Mapping of repolarization duration in normal subjects by decarto technique

E.V. Blinova¹, T.A. Sakhnova¹, O. Yu. Atkov¹, V.G. Trunov² , E.A.I. Aidu², L.I. Titomir^2
1Cardiology Research Complex, Ministry of Health of Russian Federation ,
 121552, 3rd Cherepkovskaya str. 15a, Moscow, RUSSIA
²Institute for Information Transmission Problems, Russian Academy of sciences,
101447, B. Karetny 19, Moscow, RUSSIA

Abstract: The decartogram of ventricular repolarization duration, or recovery acceleration map, was studied in 120 normal subjects. The repolarization duration shortening is represented by a dipole characteristic with the moment G components G_x, G_y, G_z in common vectorcardiographic coordinate system XYZ. The G_y component was significantly smaller in females than in males in the group elder than 30 years. In the younger, as well as in the elder group in females, the G_y component was significantly smaller than the G_z component, in males the G_y and G_z components had no significant differences.

INTRODUCTION

The main requirements of clinical electrocardiography on the acquisition and representation of data are the simplicity of the measurement procedure, diagnostic informativity of the initial signals, clear and convenient ways of visual representation of the data. The opinion of many experts in electrocardiology is that the ECG data should be represented in an electrophysiologically meaningful form with the attachment to the anatomical structures of the heart. This requirement can be favorably provided with the use of multiple-lead body surface potential mapping. However, the complexity of the measurement procedure restricts the application of this technique in clinical practice.

Dipole electrocardiotopography (abbreviated as DECARTO, or decartography) has been proposed to achieve a proper trade-off between the demands for representation of the cardioelectric processes in an intelligible-pictorial form and for as simple as possible data acquisition technique [1]. The method uses a 3-orthogonal-lead system and simplified models of the cardioelectric generator, which adequately fit the restricted measured data of orthogonal ECG. A pictorial representation of the orthogonal ECG data in a map-like form is implemented by constructing so-called decartograms which represent in a descriptive form the main electrophysiological states and characteristics of the heart. The decartograms characterizing the period of ventricular depolarization have been evaluated in a number of studies [1 etc.]. At the same time, there is a lack of papers devoted to the decartograms of the ventricular repolarization period. The aim of this study was to assess the features of the summary decartogam of repolarization characterizing the distribution of the repolarization process duration in the ventricles of normal subjects.

METHODS

We examined 120 normal subjects, 60 males and 60 females, 18 to 56 years old (mean age 32.6 ± 8.8 years). Among them there were 55 subjects (26 males and 29 females) younger than 30 years and 65 subjects (34 males and 31 females) elder than 30 years. In all subjects, the orthogonal ECG (MacFee–Parungao lead system) was recorded.  The heart rate during the record was in normal limits (60 to 85 beats per minute). A 10 s record was inputted into computer with the sampling rate 500 Hz. A representative cardiocycle was chosen and used for constructing the decartograms. The DECARTO technique allows representing the signals of orthogonal leads in a map-like form. Decartograms represent the projection of the main electrophysiological states of the heart onto the imaging sphere. The imaging sphere is a sphere enclosing the heart and having its centre at the heart midpoint. The imaging sphere is cut along the meridian facing the right side of the patient’s chest, unrolled, and projected onto a plane in such a way that each element of the sphere retains its area value on the planar projection. The resulting map is bounded by an oval with the top and bottom points corresponding to the sphere poles and the left and right boundaries corresponding to its right meridian. The front and back surfaces of the heart are projected onto the left and right parts of the oval map, respectively.

The map under consideration (so-called “recovery acceleration map”) shows the distribution of the dipole component of the depolarized state duration shortening over the heart surface (concerning the electrophysiological meaning, this characteristic is a surface distributed equivalent of the ventricular gradient vector taken with the inverse polarity). This quantity has greater values in the area with the relatively shorter action potential duration, in other words, it is the area where the recovery process takes less time as compared with the other parts of the heart.

The set of parameters used for a quantitative analysis of this decartogram included the magnitude G (in ms) and spatial components G_x, G_y, G_z (directed leftwards, downwards, and backwards, respectively) of the “recovery acceleration” vector (proportional to the ventricular gradient with negative sign). We compared the values of these parameters in males and females and in younger (before 30 years) and older (after 30 years) groups of subjects. The data were represented as mean value ± standard deviation. For comparing the groups, the Student t-test was used. The results obtained were thought to be statistically significant with  p < 0.05.

RESULTS

The values of the parameters studied in males and females younger than 30 years and elder than 30 years are represented in Tables I and II.

TABLE I
The values of the parameters studied in subjects younger than 30 years

Parameter, ms	Males	Females
G	83.2 ± 25.8	78.6 ± 24.1
Gx	43.6 ± 14.0	36.7 ± 12.6
Gy	- 26.3 ± 20.1	- 24.8 ± 18.4
Gz	34.1 ± 13.1	36.2 ± 16.2

TABLE II
The values of the parameters studied in subjects elder than 30 years

Parameter, ms	Males	Females
G	79.5 ± 22.8	77.4 ±21.2
Gx	42.6 ± 14.1	42.2 ± 15.5
Gy	- 29.7 ± 14.4	- 20.6 ± 14.3*
Gz	28.6 ± 12.5	32.4 ± 11.6

* Differences between males and females are statistically significant (p<0,05).

In the males, the G_x component was greater than G_y and G_z components (p<0.01). The G_y and G_z components had not significant differences. In the females, the G_y component was smaller than G_x component (p<0.01) and G_z component (p<0.05 in females younger than 30 years and p<0.01 in females elder than 30 years). The G_z component was smaller than G_x component in females elder than 30 years (p<0.05). In females younger than 30 years the G_x and G_z components had not significant differences.

The decartograms showing the distribution of the dipole component of the depolarized state duration shortening in typical cases are presented at Figs. 1 and 2.

Figure 1. Decartogram of 22 years old male.

Figure 2. Decartogram of 45 years old female.

There were no statistically significant differences of the parameters under consideration between the groups of subjects aged less than 30 years and more than 30 years.

DISCUSSION

Our data concerning the sex differences of the “recovery acceleration” vector are in some accordance with the data on the sex differences of ventricular gradient from [2]. In that study, there was a smaller Y-component and a larger Z-component of ventricular gradient in females than in males. In our study the G_y component was significantly smaller in females than in males only in the subjects elder than 30 years. At the same time, in the younger as well as in elder group of females the G_y component was significantly smaller than G_z component, in males the G_y and G_z components had no significant differences.

It is interesting to note that the decartograms showing the distribution of the dipole component of the depolarized state duration depict these differences in a clear form. In Fig. 2 it is seen that in the female the main differences in action potential duration are found between the right-anterior and the left-posterior parts of the imaging sphere, in other words, between the right and left ventricles. In this case, the action potential in the right ventricle is relatively longer than in the left ventricle. In the male, as it is seen in Fig.1, the vertical component of ventricular gradient is more pronounced. The main differences in the action potential duration are found between the apex and the basal regions of the heart. The action potential in the apical regions is relatively longer than in the basal regions.

It is unclear why females as compared with males have a more pronounced difference in the action potential durations between the right and left ventricles. Some studies have shown that an estrogen, in particular, the 17beta-estradiol, may prolong the action potential duration in ventricle myocytes [3, 4]. Whether it can make some biotransformation in the lungs and therefore affect the right ventricle to a greater extent than the left ventricle, should be further investigated.

REFERENCES

[1]  L.I. Titomir, I. Ruttkay-Nedecky, L. Bacharova, Comprehensive analysis of the electrocardiogram in orthogonal leads. Moscow: Nauka, 2001 (in Russian).

[2]  K. Yamauchi, I. Sotobata, “Sex and age differences in ventricular gradient”, Jpn. J. Med., vol. 30, pp. 504-508, 1991.

[3]  S. Tanabe, T. Hata, M. Hiraoka, “Effects of estrogen on action potential and membrane currents in guinea pig ventricular myocytes”, Am. J. Physiol., vol. 277 (2 Pt 2), pp. H826-33, 1999.

[4]  F. Berger, U. Borchard, D. Hafner, et al.,  “Effects of 17beta-estradiol on action potentials and ionic currents in male rat ventricular myocytes”, Naunyn. Schmiedebergs. Arch. Pharmacol., vol. 356 (6), pp. 788-96, 1997.