A study of Electrocardiographic pattern of normal school children in Palakkad District of Kerala

Background: Electrocardiography is a simple, non-invasive and relatively cheap investigative tool used for cardiac evaluation. However, there are limited electrocardiographic studies of Indian children. These differences may also be seen in children, hence, the need to develop local reference values. Methods: This was a cross-sectional study on selected 450 subjects. An 12 lead ECG was measured on all subjects. Results: There were two hundred and twenty boys   (48.3 %) and two hundred and thirty girls   (51.7 %). The mean heart rate decreased with increasing age. R wave amplitudes were higher in the left precordial leads, in keeping with left ventricular dominance. Mean values were higher in males than females in the three age-groups in most of the precordial and limb leads. In V4R, V2, andV3 highest mean R wave voltages of 0.5+0.1 mV, 1.4+0.3 mV and 1.4+0.2 mV were recorded in the 5-7-year-old. While in V5, and V6 the mean R waves were higher in the 12-15-year-old age-group (3.7+ 0.5 mV and 2.5+0.4 mV respectively).  The S-Waves showed progressive decrease in its amplitude on the left precordial leads with increasing age. Conclusion: The mean values in heart rate, QRS duration, PR interval, P wave amplitude showed higher amplitudes in  males. Similarly higher amplitudes of R-waves in male children were recorded in precordial leads V2, V3, V5 and V6 in the three age-groups.


Introduction
Electrocardiography (ECG) is a harmless, painless, noninvasive, easy to perform and relatively inexpensive technique of obtaining information on cardiac activity [1]. In interpreting paediatric ECG, integration of the clinical history, physical examination findings with the ECG report is vital to making a reasonable diagnosis. Interpretation of paediatric ECG is age-dependent and reference standards are required for comparison.
The ECG reference standards in Indian adults have been established. The current practice of comparing ECG measurements of Indian children with Caucasian values should be discouraged because it has error level; it is well established that, there are racial differences in the ECG characteristics [2][3][4][5][6] which may also be existent in children. Therefore, there is the need to develop local reference standards. Although there are pioneer works on ECG on apparently healthy children, [7,8] studies had limitations which has affected their clinical application. This study seeks to establish reference ranges and any gender differences in the ECG characteristics of children in Palakkad District, Kerala.

Methods
The study involved 450 apparently healthy children aged 5-15 years in Cherpulassery, Palakkad District, Kerala who were free of any illness at the period of this study carried out for 6 months i.e June-December 2014. ECG recording was done to all subjects in the morning hours at the Department of pediatrics, Kerala Medical College Hospital, Cherpulassery, Palakkad District, Kerala.
Those whose blood pressure were more than the 95 percentile for age, sex and height [9]; those with dysmorphic facies; those on any medication known to affect the cardiovascular system [8] and those with any chronic illness were excluded from the study.
Approval to carry out the study was obtained from the Institutional Ethics Committee. Informed consent was obtained from parents or caretakers.
The ECG machine was a BPL 108T machine with frequency of 150 Hz and sampling frequency of 1000 Hz. The machine has two speeds, 25 mm/sec and 50 mm/sec; with three levels of sensitivity at 5, 10 and 20 mm/mV. The subjects were classified into age-groups; 5-7, 8-11 and 12-15 year age-groups respectively.
PR interval was measured in lead II, from the beginning of P wave to the beginning of R wave along the horizontal line of the ECG paper. P wave duration was measured in II, from the beginning of the P wave to the end of P wave along the horizontal line of the ECG paper.
QRS complex duration was measured in lead V5, from the beginning of Q wave to the end of the S wave along the horizontal line of the ECG paper.
All measurements were done manually by visual inspection using magnifying glass. Second percentile was defined as the lower limit of normal, whilst the 98 th percentile was defined as the upper limit of normal [10].
Data entry and analysis were carried out using a computer software package; the Statistical Package for Social Sciences version 14. The mean, standard deviation of various ECG variables were determined for both sexes, and 2 nd and 98 th percentiles of measurement distribution were determined.
Heart Rate-The heart rate decreased with increasing age and the female groups had higher mean values than the male groups.
Electrical axis of the heart-The electrical axis of the heart in the frontal plane was represented by the QRS-axis. The mean QRS-axis in the three age-groups lay within the normal axis of 0 o -90 o . The axis progressively increased with age and higher mean values were recorded in the male population. P Wave Axis-In the three age-groups, the mean P wave axes lay within 0 o -90 o . The lowest mean P wave axis was recorded in the 8-11 year age-group, whilst the highest mean value was in the 5-7 year age-group.
T Wave Axis-The mean T wave axes for the three age-groups were within 0 o -90 o . The lowest mean T wave axis was 30 o + 9 o , whilst the highest mean value was 50 o + 8 o in the three age-groups. P Wave Amplitude-P wave amplitude was determined using lead II. Higher amplitudes were recorded in 8-11 year agegroup and 12-15 year age-groups. P Wave Duration-Longer duration was recorded in the older children, in the 12-15 year age-group.
PR Interval-The PR interval was recorded using lead II. The PR-interval showed variation with age. There was progressive increase in the P-R interval with increasing age and higher mean values were documented in the male population.
QT Interval-Bazett's formula was used to calculate the corrected QT-interval, which was measured from precordial lead V5. The QT and QTc increased with increasing age in the male population, while they were of almost same values in the female population.

QRS Duration-
The mean duration of the QRS complex increased with increasing age. The lowest mean value was 56 + 10 milliseconds, whilst the highest mean of 80 + 15 milliseconds was recorded in the three age-groups. Furthermore, females had relatively lower mean QRS duration when compared to the males all the three age-groups.  Q Wave-In lead II, the lowest mean Q-wave amplitude was 0.2+ 0.1 mV, whilst the highest mean value of 0.3+0.1 mV was recorded in the three age-groups. Highest amplitude was recorded in males of the 8-11 year age-group. In lead III, the lowest mean Q-wave amplitude was 0.10 + 0.1 mV, whilst the highest mean value of 0.26 + 0.1 mV was recorded in the three age-groups. Highest amplitude was recorded in the 12-15 year age-group. In lead aVF, the lowest mean value was 0.1+0.1 mV, whilst the highest mean value of 0.2+0.1 mV was recorded in the three age-groups.
Highest amplitude was recorded in the 8-11 year age-group. In lead V5, the lowest mean value was 0.2+0.1 mV, whilst the highest mean of 0.3+0.1 mV was recorded in the three age-groups. Highest amplitudes were recorded in the 5-7 and 12-15 year age-groups. In lead V6, the lowest mean Q-wave amplitude was 0.1+0.1 mV, whilst the highest mean of 0.3+0.1 mV was recorded in the three age-groups. Highest amplitude was recorded in the 5-7 year age-group. In table II, tall R waves were predominant in the left precordial leads V5 and V6. The tallest amplitude was recorded in V5, with a mean value of 3.7 + 0.5 mV in the 12-15 year age-group. Lead aVR recorded the lowest amplitude in the 5-7 year age-group with mean value of 0.2 + 0.1 mV.

R-Wave Amplitude-
Progressive increase in the magnitude of the R wave from the right sided precordial leads to the left precordial leads was recorded. In the three age-groups, males had higher R wave amplitudes in the left precordial leads than females.  S-Wave Amplitude- Table III depict the S-wave amplitudes of the study population. There were predominant S waves in the right precordial leads in the three age-groups. Highest amplitude was recorded in lead V2 in the three age-groups. There was progressive decrease in the magnitude of the amplitude when traced from V1 on the right to lead V6. In the limb leads, aVR recorded the highest amplitude amongst the three age-groups.
T-Wave Amplitude-The highest amplitude was recorded in the 12-15 year age-group; whilst the lowest amplitude recorded was in the limb lead aVR. There was a gradual increase in the amplitude in the T wave on the precordial leads with increasing age.

Discussion
ECG parameters recorded were analyzed based on agegroups; 5-7, 8-11 and 12-15 year similar to that used by Davignon et al. [10]. The lowest mean heart rates was 75 + 7 beats per minute, whilst the highest mean was 96 + 12 beats per minute in the three age-groups. The 5-7 year age-group had higher mean heart rate than the older age-groups. Physiologically the heart rate decreases with increasing age and is attributed to increased autonomic nervous system activity [11]. This observation was similar to those reported by Davignon et al [10] , Dickson [12] and Rijnbeek et al [13]. Furthermore, females had relatively higher mean heart rates than their male counterparts. These findings were similar to those reported by Edemeka and Ojo [8], Rijnbeek et al. [13], Sedat et al., [14 ]and Sun et al [15].
Mean P wave amplitude in lead II remains remarkably unchanged from birth until at least the age of 16 years. After the age of 6-12 months, the 98th percentile value is 0.25 mV. For the diagnosis of right atrial enlargement, P wave amplitude should be greater than the upper limit of normal. The 98th percentile value in this study using lead II was within 0.25 mV, which was in conformity with the reports of Davignon et al [10], Dickson [12 ]and Rijnbeek et al [13].
The P wave duration and PR interval measurements were comparable to Caucasian standards [15,16]. The PR interval increased with increasing age. The lowest PR interval value recorded was in the 5-7 year agegroup. This could be due to the fact that the heart rate has influence on the PR interval; the higher the heart rate the shorter the PR-interval [16].
Typical features of a normal Q wave are that; it has amplitude not more than 0.4 mV and a width of not more than 40 milliseconds. The mean Q waves fulfilled these physiologic characteristics in our study.
The presence of Q-wave of more than 0.4 mV alone or in association with tall symmetrical T waves in lead V5 or V6 is often accepted as a sign of left ventricular hypertrophy [10].
The upper limit of Q-wave amplitude recorded in V5 was 0.4 mV in males of all the age-groups. These were comparable to the findings of Rijnbeek et al [13] but contrasted with that of Davignon et al., [10] who documented a 98th percentile value of 0.3 mV.
This disparity may be attributed to the difference in their sampling rate. While a sampling rate of 1000 Hz was used in this study, Davignon et al [10] used a sampling rate of 333 Hz. Q wave is a high frequency wave, therefore using a low sampling rate may result in recordings of lower voltages.
The QRS duration had a lowest mean value of 56+10 ms, whilst the highest mean value was 80+15 ms in the three age-groups. This finding was comparable to those of Edemeka et al., [8 ]Davignon et al., [10] and Rijnbeek et al [13].There was progressive increase in the duration of QRS complex with increasing age which was similar to that of Macfarlene et al [17].
Tall R-waves were observed in this study. Highest amplitude recorded was in the precordial lead V5. These mean values were higher than those of Davignon et al. [10]. This trend compared with the report of Rijnbeek et al. [14] who also reported higher R wave amplitudes in the left precordial leads than those of Davignon et al. [10] This difference could be attributed to differences in sampling rate in their studies. Furthermore, higher amplitudes were equally documented in this study when compared to that of Rijnbeek et al [ 13].
Although taller R-waves have been reported to be common among Indians, [2,3,4,5] differences in modality of ECG reading should be recognized, whilst Rijnbeek et al [13] used 'Modular ECG Analysis System,manual reading was done in this study.
S-waves were predominant on the right precordial lead in the three age-groups. This is because left ventricular dominance is already established by the age of 5-years. S-waves were deeper on the right precordial leads in the 8-11 year age-group when compared to other agegroups. This trend was similar to the findings of Rijnbeek et al [13].
Similarly, higher amplitude S waves were recorded in this study for the three age-groups when compared to that of Rijnbeek et al [13]. This may be due to racial differences between Indian and Caucasian children.

Conclusion
ECG pattern in apparently healthy Palakkad school children aged 5-15 years were determined and the mean values of some of the ECG variables such as the Rwaves and S-wave were higher than those reported other studies.
However, there was a significant sex difference in the R wave amplitude in the precordial leads V2, V3, V5 and V6 in the three age-groups; the male population having higher amplitudes than the female, which is similar to previous reports.