Joydeep Ghosh, MD, FACC*,
Johnson Francis, MBBS, MD, DM† and Barry J. Maron, MD, FACC‡
*Assistant
Professor of Clinical Medicine, Mount Sinai School of Medicine,
Maimonides Medical Center, Brooklyn, New York, USA
†Associate Professor of Cardiology, Medical College
Calicut, Kerala, India
‡The
Hypertrophic Cardiomyopathy Center, Minneapolis Heart Institute
Foundation, Minneapolis, USA
Address for
correspondence: Joydeep
Ghosh, MD, FACC, Assistant Professor of Clinical Medicine, Mount
Sinai School of Medicine, Maimonides Medical Center, Brooklyn, New
York, USA.. E-mail:
troponins@aol.com
Abstract
Hypertrophic
cardiomyopathy (HCM) is an important cardiovascular disease with sudden
cardiac death as
the most
devastating presentation. Implantable cardioverter defibrillators
(ICD) are the
optimal therapy for
prevention of sudden death from ventricular tachycardia or fibrillation
of any cause.
While there is no controversy with implanting
ICDs in patients who have already survived a cardiac arrest,
identifying high-risk patients
for primary prevention in this disease remains a
challenge.
Implanting ICDs in
patients with HCM is an important clinical consideration since many
individuals could achieve normal or near-normal lifespans with this
protection.
Keywords: Hypertrophic cardiomyopathy; implantable
defibrillator; sudden cardiac death
Hypertrophic
cardiomyopathy (HCM) is the most common genetically inherited cardiac
disorder with an estimated prevalence of 1 in 500 in the general
population1. The disease is
characterised by marked
heterogeneity with respect to clinical
manifestations, natural history and prognosis. Sudden cardiac death
(SCD) is the most the devastating
consequence of the disease and HCM is now
recognized as most
frequent cause of sudden cardiac
death in pre-adolescent and adolescent children, as well as young
athletes. This is not to suggest,
however that patients who have reached midlife
or even beyond are immune from SCD; in fact, no age is considered safe
from this point-of-view2,3.
The causes of
sudden death in HCM remain controversial. While earlier theories
outflow-tract
obstruction, addressedthe current
consensus favors primary
ventricular arrhythmias,
while recognising that the pathogenesis of such
arrhythmias is multifactorial and involve complex interactions between
a combination of factors. Given the high liklihood of prevention of
sudden
death from ventricular tachycardia or ventricular fibrillation by
implantable cardioverter-defibrillators (ICD) in multiple primary and
secondary prevention trials of ischemic and non-ischemic dilated
cardiomyopathy, it is a
reasonable expectation that such
results can be replicated in patients with HCM.
The difference between HCM and
coronary artery disease is that risk-stratification in HCM
with such variable natural history is exceedingly difficult and there
exists no definitive
consensus on precisely
which patients should receive an ICD for primary
prevention. On the other hand the ICD is strongly indicated for any
patient, including those with HCM, who has survived a documented
cardiac arrest i.e.,
secondary prevention4,5.
It is
important to remember that unlike other patients with ischemic
or non-ischemic cardiomyopathy and ICD implants who have a limited
life-span owing to advanced
congestive heart failure and age, high risk HCM patients
are usually
younger and asymptomatic. Consequently, the potential of the ICD
to prolong life is much greater in the
HCM population, and therefore
it is vitally
important that patients be identified correctly
for this
therapy.
In order to create a high
risk profile, all HCM
patients should undergo an
initial comprehensive ambulatory risk stratification assessment,
including detailed personal and family history and physical
examination, 12-lead ECG, echocardiogram, ambulatory Holter
monitoring and
exercise testing. The currently recognised major risk factors for SCD
in HCM include: unexplained
syncope, (particularly when exertional or
recurrent), family history of HCM-related sudden death, identification
of high-risk mutant genes, frequent, multiple or prolonged
episodes of non-sustained ventricular tachycardia on Holter monitoring,
abnormal blood pressure response to exercise and extreme degrees of
left ventricular hypertrophy (maximum left ventricular wall thickness
of 30
mm or more), particularly in adolescents and young adults6. Invasive electrophysiology study
as a means of risk stratification has been largely abandoned in HCM
patients, primarily
because of the unknown
clinical significance of
commonly induced ventricular tachyarrhythmias in the laboratory.
Controversy
exists as to how many major risk factors are required in a
particular patient to be considered deserving of an ICD,
and some of this is uncertainty is due to the fact that the magnitude
of risk conferred
by each risk marker may not be
the same. While the
presence of multiple risk factors may convey an increased risk, a
single risk factor may be adequate
in a given patient to justify an ICD implant. ICD trials in HCM
have documented their efficacy for
sensing and terminating arrhythmias. A recent study by Maron, et al.
showed appropriate discharges triggered by VT in 23% of patients when
followed over 3 years, with an average discharge rate of 5% per year
for primary prevention and 11% per year for secondary prevention. About
60% of the patients with appropriate interventions
received multiple discharges7.
There are
certain technical considerations for the electrophysiologist implanting
ICD's in this population that deserve
mention. The first relates to
whether a single- or dual-chamber system should be employed.
Dual
chamber devices have the benefit of AV synchrony with pacing, although
for the most part pacing as a treatment modality in HCM to reduce
symptoms and outflow gradient was
largely abandoned
after the M-PATHY trial8.
More importantly, dual chamber devices provide superior SVT
discrimination. The latter is particularly important in this population
which has a high incidence of atrial fibrillation, though newer
technology (for morphology,
stability and onset discrimination) may
provide
relatively good discrimination even with single lead systems. The
single-chamber "shock box" device is probably a more appropriate option
for young patients who do
not appear susceptible to
atrial fibrillation and in whom
survival depends largely on effective ventricular arrhythmia
termination. A single chamber ICD is also indicated in all patients
with chronic atrial fibrillation, while patients with paroxysmal atrial
fibrillation would probably benefit more from dual chamber devices with
mode-switch capability. It should be emphasized
that atrial
fibrillation may be poorly
tolerated in HCM patients who already have
severe diastolic dysfunction since
LV filling is critically
dependent on effective atrial systole.
A dual chamber device with rate-drop response is
probably a better option for a HCM patient with recurrent episodes of
neurocardiogenic syncope, although
trials of pacing for
neurocardiogenic
syncope have so far provided ambiguous
results. An excellent update on
selection of ICDs for each patient has been published recently9
Lead fractures
and device recalls are particularly relevant events that affect young
HCM patients who will likely spend their whole lives with the ICD
system. Unfortunately,
these are impossible to predict in advance, and
lead removal, in particular for older
leads, is fraught with difficulties10.
Whenever possible, adding a new lead
should be preferred to extracting an old one. Though active
fixation leads that are less than 6-months old are usually removed
without difficulty, passive leads require laser excision and should
only be performed in a center with considerable experience with
such
procedures.
The number of
HCM patients who meet criteria
for an ICD implant is
increasing.
It behoves
the cardiovascular community to offer these patients all major advances
in treatment so that they can continue to lead productive lives, given
their preserved systolic
function and often minimum
symptoms. Many of the
current uncertainties about patient selection will be resolved soon
with further clinical trials, and in the future more HCM patients will
live longer lives with the aid
of more technologically advanced ICD
systems.
References
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