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The term “high resolution electrocardiogram”
(Hi-Res ECG) encompasses any technique that results in improvement of the
signal-to-noise ratio, and thus allows analysis of signals that are too small to be
detected by routine measurement techniques. Among such signals are those which
arise from areas of slow and inhomogeneous conduction in diseased ventricular
myocardium [usually referred to as late potentials (LPs)]. These potentials are small
because the activation front is slow and fractionated, or the mass of tissue undergoing
depolarization is small, or both. LPs are of clinical relevance because they may identify
a substrate for reentrant ventricular excitation1.
The Hi-Res ECG can be analyzed in
time-domain, frequency-domain, or a combination of both time- and frequency-display
in the form of spectro-temporal maps.
Time-domain analysis of the Hi-Res ECG mainly consists of the determination of 3
parameters: the duration of the filtered QRS complex (QRSD), the duration of
low-amplitude signals of <40 mV, i.e. the time that the filtered QRS voltage
remains below 40 mV (LAS40), and the root mean square voltage of the
terminal 40 msec of the QRS (RMS40). In 1991, a Task Force of the American College of Cardiology, the
American Heart Association, and the European Society of Cardiology recognized that
the definition of a LP and the scoring of a recording as normal or abnormal have not
been standardized2. Representative criteria include
that a LP exists (using a 40-Hz
high-pass filter) when: 1) QRSD is>114 msec; 2) LAS40 is>38 msec; and 3) RMS40 is<20
mV2. Time-domain analysis remains the mainstay
for analysis of the Hi-Res ECG, due
to its proven diagnostic accuracy and documented
reproducibility3. The Task Force
also attempted to define clinical indications for the Hi-Res
ECG2. These included risk
stratification for future arrhythmic events in survivors of myocardial infarction (MI), and
prediction of malignant ventricular tachyarrhythmias in patients with coronary artery
disease and syncope, or asymptomatic non-sustained ventricular tachycardia (VT). The
Hi-Res ECG could also be utilized for the evaluation of the results of thrombolytic
therapy and antiarrhythmic surgery, and for the recognition of acute rejection in
patients with cardiac transplant. Other applications were risk stratification for serious
arrhythmic events in patients with organic heart disease other than coronary artery
disease, such as idiopathic dilated cardiomyopathy, hypertrophic cardiomyopathy, right
ventricular cardiomyopathy, etc. The Task Force recommendations were recently
updated by an American College of Cardiology Expert Consensus
Document4.
It is recognized that current techniques for time-domain analysis have several
limitations. As already mentioned, there is lack of agreement on recording
techniques, such as the optimal filter characteristics and algorithms to identify
QRS onset and offset, as well as on the best numerical criteria of abnormality.
Furthermore, the ability to detect LPs in patients with intraventricular conduction
defect is reduced. The rationale for frequency-domain analysis is the observation
that the QRS, LPs, and ST segment waveforms have different spectral characteristics.
A novel approach to frequency-domain analysis, described as spectral turbulence
analysis, was published by Kelen et al5.
The hallmark of arrhythmogenic abnormality
was postulated to be frequent and abrupt changes in the frequency signature of QRS
wavefront velocity as it propagates throughout the ventricle around and across areas
of abnormal conduction, resulting in a high degree of spectral turbulence. Spectral
turbulence analysis was shown to provide an accurate marker for the
anatomic-electrophysiologic substrate of reentrant ventricular
tachyarrhythmias5.
Its reproducibility is high, and similar to that of time-domain Hi-Res ECG3.
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