, 3, and 5 years, respectively, was 98%, 76%, and 67%; survival free of death was 94%, 80%, and 75%; survival free of cardiac death was 96%, 86%; and 78%; and survival free of sudden cardiac death was 97%, 94%, and 91%.
There were statistically
significant differences among the actuarial
curves of the three groups of patients from the perspective of total
events (P=.0001) (Fig 1), death
(P=.0029), and cardiac death (P=.0031) (Fig
2). However, differences among the actuarial curves of
the three groups did not reach statistical significance considering
arrhythmic death (P=.09) (Fig 3) and sudden
death (P=.34).
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Comparing the characteristics of
survivors with the characteristics of
the patients who died during follow-up (Table 6),
the univariate analysis showed that the latter
group had a greater proportion of women (P<.001), a worse
NYHA functional status at the time of implant (P=.02), and a
more frequent use of epicardial patches (P=.001). Female
patients were similar to male patients in terms of age (59±10 versus
59±11 years), LVEF (37±13% versus 38±16%), and clinical
or induced
arrhythmias. Female patients in our series tended to be less
likely to have ischemic heart disease (33% versus 62%,
P=.15) and more likely to have noncoronary
cardiomyopathy (56% versus 28%,
P=.13). Interestingly, 7 (58%) of the 12 patients who died
suffered previous MCD versus 9 (13%) of 68 patients who survived. No
patient in this group had programmed antitachycardia
pacing. Multivariate stepwise regression
analysis with the Cox proportional hazards model showed the
following to be independent predictors of death or heart transplant
(Table 5): MCD (P=.0001), female sex
(P=.0007),
age (P=.0015), history of cardiac arrest
(P=.0026), and functional status (P=.003). The
only independent predictor of total mortality was female sex
(P=.002), since the functional status (P=.06)
and
the antecedent of MCD (P=.06) did not reach statistical
significance. Independent predictors of cardiac death were MCD
(P=.0071) and female sex (P=.018). Independent
predictors of arrhythmic death were age (P=.0013), female
sex (P=.02), and MCD (P=.023). The antecedent
of
concomitant surgery (P=.051) and a history of cardiac arrest
(P=.081) did not reach statistical significance. Finally, we
could not identify independent predictors of sudden death.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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The main finding of the present study is that the occurrence of MCD was a common clinical problem, present in 20% of our patients, and it was an independent predictor of cardiac and arrhythmic mortality in patients with an ICD.
Causes of MCDs
Recent studies24 25
showed that a single shock may
fail to convert 7% of episodes of VF and between 3% and 11% of
episodes of VT, depending on left ventricular function. At
first glance, and given the statistical nature of the defibrillation
threshold,26 the failure of the first discharge could be
expected occasionally in any given patient without having clinical
significance. A more likely explanation, given the common clinical
routine of accepting only defibrillation thresholds well below the
maximal output of the generator ("safety margin"), would be a
marginally high defibrillation threshold. In patients in whom the
defibrillation threshold could be considered marginally high, the need
for multiple discharges could be expected and might have clinical
significance. However, several observations make this explanation
unlikely in our patients. First, in >60% of the cases, the apparent
cause of the repetitive ICD firing was VT. Because the cardioversion
threshold for VT is usually much lower than that for VF,27
it seems unlikely that these patients had refractory VT. However, we
cannot exclude the possibility that some of these patients might have
had VF converted into VT or vice versa after the first ICD discharge.
Second, the defibrillation thresholds at implantation in these patients
were not significantly higher than in the remaining patients and were
in the range usually considered adequate. Third, previous
reports28 suggest long-term internal cardiac
defibrillation threshold stability. Last, 15 of 16 group A patients had
single discharges that preceded the first episode of MCD, and 14 of
them had successful single ICD discharges after the first episode of
MCD. An alternative and perhaps more likely explanation for the
development of MCD is the presence of immediately recurrent VTs. In
fact, such a phenomenon was observed in 4 of our patients after
admission. In 3 of our patients, it is likely that a full sequence of
high-energy therapy failed, since the first documented rhythm on
hospital admission was a sustained ventricular
tachyarrhythmia after these patients had experienced
multiple discharges. In a recent report,29 4% of episodes
needed more than three shocks for conversion. Interestingly enough, as
in the majority of our cases, these resistant
tachyarrhythmias were VTs and not VFs, again favoring an
immediate recurrence rather than a true high
tachycardia conversion threshold for this resistance.
The role played by possible precipitating factors in the development of MCD is unclear, since no evidence of clinical, analytic, or electrocardiographic factors was documented in the majority of patients and few of them were taking antiarrhythmic drugs. We did not routinely perform angiography to check for a progression of the coronary lesions during follow-up, so we cannot exclude its influence in the development of MCD.
ICD Discharges and Left Ventricular
Function
Recently, Levine et al18 analyzed the
predictors of first discharge in patients with automatic ICDs. Of 197
patients, 105 (53.3%) had an appropriate ICD discharge at 9.1±11.1
months of follow-up. The simultaneous presence of low
LVEF (<25%) and advanced congestive heart failure (NYHA class III or
IV) was the best-identified predictor of early ICD discharges.
Other authors15 17 also demonstrated the direct
influence
of depressed LVEF in the occurrence of appropriate ICD discharges.
However, to the best of our knowledge, there are no studies that
distinguish between single and multiple consecutive, high-energy
ICD discharges. In our series, we observed that patients with MCD had a
significantly lower LVEF than patients with single ICD discharges or
patients without ICD firing during the follow-up period. Moreover,
although the functional status was worse in patients with multiple ICD
discharges, LVEF was the only independent predictor of multiple ICD
firing in our series of patients. These data are consistent
with the findings of Swerdlow and coworkers,30 who
documented a similar relation between left ventricular
function and subsequent arrhythmic events in patients who survived an
episode of sustained ventricular
tachyarrhythmia.
Another interesting finding of our study was that in patients with MCDs, epicardial patches were more frequently used with the defibrillators. However, it was not an independent predictor in the Cox analysis, suggesting, as has been previously reported,14 20 that the longer the follow-up (the use of epicardial patches was the rule in the late 1980s but has been extremely rare since that time), the higher the incidence of MCDs. This is reinforced by the fact that both populations (with and without epicardial patches) had comparable clinical characteristics and survival curves, excluding a bias in the patient selection. Whether the cause of this finding was a deterioration of left ventricular function or simply the result of a longer time at risk cannot be answered by our data.
ICD Discharges and Mortality
Several
groups1 2 3 4 5 6 7 8 9 10 31
have reported on the low
sudden-death mortality and the relatively low overall mortality in
patients who received an ICD. Such reports have been criticized on the
grounds that survival analyses considered all ICD recipients as
a single group10 and that patients with discharges might
have to be considered separately. Because left ventricular
function is an important determinant of outcome in patients with VF or
VT32 33 34 35 36
and impaired left ventricular function
is associated with the development of ICD discharges,18 it
is not unexpected that patients with ICD discharges have worse
prognoses than patients free of them during their follow-up. Zilo
et al15 analyzed data from 56 patients who
underwent initial implantation of an ICD and compared the 32 patients
who experienced spontaneous shocks with the 21 patients without shocks;
the former group of patients had a lower mean LVEF (27±14% versus
36±15%, respectively) and a lower 3-year cumulative survival rate
(69% versus 93%). In data reported by the Bilitch Registry
Group,17 patients who had received a shock had a trend
toward a worse long-term prognosis, and in the study by Tchou et
al,16 sudden-death mortality was significantly higher
in patients who had received appropriate ICD shocks. However, other
series failed to find prognostic implications for ICD
discharges.16 17 18 19 20 21
In the present report, we observed that the prognosis of patients with MCD was worse than the prognosis of patients with single or no ICD discharges. Moreover, patients with single but not multiple ICD discharges did not seem to respond differently than patients without discharges. These findings seem to favor the hypothesis that the electrical instability represented by the need for several consecutive ICD discharges for termination of ventricular tachyarrhythmias (because of the existence of a refractory or immediately recurrent arrhythmia) has prognostic significance. More of our patients who received multiple appropriate shocks died than did patients who received single discharges or no shocks. Furthermore, patients with MCD suffered earlier appropriate ICD discharges than the group of patients with single ICD discharges. This short period of time between the implant and the first valid shock is similar to that described by Myerburg et al37 in patients with ICDs who subsequently died during follow-up. In fact, despite a worse functional status and a lower LVEF in the group of patients who died during follow-up, the occurrence of MCD was, in our study, an independent predictor of cardiac and arrhythmic death.
Recently, two
different series seem to confirm our
results.38 39 Kluger et al38 studied the
incidence and prognosis of cluster shocks (defined as 
3 shocks within
15 minutes >72 hours after implantation) in 131 consecutive patients
with an ICD. After a mean follow-up of 24±22 months, 23% of the
patients had experienced cluster shocks due to ventricular
arrhythmias, and an additional 8% of patients had experienced
cluster shocks due to undetermined causes. Time from ICD implant to
occurrence of cluster shocks averaged 9±10 months. Patients with
cluster shocks tended to have a lower LVEF and showed a greater
short-term mortality (relative risk of 4.4) compared with isolated
shocks. Kohno et al39 followed up 133 patients who
underwent implantation of a third-generation ICD during 24±17
months. Cluster shocks, defined as 5 ICD shocks per hour or 8 per
24 hours, occurred in 42 patients. Compared with the remaining
patients, those with cluster shocks had lower LVEF and higher total
cardiac mortality.
It can be suggested that patients with ICD discharges are in fact a heterogeneous group. Among these patients, those whose ventricular arrhythmias need multiple consecutive discharges may respond worse than those in whom the first discharge terminates all arrhythmic episodes. This may help explain the apparent discrepancies in prognostic significance of appropriate ICD discharges found in different studies. Series in which ICD discharges were found to have prognostic significance might have had more patients with MCD than those that did not find such significance.
Other Predictors of Death
The finding that female sex is an
independent predictor of death
was unexpected. Five of the 12 patients who died during the
follow-up were women. The clinical profile of patients included in
the present report and the long-term mortality observed in our
study are similar to other studies previously
published.6 8 36 37 38 39 40
Women represent a minority
among ICD implantees, and the proportion of women in our series (11%)
was even lower than in other published studies (19% to
27%).5 8 19 In two studies, female sex
has been
demonstrated to be an independent clinical predictor of ICD shocks: the
CASCADE trial,41 which studied survivors of
out-of-hospital VF, and a multicenter approach,42
which studied survivors of an episode of VF who had coronary
artery disease without acute myocardial infarction. Only one
series,43 to our knowledge, studied specifically the
outcome of women treated with ICDs. During a follow-up of 17±12
months, women compared with men tended to have fewer episodes of VT or
VF that required ICD interventions and had lower mortality. However,
compared with the women in our study, women presented in the
study of Kudenchuk et al43 were younger (50±19 versus
59±10 years old) and had a better LVEF (44±17% versus
37±13%).
These differences could justify the greater mortality of women in our
series. In other cardiovascular disorders, ie,
myocardial infarction, women have higher unadjusted mortality and
morbidity rates compared with men in both short-term and
long-term
follow-up.44 45 46 47 48 49 50 51 52
However, whereas some
studies found that female sex is an independent predictor of
mortality,44 others indicated that the higher mortality
observed in women is due to differences in baseline
characteristics.45 46 47 In addition, the
sex of the patient
may influence physicians' decisions about the use of
diagnostic and therapeutic procedures. Several studies
suggested that physicians undertake a less aggressive approach to the
diagnosis and treatment of coronary disease in women, with less
frequent use of invasive cardiac
procedures49 50 51 and later
referral for bypass surgery.52 In fact, in the present
study, we cannot exclude a selection bias that favored referral for
evaluation and/or ICD implantation in female patients with a poorer
prognosis.53
An inverse correlation between age and the probability of death or cardiac transplantation could be due to the age limitation for candidates for heart transplantation (65 years at our center). More difficult to explain is that the same correlation has been observed in relation to arrhythmic death, although some studies suggested that the proportion of sudden death is highest in the younger age groups.54
Other findings in the present study could suggest the existence of more refractory ventricular arrhythmias in patients with nonischemic cardiomyopathies. Death occurred in 4 of the 47 patients with a previous myocardial infarction and in 7 of the 25 patients with some other form of cardiomyopathy (P=.03). Four of the 6 patients with MCD and nonischemic cardiomyopathy died during follow-up in contrast to 3 of 10 patients with ischemic heart disease.
The use of the two major ICD technologies (patch system versus endocardial system) did not appear to influence outcome in our series. Although the ICDs of patients who died more often used epicardial patches, Cox analysis did not identify the ICD lead system as an independent predictor of survival. This finding, in addition to the fact that no statistically significant differences were found when the clinical characteristics and the survival curves of patients with epicardial and endocardial lead systems were compared, suggests that a longer follow-up period in patients with epicardial patches may be responsible for the higher mortality in these patients. This is in accordance with findings in other reported series in which, excluding perioperative mortality, there appear to be no clinically meaningful differences between patients who received endocardial leads and those who received epicardial patches.55 56
Study Limitations
The electrocardiographic documentation of
cardiac rhythm at the
time of ICD discharge was not generally available. Our definition of
"appropriate" is, therefore, of limited validity. We know that
the presence of syncope is not synonymous with VT and that episodes of
VT that precede ICD firing can be associated with no
symptoms.57 58 59 However, we chose a
definition that at
least did not overestimate the frequency of true appropriate ICD
discharges. The expansion in memory capabilities and the addition of
more parameters to be stored by ICD generators that have
recently been implemented (ie, intracardiac electrograms) will help to
interpret the cause of multiple consecutive discharges.
We did not measure defibrillation thresholds after episodes of MCD. Although for reasons previously mentioned, we feel it is unlikely that a pure rise in defibrillation threshold was the cause of these episodes, we cannot exclude the possibility that in some of the patients, the defibrillation threshold had increased.
Clinical Implications
It is well recognized that despite an
important reduction in
sudden-death rate by implantable defibrillators, there is still a
significant cardiac mortality in this patient population. The findings
of the present study suggest that the appearance of MCDs is a
marker for a bad prognosis in patients with an ICD. In fact, according
to our data, it could predict death on a relatively short-term
basis. In the majority of patients, the time elapsed between the
development of MCDs and death was, however, long enough to allow for
additional interventions. In our patients, no important changes in
therapeutic regimen were performed when MCDs were noted, because the
prognostic significance of such a phenomenon was unclear to us.
However, in view of our findings, and if our findings are confirmed in
larger series of patients, the presence of MCDs could be considered an
indicator of the need for alternative therapies in an attempt to
improve such a bad prognosis. Those therapies would include cardiac
transplantation in those patients who are potential candidates. In the
remaining patients, revascularization, more intense
therapy of congestive heart failure, or even more aggressive
antiarrhythmic therapy could be considered.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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The authors are indebted to Fe De Miguel and Estrella Munilla for excellent patient care.
Received July 17, 1995; revision received September 28, 1995; accepted October 4, 1995.
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References |
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