Correlation of the vector magnetocardiogram with the vector ...

Correlation of the Vector Magnetocardiogram with the Vector Electrocardiogram in Normal Subjects and in Anterior and Inferior Myocardial Infarction

Juha Nousiainen, Sakari Oja and Jaakko Malmivuo Ragnar Granit Institute of Biomedical Engineering, Tampere University of Technology, P.O. Box 692, SF-33101 Tampere, Finland

Abstract

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The relationship between the unipositional vector

III. METHODS

magnetocardiogram (VMCG) and the Frank vector electrocar­ diogram (VECG) was studied with a stepwise multiple regression

The VMCGs were recorded using the unipositional lead

analysis in 200 normal subjects and in 144 patients with myocar­

system [2,3,4]. Shortly, the three orthogonal components of

dial infarction (MI). The calculated squared multiple correlation

the magnetic field were recorded inside a magnetically shield­

coefficient showed that at most 45% of the variation in the normal VMCG could be explained by the VECG. In MI group

ed room with a frrst-order vector gradiometer at a single point above the heart, corresponding the electrode location V of

2

the predicting value of the VECG was smaller.

the standard 12-lead ECG lead system. The VECGs were recorded using the Frank lead system. The lead II of the

1. IN1RODUCTION

standard 12-lead ECG system was recorded as a common

The relationship of the magnetocardiogram (MCG) and the electrocardiogram (ECG) has been an object of many theoret­ ical and experimental studies. Our clinical studies have dem­ onstrated that vector MCG and vector ECG contain com­ plementary information about the pathological condition of the heart [1]. In contrast, some similarities in the lead fields of the unipositional VMCG leads with the VECG leads ob­ served in model studies [2] give rise to expect some redun­ dance in the VMCG and the VECG which was actually observed in the morphological study of the normal and abnor­ mal material [3]. In this paper, the relationship between the VMCG and VECG is approached by means of a multiple regression analysis, applied for the normal subjects and patient with myocardial infarction (MI). It gives quantitative information

time reference from each subject simultaneously with the VMCG and VECG recordings. The amplified and bandpass filtered (0.01-100

representative QRS-complex. The instantaneous QRS-ampli­ tudes were determined in the x- (postero-anterior), y- (right­ to-left), and z- (infero-superior) components of the cardio­ grams using a technique of time-normalizing the QRS-wave. The influence of the VECG predictor variables to the dependent VMCG variables was studied during the QRS­ complex with a stepwise multiple regression analysis of the BMDP software. The criterion was to yield a maximum squared multiple correlation coefficient

ent spatial sensitivities.

IV. RESULTS

A. Normal subjects The R2 between the VMCG

X-,

pendent variables and the VECG

II. MATERIAL

(R2) by selecting the

best subset of predictive variables.

variability in the dependent (MCG) variables. The physiologi­ common electrical cardiac source that they record with differ­

signals

converter [3]. The digitized signals were averaged to find the

of how much the independent (BCG) variables can explain the cal basis of the regression analysis of these methods is their

Hz)

were acquired at a sampling rate of 500 Hz with a 12 bit NO

y-, and z-amplitudes as de­ X-,

y-, and z-amplitudes as

independent variables is shown in Fig. 1. In the whole normal

The normal material of this study consisted of 200 healthy subjects (116 male and 84 female subjects), aged from 16 to 78 years (mean, 46 ± 15 years). The MI material comprised of 92 patients with old inferior MI aged from 39 to 74 years (mean, 58 ± 9 years) and 72 patients with old anterior MI aged from 33 to 84 years (mean, 59 ± 10 years). The study material has been described in detail in our previous works [1,3].

group maximally about 41%, 27%, and 45% (in the male group 48%, 42%, and 54%, respectively) of the variation in the VMCG

X-,

y-, and z-amplitudes, respectively, could be

explained by the common effect of the VECG

X-,

y-and z­

amplitudes. In the female group the corresponding values were smaller. The relative importance of the

X-,

y-, and z-components of

the VECG to predict the instantaneous VMCG amplitudes is also presented in Fig. 1. In all the three cases there was only one characteristic counterpart for the VMCG leads among the VECG leads. The instantaneous amplitudes of the VMCG

This work was supported in part by the Academy of Finland.

0-7803-0785-2/92$03.00 ©IEEE

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X-

lead correlated most with the VECG z-amplitudes. At most 39% (in the male group, 45%) of the variation in the VMCG x-amplitude was due to the linear effect of the VECG z­ amplitude. In the VMCG Y-Iead at most 27% ( in the male group, 41%) of the variation was explained by the z-ampli­ tude of the VECG. The VMCG z-amplitudes correlated most with the VECG y-amplitudes, the highestR2 value being 0.41. R2 0.5

ClI VECG Z

0.4

o VECG Y

0.3

• VECG X

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0.3 0.2 0.1

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0.5 0.4 0.3 0.2 0.1 0

Time-normalized QRS-duration (%) Fig. 1. Relative contribution of the X-, y-, and z-lead amplitude of the VECG

(R2) in a) X-, b ) Y-, and c) Z­

leads of the VMCG during the time-nonnalized QRS-complex in 200 nonnal . subjects.

The correlation between the spatial vector magnitudes of the VMCG and VECG was surprisingly weak. The highest corre­ lations (R2=0.39 in the whole group, R2=0,44 in males and R2=0.25 in females) were found in the mid-QRS. B.

In inferior MI the correlation between the VMCG x- and VECG z-amplitudes increased clearly from the normal case during the initial QRS despite of no clear change in the VMCG from the mean normal curve. In contrast, during the terminal phase of the QRS-complex when the VMCG x­ amplitudes in the inferior MI differ clearly from the normal values, the VECG z-amplitudes explained greater proportion of the variability in the VMCG x-amplitudes than in the normal subjects. In the VMCG Y-Iead the predicting value of the VECG z-amplitudes has increased slightly compared with the normal case. In the VMCG Z-lead the predicting value of the VECG Y-Iead has clearly decreased during the initial and terminal phases of the QRS. V. DISCUSSION AND CONCLUSION

90

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to the squared multiple correlation coefficient

C. Inferior MI

The results of the this correlation study between the normal VMCG and VECG are in accordance with our previous results of lead field studies [2] and morphological studies [3]. The VMCG X-lead and the VECG Z-lead, the VMCG Y-Iead and VECG Z-lead, and the VMCG Z-lead and the VECG Y­ lead, respectively, contain some common and redundant information about the depolarization of the ventricles. However, the value of the VECG to predict the normal variability in the VMCG was only momentarily at most 45%. This means that more than half of the variability in the normal VMCG is independent of the VECG. Age of the subjects has shown to be most important non­ cardiac factor affecting the variability of the normal VMCG parameters [1,3]. However, an addition of age among the independent variables increased only slightly the R2 values and did not change considerably the formerly observed pre­ dicting values of the VECG parameters. We have reported characteristic changes in the VMCG due to MI from the normal case [1]. In anterior MI these changes were associated with the clear decrease in the R2 values between the VMCG and VECG. In inferior MI these changes were mostly associated with the increase in the R2. Our results support the necessity to develop the unipos­ itional VMCG lead system further to improve its sensitivity distributions in the heart region and thus to get more infor­ mation about the electrical activity of the heart independent of the VECG. REFERENCES

Anterior MI [ 1]

In anterior MI the importance of the VECG z-amplitude to predict the VMCG x-amplitude was instantaneously increased from the normal case in the beginning of the QRS-complex. The small correlation between the VMCG y-amplitudes and the VECG z-amplitudes was disappeared. A clear decrease in the R2 values between the initial VMCG z- and VECG y­ amplitudes was associated with the prominent difference in the VMCG between the normal case and anterior MI.

O.S.

Oja,

J.

J. Malmivuo, and A. Uusital0, in anteroseptal and inferior infarctions," in

Nousiainen,

"Magnetocardiogram

A dvances in Biomagnetism '91, M. Hoke et al., Eds. Amsterdam: Elsevier, 1992, in press. [2]

H. Eskola, On the properties of vector magnetocardiographic leads.

[3]

Tampere: Tampere University of Technology, 1983. J. Nousiainen, Behavior of the vector magnetocardiogram in normal subjects and in some abnormal cases. Tampere: Tampere University of Technology, 199 1.

[4]

J. Malmivuo, On the detection of the magn£tic heart vector - An appli­ cation ofreciprocity theorem. Helsinki: Acta Po1ytechnica Scandinavica, 1976.

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