The implantable loop recorder: A tool that is
"here to stay"
Carel
C. Cock,
MD,
PhD
Department of Cardiology,
VU University Medical Center, Amsterdam, The Neteherlands.
Address
for correspondence: C.C. de Cock, MD, Department of Cardiology, VU
University Medical Center, Boelelaan 1117,
1081 HV, Amsterdam, The Neteherlands. Email: CC.dCock@vumc.nl
Introduction
Syncope
is a common clinical disorder accounting for 3 % for emergency room
presentations and 1 % of hospital admissions 1. Moreover it has not always a
benign course, with mortality rates up to 33 % at 1 year in patients
with a structural cardiac disorders 2, 5. In addition
costs for investigation for syncope is substantial and about 25 % of
all patients remain undiagnosed 6.
Investigation of patients with recurrent unexplained syncope may
include electrocardiography and treadmill exercise testing, neurologic
testing, tilt table testing, ambulatory Holter monitoring and
electrophysiological testing.
This
review will briefly discuss these current available strategies and
focus on the usefulness of a recently introduced tool using an
implantable loop recorder (ILR) in the treatment of patients with a
recurrent unexplained syncope.
Clinical history and physical examination
A
careful history taking and clinical examination in addition to
electrocardiography is essential for patients referred for recurrent
syncope. History should focus on postural symptoms, palpitations,
family history (long QT syndrome, Burgada syndrome, cardiomyopathy) and
should include the use of medication particularly in the elderly
patients 7, 8, 9. Physical examination
should focus on (orthostatic) blood pressure, cardiac murmurs and
specific cardiac disorders e.g. aortic or mitral stenosis.The yield of
these initial evaluation however is low with less than 50 % of patients
having a primary diagnosis made.
Electrocardiography
Electrocardiography is essential in the work-up of patients with
unexplained syncope but may reveal a direct cause in only 5 % of
patients. Pre-excitation patterns, a long QT- interval, the recently
reported Burgada syndrome and characteristic features in patients with
arrhythmogenic right ventricular dysplasia should all be considered 7, 8, 9.
Echocardiography and exercise testing
Echocardiography may reveal structural heart disease e.g.
cardiomyopathy but the diagnostic yield is low in patients without a
cardiovascular history and normal physical examination. Although more
than half of patients with unexplained syncope presented with an
abnormal echocardiogram, no additional value could be demonstrated in a
large series of patients reported by Recchia et al 10. In addition a
low diagnostic yield of exercise testing was found.
Tilt table testing
Recently a
large number of studies reported on the use of tilt table testing for
patients
with recurrent syncope 11
12, 13, 14
resulting in an expert consensus on the use of tilt table testing by
the American College of Cardiology in 1996 15. An international
classification for tilt testing for patients with induced
neurocardiogenic syncope proposed in 1992 indicated that several
subtypes (i.e. type 2a and type 2b) are candidates for pacing therapy
for recurrent syncope. Head-up tilt table testing provides the highest
yield from all diagnostic procedures in patients with recurrent
syncope.
However still the majority of patients cannot be detected using this
test 1.
Connolly et al.16
recently studied the
value of pacing in patients with neurocardiogenic syncope and reported
a
85 % reduction of symptoms in patients randomly assigned to DDD pacing
with
a rate drop response algorithm. However Benditt 17 commented in an editorial
referring
to this study stating that "the relationship between observations
during
tilt table induced faints and the pathofysiology of spontaneous
syncopal
events in the same patients had yet to be studied adequately".
Nonetheless,
tilt testing can provided useful information on these patients and can
be
used to select patients responsive to cardiac pacing.
Electrophysiological studies
The
diagnostic yield of electrophysiological testing is highly dependent on
patient selection. In patients with structural heart disease the yield
is more than 50% whereas in patients without structural heart disease
and a normal EKG diagnostic yield may be around 10% 5. In addition, in
patients with an abnormal EKG this type of investigation can reveal
ventricular arrhythmias or bradycardia in 17% and 19% of patients
respectively 2, 18. However, a negative test can not rule out an
arrhythmic
cause and has low predictive value, particularly in patients with
non-ischemic
cardiomyopathies, the long QT syndrome and the Brugada syndrome.
Implantable loop recorders
Recently, implantable loop recorders have become available to study
patients with unexplained recurrent syncopal events. This device has a
solid state loop memory capable of storing electrocardiographic events
up to 40 minutes before and 2 minutes after the activation. It can be
easily placed subcutaneously under local anaesthesia and has a battery
life of 15 to 18 months.
Krahn
and co-workers 19
were among the first to describe a high diagnostic yield of the
implantable loop recorder (ILR) in 16 patients with recurrent syncope.
Extensive investigations including electrophysiologic studies,
treadmill testing, 48-hours ambulatory monitoring and tilt table
testing failed to obtain a definite diagnosis in these patients. In 94
% of cases recurrent syncope had occurred after implantation of the
device revealing a arrhythmogenic cause in 60 %. Consequently in 40% of
these patients no arrhythmias were detected. In all patients with an
arrhythmogenic cause, successful therapy was implemented.
A final
analysis was recently presented extending the follow-up to 40 ±
10 months 20.
From the 24 patients included, 52 % presented with an arrhythmic cause
with bradycardia in the vast majority of patients. Treatment was
directed at the underlying cause in the 18 patients who received
specific diagnosis. During follow
up, syncope did not occur in 16 of the18 treated. In 3 patients who
underwent explantatition of the device after battery end-of-life
without recurrent syncope, no recurrence was seen . The same group also
presented a large
series of 85 patients with recurrent undiagnosed syncope despite
extensive
evaluation 21.
Patients were eligible if they had at least to syncopal episodes within
the previous 2 months or if they had a single syncope with a history of
presyncope. In all ILR was implanted which resulted in detection of
rhythms
in 21 patients, the vast majority (18 pts) having a bradycardia. Of
interest,
in 29 patients no arrhythmia was detecteted despite symptoms. In
addition,
the inability to freeze the device occurred in 8 patients. Patients
with
syncope were more likely to record a arrhythmia during symptoms
compared
to patients with a history of presyncope (70 % vs 24 % p = 0.005).
A
rather high proportion of patients (32 %) had no symptoms despite
prolonged monitoring. This spontaneous resolution was also reported by
others 20 who
noted a 57 % - 80 % resolution of symptoms. Van Nierop et al 22 has recently
reported on 35 patients who underwent implantation of an ILR because of
recurrent syncope or presyncopal events. In 83 % of patients a
symptom-rhythm correlation could be studied, revealing a clear
arrhythmia in 23 %, half of them having bradycardia. Resolution of
symptoms was presented in 70 % whereas the mean annual syncopal event
rate was significantly decreased after implantation of the loop
recorder (4.7 ± 2.4 vs 1.3 ± 0.7, p < 0.01). These
patients were significantly younger and most of them were women. In 11
% patients were not capable of activating the device to save the event.
This relatively large
proportion of patients not capable of a proper activation of the loop
recorder
has led to the introduction of a second generation loop recorders
capable
of storing events after detection of a (programmable) heart rate.
Although
this may seem advantageous it might also introduce confounding factors
since
rhythms may be detected which are completely without symptoms. The
predicted
value of these rhythms have not been studied and should be interpreted
with
great caution 23.
Interpretation of the stored arrhythmia may also be difficult and is
highly depended of the quality registration of QRS amplitude and P-wave
morphology. Zellerhof and co-workers 24 compared the stored ECG in 4 different
positions in 56 consecutive patients. The best ECG quality (defined as
the highest QRS amplitude and the best visible P wave, QRS duration and
QT interval) was found in the left or right sternal position with a
horizontal position of the ILR.
Limitations of the device
With
increasing healthcare costs, a proper selection of patients for
implantation of an implantable loop recorder is mandatory. Total costs
of the investigation of patients with recurrent syncope is high 6 because of extensive diagnostic
testing. Krahn et al 25
demonstrated that ILR implantation could reduce costs in a pilot study
of 24 patients referred for recurrent syncope.
Although symptom-rhythm correlation can be obtained in the vast
majority of patients, about 25 % of all patients remain without
definite diagnosis despite the use of ILR. In addition a relatively
large proportion of patients failed to freeze the device ranging from
11 % to 36 % of all studies which underscores the need for proper
patient selection 21,
22. Previous studies demonstrated that
patient selection should focus on those subsets with frank syncope
which have a significantly higher diagnostic yield as compared to
patients presenting with pre-syncope.
With
the increasing use of the device other negative aspects have been
demonstrated. De Cock and co-workers 26 investigated the potential interference from
electronic article surveillance devices and found that interference may
occur causing malfunction of this device. Cellular phones did not
produce any interference. Finally the device is not able to monitor
blood pressure, which can play a key role in the assessment of
vasovagal syncope. It has been hypothesized that in the near future
these devices will be capable of recording, apart from the arrhythmia,
other physiological parameters (such as blood gas analysis) which may
further increase the diagnostic yield 27, 28.
In conclusion, the implantable loop recorder has
been demonstrated to be a very useful tool in the work-up of patients
with unexplained recurrent syncope "that is here to stay". Undoubtedly
future development of the device will help us in the treatment of these
patients.
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