|Year : 2014 | Volume
| Issue : 2 | Page : 112-123
Normal limits for pediatric electrocardiogram in Ilorin, Nigeria
Afolabi Joseph Kolawole1, SI Omokhodion2
1 Departments of Paediatrics and Child Health, University of Ilorin Teaching Hospital, Ilorin, Nigeria
2 Department of Paediatrics, University College Hospital, Ibadan, Nigeria
|Date of Web Publication||3-Oct-2014|
Afolabi Joseph Kolawole
Department of Paediatrics and Child Health, University of Ilorin, Ilorin Teaching Hospital, PMB 1459, Ilorin
Source of Support: None, Conflict of Interest: None
Background: Proper interpretation of electrocardiogram (ECG) in children relies on comparisons with standards derived from normal population. Comprehensive data on ECG in Nigerian children are lacking.
Objective: To determine the ECG reference values for Nigerian Children.
Subjects and Methods: A cross-sectional study was conducted on 1500 apparently healthy Nigerian children between 0-12 years at Ilorin. The ECG was recorded at a sampling rate of 1000Hz with V 3 R replacing V 3 and the ECG parameters were measured by hand.
Results: The means and standard deviations of the lead independent ECG indices are as follows: Heart rate, 117 ± 23; QRS duration 0.05 ± 0.01 PR Interval 0.13 ± 0.02, QRS axis 63.80 ± 37.6, P wave axis 46.6 ± 18.4, T wave axis 44.5 ± 18.5, P wave duration 0.07 ± 0.01, P wave amplitude 1.4 ± 0.48. These values are similar to those of the Caucasian children. The mean amplitude values of Q, R, S and T waves are also similar to the Caucasian Children. The 98 th percentile values in some of these leads are substantially higher than that of the Caucasian children. This calls for caution when interpreting ECG in Nigerian children.
Conclusion: ECG normal limits have been established in Nigerian children at Ilorin. Similar studies should be conducted in other geo-political zones in Nigeria so that ECG reference values for Nigerian children could be established.
Keywords: Electrocardiogram, normal limit, Nigerian children
|How to cite this article:|
Kolawole AJ, Omokhodion S I. Normal limits for pediatric electrocardiogram in Ilorin, Nigeria. Nig J Cardiol 2014;11:112-23
| Introduction|| |
Proper interpretation of electrocardiogram (ECG) relies on comparisons with standards derived from normal population. Many authors have demonstrated that ECG could be influenced by age sex, nutrition and race. ,,,
Abnormalities have been reported in the ECG of Negroes (Americans and Africans) in comparison with Caucasians. For example, the persistence of the juvenile pattern that occurs as a normal variant in some adults. , has been found to be more common in Negroes than the Caucasians.  Other normal variants that are prevalent in Negroes are the ST depression, ST elevation with or without tall T waves and high R and S wave amplitudes in the chest lead. 
Several studies have been carried out, to determine the normal limits for the pediatric electrocardiogram amongst the Caucasian children. ,,,,, In some of these studies, certain imperfection limits their practical applicability. Some of them lack details and the ECG parameters were measured by hand. Hand measurement in a large population study is time- consuming, but useful in small population study and in clinical practice, especially in the newborn where hand measurement is superior to electronic one.  In contrast, computer analysis of digitized ECG allows for more accurate measurement, although it may miss some abnormalities or even identify some where none exists. Therefore, this study needs a well-developed computer ECG program, which at present has not been developed in Nigeria, a country with dirt of data on pediatric ECG.
The most comprehensive study has been that of Davignon et al. in which the ECG of 2141 children aged 0-16 years were recorded in Quebec Canada. The ECG was digitized at a sampling rate of 333 Hz and normal limits were determined using computer- assisted measurements. However, in a study of 1780 children, Macfarlane et al. recorded ECGs at a sampling rate of 500 Hz and demonstrated that the 98 th percentile of normal amplitude could be up to 46% higher than those published by Davignon et al. This was corroborated by Rijnbeek et al. in 2001. They recorded ECG in 1912 healthy Dutch children aged 11 days to 16 years at the highest sampling rate of 1200 Hz and noted significant differences in normal limit of many ECG measurements from those reported earlier. Significant sex differences were also demonstrated for amplitude measurements and QRS duration. Recent work by Semizel et al. in Turkey, demonstrated similar finding with Rijnbeek.  They recorded ECG in 2241 healthy Turkey children aged from 1 day to 16 years, at sampling frequency of 500 Hz and showed that the gender differences in amplitude and duration particularly in the adolescent period exist. These differences reported were attributed to the sampling frequencies used.
Although several studies have been conducted to determine the normal limits for the pediatric ECG among the Caucasians, there is paucity of data for Nigerian children. The current practice has always been to base the interpretation of ECG in Black children on the Caucasian standard. This practice should be discouraged because racial differences had been reported in ECG. For example, persistence of juvenile pattern is more common in Negroes than Caucasians.
ST segment variation with race has also been noted. There is unusual elevation in the precordial leads. Increase precordial QRS voltages that may mimic left ventricular hyperthrophy is a common ECG finding in blacks when compared with the Caucasians. Most of the studies on ECG in Nigerian children compared their findings with controls matched for age and sex without reference standard for comparison. , Ifere,  working in Zaria in northern Nigerian recorded ECG in 425 healthy Nigerian children from 0-5 years to determine the cardiac rhythm, heart rate and presence of arrhythmia but did not establish normal limits for those children. Edemeka and Ojo  in Sokoto attempted to establish some reference values in ECG in children. They recorded ECG in 230 Nigerian children aged from 5-14 years. Small sample size and a narrow age range limited this study. He did not cover the age range between 0-5 years, where a lot of ECG changes exist in children. Moreover, he used a single channel ECG machine and did not state the sampling frequency used. It was therefore difficult to establish a comprehensive reference values from such studies. There is currently no comprehensive data on ECG standard in Nigeria children, and hence this study is undertaken to establish normal limits of ECG in Nigerian children to encompass a wider age range using a large sample size.
| Subjects and methods|| |
A total of 1500 apparently healthy Nigerian Children ranging from 0-12 years were recruited. They consisted of newborns, infants, toddlers, primary school and junior secondary school children. The newborns were recruited from labor wards, obstetric wards and postnatal clinics of the University of Ilorin Teaching Hospital, the toddlers from the immunization center of various health facilities at Ilorin, the young children between 1-3 years from day care and Nursery and Primary Schools and the older children from Junior Secondary school. These locations were chosen because these were the areas large population of children could be found. Appropriate permission was taken from these institutions and written informed consent was obtained from the parents. Ethical Committee of the University of Ilorin Teaching Hospital approved the study.
The inclusion criteria were; Nigerians age 0-12 years.
Have no evidence of cardiac diseases or any other conditions that may affect the cardiovascular functions such as pneumonia, diarrhea, asthma, acute glomerulonephtis etc.
Have not received any medications that could adversely affect the cardiovascular functions in the last 72 hours. e.g. antimalaria.
Children with chronic illness, presence of protein energy, malnutrition, perinatal asphyxia, prematurity and post maturity were excluded from the study.
The subjects were selected by multistage stratified sampling method. Three public, nursery and primary schools, secondary schools, and University of Ilorin Teaching Hospital were randomly selected. Subjects were then recruited from each study area using proportional and systematic sampling technique.
Each subject had a complete physical examination, the body temperature, blood pressure, the age, weight and height, were determined. Blood pressure was measured using a mercury sphygmomanometer (Accoson, made in England) with the child sitting comfortably as recommended in the seventh report of the joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure.  Blood pressure measurement was not taken in the children under the age of three years, because of lack of appropriate cuff. The height of children two years of age and older were measured in the standing position using a standiometer (Seca) to the nearest 0.1 cm. An infantometer (Seca) was used to measure the recumbent length of children less than two years. The infants were weighed unclothed on a weigh master infant scale (Weigh Master Company; reading England) that has been checked for zero error before each weighing and was cross-checked periodically with standard weight. The older children were weighed in their under pants on a bathroom scale. The weight measurement was recorded to the nearest 0.1 kg. Body mass index (BMI) was calculated using the following formula
BMI = wt (kg) ⁄ h 2 (m 2 ).
A 12-lead ECG was recorded using a portable heat writing AT-1 3-channel electrocardiograph (Schiller Cardiovit AT-1 CH-6340 BAAR) with frequency range of 0-150 Hz and sampling frequency of 1000 Hz. Special newborn disposable electrodes and pediatric cuffs were used. The electrodes were positioned as recommended by the American Heart Association.  V 3 R was placed at a similar position as V 3 on the right side of the chest.
Conventional 12-lead ECG was recorded with the control switches set at a calibration of 10 mm/Mv and paper speed of 25 mm/sec. Five cardiac cycles were recorded per ECG lead. In addition lead II was taken at a paper speed of 50 mm/sec to serve as rhythm strip. Where there were high voltages the calibration was set at 5 mm/mv. The following measurements were manually taken from the ECG tracing paper; heart rate, frontal plane QRS, P and T wave axes (determined by the Hexaxial reference system),  PR interval, QRS duration, P wave amplitude and duration, QT duration and RR interval, which were the intervals preceding the measured QT duration in lead II. Wave amplitude measurements were determined using PQ or TP isoelectric line as baseline. Q, R, S, and T waves were measured in each lead. Ventricular activation time QR was measured from the beginning of QRS complex to the peak of R wave in lead II.
Five ECG recording from every one hundred were randomly selected and interpreted by a senior pediatric cardiologist for quality control. This was found to be 98% concordant with the researcher's interpretation.
Data entry and analysis were carried out using statistical package for social sciences (SPSS version 10) software. The relationship between ECG wave amplitude, duration and other ECG indices used in pediatric cardiology, the heart rate, age, weight, BMI and sex were studied. The mean and standard deviation, minimum and maximum values of the variables were determined. The 2 nd and 98 th percentile of the measurement distribution were taken as lower and upper limits of normal. Tables and percentile graphs were generated.
| Results|| |
A total of 1500 healthy children were studied. There were 723 males and 777 females with male to female ratio of 1:1.1. The distribution of the study population according to age and gender is shown in [Table 1].
[Table 2] shows the lead independent ECG indices in different age groups. The mean heart rate in the first one week of life was 128 ± 16 beats/minute, peaked at 1-3 months and decreased gradually from 3-6 months until the last age group [Figure 1]. Wide variability exists in the heart rates in all the age groups.
|Figure 1: Percentile curve of heart rate (beat/minute) by age group. The heart rate increases from birth, reaching the peak at 1-<4 weeks and decreases from about 3 months onwards until the age of 12 years|
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The mean QRS axis was directed to the right in the first 1-<4 weeks of life and shifted leftward after wards. The P wave axis and T wave axis were stable in all the age groups except in 0-<7 days, where a maximum of 179° was observed in T wave axis occurring in 4% of subjects in this age group.
QRS duration, PR interval, and P wave duration increased gradually with age (P = 0.00). PR interval showed very little change until 1-<3months when it gradually increased. PR interval decreases with increase in heart rate. [Figure 2] The P wave amplitude ranges between 0.05 mv and 0.25 mv in all the age groups. Extreme value of 0.3 mv was observed in 6-<8 years and 12 years. Negative P wave was found in 0-<7 days in 1.3% of the newborn.
QT duration increased at 1-<3 months, plateaued between 3-<6 months and 1-<3 years then it gradually increased until the age of 12 years. The QT interval decreased steadily with increase in heart rate [Figure 3]. The QT C was calculated using Bazett formula (QT c = QT∕ √RR).
|Figure 2: Percentile curve for PR interval by heart rate. The PR interval decreases gradually with increase in the heart rate|
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|Figure 3: Percentile curve for QT duration by age groups. The mean QT duration decrease steadily with increasing|
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The mean QTc is 440 ms. Range is between 400-480 ms in all age groups [Figure 4].
|Figure 4: Percentile curve of QTc by age groups. The corrected QT (QT/FRR) was found to be relatively constant with a mean of 0.44 the range being 0.40-0.48|
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[Table 3] shows the Q waves amplitude in Lead II, III aVf, v 5 and v 6 . Q wave is produced by the depolarization of the ventricular septum and is a negative deflection. Zero deflection indicating absent Q wave was excluded from the statistical analysis of the data. Mean values did not exceed 0.4 mv in all the leads. The upper limit in lead II and III is as large as 0.87 and 0.95 mv, respectively. In V5 and V6 the upper limit of 0.72 mv in 3-<6 months, 0.6 mv in 6-<12 months and 0.6 mv in 1-<3 years were observed. Q waves increased gradually with age in V5 and V6 until 6-<12 months of age. After this age, the upper limit of normal is relatively constant in older age groups.
[Table 4] and [Table 5] show the normal limit for R and S wave amplitude, respectively, in all the leads. R wave amplitude decreased with age in the right precordial lead (RPL) and increased with age in the left precordial leads (LPL). S wave amplitude increased with age in the RPL and decreased with age in the LPL. [Figure 5],[Figure 6],[Figure 7],[Figure 8],[Figure 9],[Figure 10],[Figure 11] and [Figure 12].
|Figure 5: Percentile curve of R wave amplitude by age groups in lead V3R. There is high amplitude R wave between 0 - < day to 1 - < 3month. After this age there is a sharp decrease in the R wave amplitude|
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|Figure 6: Percentile curve of R wave amplitude by age groups in lead V1. High amplitude R wave in age 0 - < 7 day to 1 - < 3 years decrease from the age of 3-5 years until age 12.|
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|Figure 7: Percentile curve of R wave amplitude by age groups in lead V5. R wave amplitude increases gradually with increasing age|
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|Figure 8: Percentile curve of R wave amplitude by age groups in lead V6. R wave amplitude increases gradually from birth to age of 12 years|
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|Figure 9: Percentile curve of S wave amplitude by age groups in lead V3R. S wave amplitude increase gradually with increasing age|
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|Figure 10: Percentile curve of S wave amplitude by age groups in lead V1. S wave amplitude showed a sharp increase from age 1-3months until the age of 12 years|
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|Figure 11: Percentile curve of S wave amplitude by age groups in lead V5. S wave amplitude showed a marked decline from the age of 3-<5 years|
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|Figure 12: Percentile curve of S wave amplitude by age groups in lead V6. High S wave amplitude at births gradually decreases until the age of 12 years|
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[Table 6] shows the distribution of T-wave amplitude in the precordial leads. The mean in V 3 R and V 1 is negative in 0-<7 days but approaches zero in other age groups. In the LPLs the mean progressively increased from 0-<7 years to 3-<6 months. Thereafter, it was relatively constant until 3-<5 years when it showed an increase.
| Discussion|| |
Recent comprehensive reports on normal limit based on population of apparently healthy children are those emanating from Davignon et al.,  Macfarlane et al.,  Rijnbeek et al.,  Semizel et al. and Sun et al. As far as we know, this study is the first most comprehensive study of pediatric ECG in Nigerian children.
In this study, the mean heart rate of 128 beats/minute in the first week of life increased in the 1-<3 months before it gradually decreased in the older age groups. (P = 0.00) This trend was similar to the work of Davigion et al., who also reported the occurrence of highest rates between 1-3 months of life. In contrast to this finding, Ifere,  in Nigeria observed that the heart rate increased after the first day, reaching its highest rates between 1-<3 days of life with a gradual decline after one week. This difference might be due to his small sample size and narrow age range.
The relatively right ventricular hypertrophy of the neonate regresses over the first few months of life. There was a relatively rapid change in the axis after the age of 3 months. This finding is similar to the work of Davignon et al.,  Rijnbeek et al. and Semizel et al. Right axis deviation up to +180° was found in the newborn and changed to less than +90° at 6 months -<3 years.
Superior axis observed in this study occurred in 2.2% of the study population. Various workers also made similar observation. , Superior QRS axis is often associated with structural congenital heart disease most frequently atrioventricular septa defect, ventricular septa defect and left bundle branch block.  However, superior axis has been reported in normal healthy children. Jones et al. reported 6 neonates with superior axis. These newborn had no evidence of structural heart diseases. They were followed up over a period of time, and only one of them had a persistent superior axis at three months. Edemeka and Ojo  reported superior axis of 4.06% in their series. A.R. Walker and B.F. Walker  had earlier made similar observation. Therefore, the possible interpretation is that the superior axis observed in our study may be a variant of normal. Unfortunately, echocardiography, which would have confirmed this was not carried out in this study.
QRS duration, PR interval and P wave duration, increased gradually with age. QRS duration was measured in lead II because in our series, lead II is one of the leads where the beginning of QRS was more sharply defined and thus, easier to identify and measure. Davignon et al. measured QRS in their series in V 5 because of similar reason. The mean QRS duration of 40 ms in the first age group increased gradually to 70 ms at the age of 12 years. This trend is in agreement with those observed by other workers. ,, Davignon et al. measured QRS duration in lead V in contrast to lead II used in this study. The measurement of QRS duration in lead II may therefore be slightly shorter than in lead V. However, this difference has not significantly affected the normal range obtained in this study. Rijnbeek et al. reported a mean value of 67-78 ms between 0-<16 years in males and 67-87 ms in females. These values are higher than those of Davignon's and the present study. This is not surprising because Rijnbeek et al. determined QRS duration in all the leads yielding longer QRS duration. The values of QRS duration measured by the computer using multiple, simultaneously recorded leads can be expected to be wider than that measured using a single lead because the beginning or end of the QRS may deviate from the base line only slightly or not at all in some leads. 
P wave amplitude does not change significantly during childhood. The study revealed that the mean P wave amplitude of 0.13 mv at birth varied very little with mean of 0.15 mv at the age of 12 years. The upper normal limit in this study was 0.25 mv. This was the finding of other workers. ,, Higher P wave amplitude of 0.30 mv occurred in few subjects in this study and were outside the 98 th percentile. Rijnbeek et al. made similar observation. The normal P wave amplitude could be taken as 0.05-0.25 mv in Nigerian children.
QT interval increased as the heart rate decreased. The corrected QT duration is large, the mean value remained at 410 ms in most of the age groups except in 1-<3 months and 3-<6 months. The range is 350-490 ms. Davignon et al. observed mean QTc of between 400-480 ms with slightly higher values in the first day of life. The observation in the present study is at variance with those of Edemeka and Ojo,  who reported lower values of 250-390 ms. Jaiyesimi , reported QT C of 350-440 ms. These two studies were conducted in Nigeria, 25 years ago and were limited by small sample size and narrow age range. The disparity could also be accounted for by the improvement in the present day electrocardiograph and the improved physiological and nutritional statuses of these children in the present study. Further studies will help to throw more light on this.
Q wave amplitude observed in this study is substantially higher than those reported in earlier studies. For example, in children aged 3-<5 years, an upper normal limit of the Q wave amplitude of 0.60 mv observed in this study is higher than 0.3 mv, as reported by Stong et al. Davignon et al.,  0.34mv of Semizel et al. and the 0.54mv reported by Rijnbeek et al. in leads V 5 and V 6 . Rijnbeek et al. obtained Q wave amplitude of 0.56 mv in V 6 in boys and 0.49 mv in girls in 1-<3 years age group as against the 0.60 mv obtained in this study. The values obtained by these workers in the corresponding age groups are lower than those obtained in the present study. The upper normal limit in the present study is also higher than those reported by Semizel et al. in most of the age groups in the left chest leads.
Rijnbeek et al. attributed higher amplitude obtained in their study to higher sampling rate, which we also used in this study. It is difficult to explain why the value obtained in this study is higher than those of Rijnbeek's study. Racial factor might be a possible explanation.
R and S wave amplitudes in the precordial leads are important parameters in the diagnosis of both right and left ventricular hypertrophy. ,,,,,,, There is considerable difference in the R and S wave amplitudes compared with those reported earlier. For example the upper normal limit of R wave amplitudes in leads V 3 R, V 1 and V 4 is higher than those of Davignon et al. and Rijnbeek et al. for all age groups.
Davignon et al. reported an upper normal limit of 4.5 mv in V4 for children aged 3-5 years compared with 3.27 mv of Rijnbeek et al. and 5.4 mv in this study: R wave amplitudes in V 4 larger than 3.5 mv were exceptional in the studies of Davignon et al. and Rijnbeek et al., whereas in this study R wave amplitude up to 5.8 mv upper normal limit occurred in 3.4% of children aged 8-12 years in V4. Similarly, S wave amplitudes in the left precordial leads is lower than those of Rijnbeek et al. but the mean in the right precordial leads is higher.
There were also higher observed upper normal limits in the S wave amplitudes. Values as high as 3.9 mv obtained in this study in children aged 6-12 months in V2 is higher than 2.78 mv of Rijnbeck et al. and 3.12 mv of Davignon et al. High voltage R and S wave amplitudes as observed in this study have been recorded in serial studies in South Africa and differences have been known to exist between the Bantu and the Caucasian. 
Evidence existed that the higher R and S wave amplitudes in Bantu were primarily of ethnic origin. Environmental factors were also suggested. Masica and associates  also found differences in SV 1 voltage between Negro and White children aged 5-<7 years. In total, 95% of their subjects in their study were Negros, suggesting that increased precordial voltage in apparently normal healthy children may be related to race. It is also known that body build may affect the amplitudes of the R and S waves in the precordial leads.  Thin chested normal individuals and persons with large ventricular mass may exhibit high voltage on ECG, whereas, thick body fat reduces the amplitude of the voltage over the precordium.  It has been shown that as children grow older and develop increasing body size, the electrocardiograph of precordial voltage decreases. Walker et al. who made a similar observation in children and adolescent pointed out the importance of developing age adjusted ECG criteria for left ventricular hypertrophy.
Therefore, the observed higher values in this study might be due to body build and racial factors, since the Caucasian children tend to be bigger than their Negroid counterpart.  It is therefore necessary to exercise caution when determining ventricular hypertrophy using voltage criteria. In such cases, echo cardiographic examination, which is more specific for ventricular hypertrophy should be an added investigation.
| Conclusion|| |
In this study, we have provided ECG normal limits for Nigerian Children aged 0-12 years of age at Ilorin. The values in some ECG parameters are similar to those reported amongst the Caucasians, but some differences exist in others.
The ECG parameters were hand measured from ECG tracing papers. Despite this limitation, normal limits of some of the ECG parameters were shown to be similar to those digitized by computer programs. The Q, T, R and S wave amplitudes were higher than those reported among the Caucasian and there is a need to exercise caution when interpreting ECG in Black children. There is also a need to carry out similar studies in other Geo-political zones of Nigerian, so that ECG reference values for the Nigerian Children could be established. More age specific longitudinal studies are advised to study the evolution of ECG with age, and efforts should be made to develop a computer-based pediatric ECG data bank for the interpretation of ECG in Nigerian children.
| Acknowledgment|| |
Staff Nurse (Mrs.) Mojisola Sodiq for assisting in the anthropometric measurement and Mr. Isiaka Oloyede who assisted in data analysis.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]