Features that would be diagnosed as abnormal in an adult's electrocardiogram may be normal, age related changes in a paediatric trace. The explanation for why this is so lies in how the heart develops during infancy and childhood. At birth the right ventricle is larger than the left. Changes in systemic vascular resistance result in the left ventricle increasing in size until it is larger than the right ventricle by age 1 month. By age 6 months, the ratio of the right ventricle to the left ventricle is similar to that of an adult. Right axis deviation, large precordial R waves, and upright T waves are therefore normal in the neonate. The T wave in lead V1 inverts by 7 days and typically remains inverted until at least age 7 years. Upright T waves in the right precordial leads (V1 to V3) between ages 7 days and 7 years are a potentially important abnormality and usually indicate right ventricular hypertrophy. The QRS complex also reflects these changes. At birth, the mean QRS axis lies between + 60 deg and + 160 deg, R waves are prominent in the right precordium, and S waves are prominent in the left precordium. By age 1 year, the axis changes gradually to lie between + 10 deg and + 100 deg.
The resting heart rate decreases from about 140 beats/min at birth to 120 beats/min at age 1 year, 100 at 5 years, and adult values by 10 years. The PR interval decreases from birth to age 1 year and then gradually increases throughout childhood. The P wave duration and the QRS duration also increase with age. The QT interval depends on heart rate and age, increasing with age while decreasing with heart rate. Q waves are normally seen in the inferior or lateral leads but signify disease if present in other leads.
| Age | PR interval (ms) | QRS duration (ms) | R wave (S wave) amplitude (mm) | |
| Lead V1 | Lead V6 | |||
| Birth | 80-160 | < 75 | 5-26 (1-23) | 0-12 (0-10) |
| 6 months | 70-150 | < 75 | 3-20 (1-17) | 6-22 (0-10) |
| 1 year | 70-150 | < 75 | 2-20 (1-20) | 6-23 (0-7) |
| 5 years | 80-160 | < 80 | 1-16 (2-22) | 8-25 (0-5) |
| 10 years | 90-170 | < 85 | 1-12 (3-25) | 9-26 (0-4) |
Normal 12 lead electrocardiogram from 3 day old baby boy showing right axis deviation, dominant R wave in leads V4R and V1, and still predominantly upright T wave in V1. Persistence of upright T waves in right precordial leads beyond first week of life is sign of right ventricular hypertrophy
Electrocardiogram from 12 year old (late childhood) (axis is now within normal "adult" range and R wave is no longer dominant in right precordial leads)