X. FREQUENCY ANALYSIS OF THE ECG DURING VENTRICULAR FIBRILLATION
WITHOUT ARTIFICIAL CORONARY PERFUSION
1 Introduction
Frequency analysis of cardiac electrograms of dogs with artificially
induced ventricular fibrillation during complete cardiopulmonary bypass
and coronary perfusion indicated a great stability of the statistical properties
of the signal over periods of several hours (chapter IV
).
Ventricular fibrillation is not anymore induced during open heart surgery,
so the only available patient data for this study come from recordings on
magnetic tape of ECG's of patients with "spontaneous" ventricular fibrillation
without coronary perfusion. The increasing hypoxia of the heart of these
patients clearly does not form a sound basis for the assumption of stationarity
of the analyzed signal. To form a link between the analysis of the stationary
ventricular fibrillation in dogs with artificial coronary perfusion and
the analysis of the essentially nonstationary ventricular fibrillation in
patients, ventricular fibrillation was induced in two dogs without coronary
perfusion and their ECG was analyzed.
The data for this part of the investigation were obtained by copying
analog tapes containing recordings of standard ECG leads via conventional
electrodes and ECG amplifiers. Signals of a sufficiently long duration
became available of two dogs from the Laboratory of Experimental Cardiology
of the University of Utrecht, 1 episode of ventricular fibrillation in
a healthy human heart caused by an incident during catheterization in the
University Hospital and of 20 episodes of ventricular fibrillation in 11
patients from the coronary care unit of the St. Antonius Hospital, Nieuwegein.
Fourier transforms were calculated according to the methods described
in chapter III .
2 Ventricular fibrillation in a healthy human heart
The case is ventricular fibrillation in a 60 year old man with aorta
stenosis and an otherwise healthy heart. The fibrillation started during
the introduction of a catheter in the right ventricle. The ECG during ventricular
fibrillation is shown in the next figure.
spectral analysis of VF in a healthy human heart
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| fig. 10.1: fibrillation in a healthy human heart |
fig. 10.2: almost Normal amplitude histogram of ECG |
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| fig. 10.3: auto power spectrum of ECG |
fig. 10.4: autocorrelation function of ECG |
The histogram fig. 10.2 looks like that of Gaussian white noise, although
normality has been rejected by the Kolmogorv-Smirnov test. The auto power
spectrumin fig. 10.3 contains two rather broad peaks with a basic frequency
of 5 Hz . The autocorrelation function in fig. 10.4 damps to zero within
1 second.
3 Dynamics of spectra
In cooperation with Annelien Roenhorst
3.1 Methods
In order to see what changes would occur in course of time, an estimation
of the auto power spectrum was made by calculating a moving average over
10 consecutive blocks (20 seconds) of Fourier transforms. Using the DISSPLA
graphics software package a pseudo three-dimensional impression of the moving
average of the power spectrum was made. Contour lines of these three-dimensional
images were drawn at 5 dB levels.
3.2 Analysis of dog data
The dynamics of the spectra in 2 healthy, intact dogs followed the same
pattern: fast and fairly regular ventricular fibrillation in the first
10 seconds followed by a slowing down and an increasing irregularity, see
the ECG of one of these dogs in the next figures.
fig. 10.5: several instances of ECG of a dog during
unassisted VF
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In this dog the fibrillation starts with a basic frequency
of 11 Hz and a higher harmonic of 22 Hz, then slows down to 4.5 Hz, remains
rather stable and regular between 60 and 140 seconds after the onset of
ventricular fibrillation as indicated by the clear higher harmonic around
9 Hz and becomes slower and more irregular after 150 seconds |
fig. 10.6: three-dimensional representation of
powerspectra of ECG in 10.5
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fig. 10.7: contour lines of figure 10.6
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Especially the three-dimensional representation indicates clearly the
decrease in amplitude of the higher harmonic.
Contrary to the experiments described in chapter IV
, ventricular fibrillation under these circumstances cannot be considered
as a stationary process.
3.3 Analysis of patient data
Up till now 20 occurrences of ventricular fibrillation in 11 coronary
care patients became available for analysis. In general, if there is a sufficiently
long period of ventricular fibrillation (> 20 sec's), the power spectrum
shows a peak at the basic frequency of fibrillation plus at least one higher
harmonic. If the heart is fairly regularly beating, normally or in tachycardia
the largest peak in the power spectrum corresponds to the mean heart rate
(comparable to the mean R-R interval) and higher harmonics are present.
In 10 cases the so-called fibrillation frequency was twice the frequency
of the beating heart just before ventricular fibrillation; sometimes this
frequency was seen as a subharmonic in the spectrum during ventricular fibrillation.
Two examples of the analysis will be shown and commented upon; see table 10.1 for patient codes and summary of results.
- A2: The first example is ventricular flutter and ventricular fibrillation
of 83 seconds duration, which arose after a premonitory tachycardia in
a 50 year old woman.
fig. 10.8: ventricular flutter and fibrillation
after tachycardia
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fig. 10.9: three-dimensional representation of
spectra of figure 10.8
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fig. 10.10: contour lines of figure 10.9
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A part of the ECG after A-D conversion is shown in figure 10.8. The
power spectrum (fig. 10.9 and fig. 10.10) shows how the basic fibrillation
frequency doubles at the onset of the flutter. With increasing anoxia the
fibrillation becomes less regular and the sub-harmonic of the fibrillation
frequency appears in the spectrum.
- J1: The second example consists of tachycardia and ventricular fibrillation
in a 76 year old man with an old anterior-septal infarction.
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| fig. 10.11: ventricular fibrillation after tachycardia |
fig. 10.12: three-dimensional representation of spectra
of figure 10.11 |
fig. 10.13: contour lines of figure 10.12
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The transition from tachycardia to ventricular fibrillation is shown
in figure 10.11. The power spectrum (fig. 10.12) during tachycardia contains
only one clear peak at the heart rate frequency (2 Hz or 120 beats/minute).
At the start of ventricular fibrillation the power spectrum changes abruptly
into the typical ventricular fibrillation spectrum with peaks at 4, 8.5,
13, 17, 21.5, 26 and 30 Hz. The contour chart (fig. 10.13) indicates the
presence of the original tachycardia peak of 2 Hz during ventricular fibrillation.
Taking into account a bandwidth of aproximately 0.5 Hz, this peak can be
considered as half the "basic" frequency of 4 Hz and the other peaks as
harmonics of the basic fibrillation frequency.
3.4 Discussion and conclusion
No conclusions will be drawn with respect to correlation between clinical
data and type or form of the power spectrum or with respect to reliability,
because this is not a clinical investigation.
The frequency content of the ECG of patients during ventricular fibrillation
has been investigated directly Agizim
1976 , Nygårds 1977
and Hulting 1979 and indirectly
(by filtering) Kuo 1978 in order
to design a reliable automated ECG monitoring system, that will detect ventricular
fibrillation quickly without mistakes. The dynamics of the spectral characteristics
of ventricular fibrillation in dogs during dying and resuscitation of the
body have also been studied Tabak 1980
, Martín 1983 and Martín 1986 . The spectrum
published by Agizim et al. (1976) looks like the spectrum of white noise,
but the other authors show spectra, principally looking like the spectra
in this study. Tabak et al. (1980) investigated the dynamics of the auto
power spectrum during induced ventricular fibrillation in 46 dogs. They
classified ventricular fibrillation into three stages:
- the spectrum contains 1 peak shifting from 12 Hz at the start to
9 Hz at the end of this stage;
- the spectrum shows two peaks, the one with the lower amplitude looks
like a continuation of the peak of stage 1 and shifts in this stage from
9 to circa 7.5 Hz and the other shifts from approximately 4.5 to 3.5 Hz;
- just as in stage 1 the spectrum contains only 1 peak, but this peak
can now be considered as a continuation of the low frequency peak of stage
2 and shifts from circa 3.5 Hz to almost zero.
These stages are illustrated in the next figure.
fig. 10.14: stadia of VF without coronary perfusion
according to Tabak (1980)
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The results of Tabak resemble the results of this study in the number
of stages, but not in all the characteristics of the stages. The fact that
Tabak and co-workers did not find higher harmonics in the spectrum of the
ECG at the start of ventricular fibrillation (stage 1) is a direct consequence
of their sampling frequency of 50 Hz. As these higher harmonics disappear
in the beginning of stage 1 fig. 10.7 , they never
enter into their picture. Very interesting is Tabak's suggestion that ventricular
fibrillation in stage 2 is not a slowed down version of ventricular fibrillation
in stage 1, but that the lowest frequency component is new. Expressed in
terms of the model developed in chapter V, par. 4
this would imply that the two synchronized groups of cells are during
stage 1 of equal size, but unequal in stage 2. Their figure 2 as redrawn
in figure 10.14 seems a very idealized picture, based upon a Fourier transform
of 25 Hz in 32 points, so whether really a linear decrease of fibrillation
frequency is present (contrary to fig. 10.7 ) is
doubtful.
Martín and coworkers show the same type of spectra as presented
in this work and they also indicate a non-linear decrease in fibrillation
frequency during anoxia Martín
1986 . Apart from the evidence given in the previous chapters, fig. 10.10 and fig. 10.13
again strongly suggest that ventricular fibrillation can be considered
as a tachycardia with myocardial cells organized in two groups in anti-phase.
The results pictured in fig. 10.7 may also be reinterpreted
as a slow decrease of the fibrillation frequency from 5.5 Hz to 4.5 Hz accompanied
by the fundamental frequency becoming visible due to an increasing unequal
distribution of cells over the anti-phase groups.
The time course of the power spectrum of the ECG seems a sensitive tool
to detect ventricular fibrillation and to investigate different types of
ventricular fibrillation. During ventricular fibrillation the heart of
the patients appears to behave more steady than the heart of dogs.
4 Conclusions
Too few patients have been investigated for firmly based conclusions
and the spectral dynamics of ventricular fibrillation in the two dogs were
only studied to get a link between the results of the analysis of ventricular
fibrillation during coronary perfusion and the results in patients. Nevertheless
ventricular fibrillation in patients seems to fit the same model as ventricular
fibrillation in dogs, which implies that the official WHO definition
("irregular disorganized electrical activity...", WHO/ISFC Task Force
(1978)) does not reflect intrinsic characteristics of ventricular fibrillation.
Also the distinction between flutter and fibrillation seems more a distinction
between stages of fibrillation than between different syndromes.
The difference between the "healthy" heart and the diseased heart will
be discussed in chapter XII .
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