QT dispersion and heart rate variability.

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European Heart Journal (1996) 17, 165-166
Editorials
QT dispersion and heart rate variability
See page 258 for the article to which this Editorial refers
Changes in autonomic nervous system control are
often found in patients with left ventricular dysfunc-
tion and heart failure1'1. Heart rate variability and
ventricular repolarization characteristics are par-
ameters modulated by the autonomic nervous system
and they may provide a non-invasive tool for obtain-
ing reliable and reproducible information on the
autonomic nervous system for the purpose of risk
stratification in patients with congestive heart failure.
There is strong evidence that non-uniform
recovery of excitability may be essential in trigger-
ing malignant ventricular arrhythmias. A surface
ECG is useful in clinical practice for studying the
heterogeneity of ventricular repolarization, and the
QT interval, which encompasses ventricular depolar-
ization and repolarization, is used to measure the
duration of ventricular repolarization. Changes in the
repolarization wave (T wave) are also important.
The heterogeneity of repolarization may be
both spatial and dynamic'21. Spatial heterogeneity is
defined as the dispersion of repolarization durations
in simultaneously recorded leads (QT dispersion).
The importance of QT dispersion as a marker of
arrhythmic risk in different clinical settings, including
chronic heart failure'3', has been confirmed since the
pioneering paper by Day et al.l4]. As QRS duration
may contribute to arrhythmic risk, measurement of
the dispersion of the JT interval, which truly repre-
sents ventricular repolarization, has been considered
a better predictor of cardiac death'51. This has led to
questioning the predictive value of QT dispersion as a
marker of poor outcome.
Dynamic changes in the repolarization pattern
(duration and morphology) have also been con-
sidered predictors of risk. Peaks of QTc >500 ms are
detected on Holter tapes more frequently in post-
myocardial infarction patients with ventricular tachy-
cardia during follow-up than in post-myocardial
infarction patients without arrhythmic events'61. Thus
QTc peaks also appear to be a marker of poor
outcome after myocardial infarction. Using new tech-
niques, mild changes in the morphology of repolar-
ization waves detected on surface ECGs have been
reported to be markers of poor prognosis171. In some
patients, particularly with the long QT syndrome and
in the presence of important acute ischaemia, QT
dispersion and beat-to-beat changes in the morphol-
ogy of repolarization may be observed'21. However,
manual measurement of QT dispersion and particu-
larly of dynamic QT changes on Holter tapes'61 is a
time-consuming process. This drawback may be over-
come by using algorithms to measure automatically
the QT interval and T waves'81. The use of digitized
12-lead ECGs, may, in the near future, allow
computer analysis of QT dispersion, much as QRS
and QT duration are now analysed automatically on
many new ECG recorders'2'71.
Early heart rate variability studies, focussing
on myocardial infarction patients, found that
decreased heart rate variability correlated with poor
prognosis'11. Later, in other studies various authors
demonstrated that patients with congestive heart fail-
ure had decreased heart rate variability that seemed
to be independent of aetiology and the presence of
ventricular arrhythmias. However, there have been
conflicting results using heart rate variability to
stratify risk in patients with congestive heart failure,
with some papers reporting a positive relationship
and others not'1'91.
Thus the role of autonomic nervous system
disturbances in sudden death is currently evolving.
It seems clear that different parameters related to
autonomic modulation of heart rate and ventricular
repolarization (QT dispersion and dynamic changes
in repolarization — QT interval and T wave morph-
ology) may play an important role in risk stratifi-
cation for different disorders (long QT syndrome,
post-infarction patients, etc.) but their value is not
completely clear in patients with congestive heart
failure.
With this in mind, the group from St. George's
Hospital, who have already made pioneering contri-
butions to this field, carried out the study published in
this issue'101. From a methodological point of view, it
is important to emphasize that both QT dispersion
and heart rate variability were evaluated on standard
12-lead ECGs. This has potentially important impli-
cations for clinical practice because it avoids using
0195-668X/96/020I65 + 09 $18.00/0
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166 Editorials
data derived from 24-h Holter recordings, which
requires laborious manual editing or complex and
never perfect, computing software. Given the fact
that correction of the QT interval for heart rate may
be misleading, the QT interval was not corrected for
heart rate in this study. An important limitation of
using heart rate variability and QT parameters to
stratify risk in patients with advanced heart disease
is that patients with arrhythmias (such as atrial
fibrillation) or bundle branch block are not eligible.
In the paper by Fei et al.[i0] 55% of patients were
excluded for these reasons. Obviously this biases the
conclusions and certainly limits the value of the
results.
Fei et al.[]0] showed that reduced RR variation
on standard 12-lead ECGs had important prognostic
implications in patients with congestive heart failure
secondary to idiopathic dilated cardiomyopathy. In
contrast, there were no significant differences in QT
dispersion between survivors and patients who died
or received transplants during follow-up.
Further studies are warranted to validate the
role of heart rate variability detected on surface
ECGs in other populations, such as patients with
post-myocardial infarction or other heart diseases.
Furthermore, heart rate variability measured on stan-
dard 12-lead ECGs should be compared with heart
rate variability results obtained on Holter tapes using
different techniques. The St George group recently
published a paper analysing heart rate variability on
long-term Holter recordings in which they concluded
that the condition did not help to identify patients
with congestive heart failure at risk of sudden
death191.
The other important conclusion of the Fei
et al. paper1'01 is that QT dispersion, at least as
evaluated with this methodology, did not predict
death in the group of patients with congestive heart
failure, considered eligible for the study. This finding
contrasts with the results of the study by Barr et alP\
In the evolving field of the use of heart rate
variability and QT parameters for risk stratification
in patients with congestive heart failure, it is im-
portant to consider the following: (1) an important
limitation is that many patients (more than 50% in
the Fei group) are ineligible, (2) heart rate variability
can be measured on surface ECGs, and (3) heart rate
variability parameters were more predictive of prog-
nosis than QT dispersion, at least as assessed with the
Fei et al. methodology1101 in patients with idiopathic
congestive heart failure, (4) It is necessary to clarify
the different results obtained by Holter and surface
ECG regarding the role of HRV to stratify risk of
sudden death in patients with congestive heart failure.
We consider that further studies comparing
heart rate variability measurements on 12-lead ECGs
and Holter tapes, spatial (QT dispersion) and dy-
namic heterogeneity of repolarization and new tech-
niques that permit the inclusion of more patients in
the studies are needed to establish the real value of
these parameters in stratifying risk in patients with
congestive heart failure and other cardiac diseases.
A. BAYES DE LUNA
X. VINOLAS
Hospital Sant Pau, Barcelona, Spain
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