|
Ventricular function
Left ventricular function is considered one of the best predictors of cardiac mortality
and morbidity after MI and many other risk stratifiers, such as the SAECG and
electrophysiologic study, have been found to be al least partially determined by the
extent of left ventricular dysfunction. Bigger et al1 reported that after adjusting for
other variables left ventricular ejection fraction < 30% was an independent predictor
for subsequent cardiac death (3.5 times that of patient with higher left ventricular
ejection fraction). However, left ventricular dysfunction is not a very sensitive marker
of AD2. Results from a large series of high risk patients with acute MI (the TRAndolapril
Cardiac Evaluation study) showed that even in patients with severe depressed left
ventricular systolic function around one-third will die suddenly2. In this study left
ventricular function appears to be the best method of predicting death whereas other
methods (particularly HRV) appear better for detecting AD.
Ventricular arrhythmias
It has been well demonstrated that the presence of frequent ventricular ectopic
beats during Holter monitoring is an independent predictor of cardiac death, AD
and major arrhythmic events after MI1,3,4. An S-shaped curve describes the
relationship between mortality rate and the frequency of ventricular ectopic beats5.
More recently the GISSI-2 results confirmed the prognostic value of ventricular
arrhythmias also in the fibrinolytic era6. In this study frequent or complex ventricular
arrhythmias were associated with a twofold to threefold increase in total and sudden
mortality risk, irrespective of the presence of signs of cardiac damage.
However, the predictive significance of ventricular arrhythmias has been questioned.
Spontaneous variation from day to day in the incidence of ventricular ectopic beats
makes interpretation of the results subject to large errors and the positive predictive
accuracy is low.
Signal averaged electrocardiography
Ventricular late potentials, detected by use of the SAECG, suggest the presence of
slow conduction and have been shown to predict both spontaneous and inducible
ventricular tachycardia and AD7-11. On the other hand the choice of the algorithm
of analysis affects the results of the SAECG and limits the relative interpretation12,13.
El-Sherif et al14 demonstrated that among the time-domain SAECG indices, QRS
duration at 40 Hz is the best prediction criteria for serious arrhythmic events in the first
year after MI. More recently has been suggested that the combined time- and
frequency-domain analysis of the SAECG could improve its overall predictive
accuracy15. In a population of 602 post-MI patients,
Vazquez et al15 showed that
the positive predictive accuracy for ventricular tachycardia of combined time-domain
and spectral turbulence analysis was 35.8% and increased to 51.2% in the subgroup
with a left ventricular ejection fraction < 40%.
Heart rate variability and baroreflex sensitivity
In the last 10 years there has been increased interest in the analysis of the autonomic
control of the cardiovascular system because of the clearly demonstrated association
between autonomic nervous system and AD. Both HRV and BRS are measures of the
sympatho-vagal balance and can be useful in the risk stratification after MI. The
predictive significance of HRV was recognized until 1987, when Kleiger et al16
reported that decreased HRV was associated with increased mortality in 808 survivors
of acute MI. More recently Farrell et al17 showed that impaired HRV was the most
powerful independent predictor of arrhythmic events and moreover the same group
demonstrated that HRV is better than left ventricular ejection fraction to predict AD18.
However, the lack of standardization in the measurement, analysis and interpretation
of HRV indexes limit the clinical use of HRV. A recent effort in this regard is the report
of the Task force of the European Society of Cardiology and the North American
Society of Pacing and Electrophysiology19.
BRS is decreased in patients following MI and depressed BRS identifies a subgroup
at higher risk of arrhythmic events20. On the other hand,
the ATRAMI study21, a
multicentre international prospective study, demonstrated that after MI the analysis
of BRS has significant prognostic value independently of left ventricular function and
of ventricular arrhythmias and that it significantly adds to the prognostic value of HRV.
QT dispersion
The dispersion of ventricular repolarization, considered as the difference in
repolarization duration among several electrocardiographic leads (QT dispersion),
was proposed as a measure of repolarization heterogeneity and an increased QT
dispersion was found to be associated with an increased incidence of malignant
ventricular arrhythmias22-24. However, the role of QT dispersion for risk stratification
of AD remains controversial which, in large measure, may be due to methodological
discrepancies. Recently, Zabel et al25, in a prospective long term follow-up study,
showed that determination of QT dispersion from the surface ECG, even when
performed with the best available methodology, failed to predict subsequent risk
in post-MI patients. In our experience26 an increased dispersion of ventricular
repolarization (JTc dispersion greater than 127), measured on standard ECG in
160 patients during the second week after acute MI, provided an independent
index predicting non-sudden cardiac death during a mean follow-up of 27.7
months (range 18-48).
T wave alternans
In the last years there was a growing interest in TWA, an alternation of the
configuration and/or duration of the repolarization wave of the ECG. TWA
seems reflect underlying dispersion of repolarization in the ventricle27 and
it could represent an important marker of vulnerability to ventricular tachycardia28.
In a recent prospective multicenter study TWA predicted more accurately future
arrhythmic events compared to electrophysiologic study29.
|
