Interpretation of electrocardiograms in infants and children.

Electrocardiography is a useful tool in the management of paediatric heart disease. Interpretation requires practice due to changing norms which are brought about by the physiological changes that occur in the circulatory system in this age group. This article outlines these normal changes.

1. The Normal Electrocardiogram

As the child develops from the foetus to the neonate, infant, child, adolescent and adult, growth and development result in major changes of body size and shape, and with this the size and position of the heart relative to the body and cardiac physiology.

The most dramatic of these changes occur at birth and within the first year of life. In order to be able to interpret paediatric electrocardiograms (ECGs) it is therefore imperative, to have an understanding of these events.

Prior to birth, the ventricular pressures are equal and the pulmonary resistance is higher than the systemic, resulting in the right ventricular (RV) size and mass being larger than the left ventricular (LV) from 35 weeks gestation.

The placenta which receives 55% of the combined cardiac output provides a low resistance circuit to the systemic circulation. Right to left shunting occurs from the pulmonary artery to aorta via the patent ductus arteriosus (PDA) and from the right atrium to the left atrium via the foramen ovale. The small pulmonary arteries have more muscle than the small systemic arteries and pulmonary vasoconstriction further elevates the pulmonary resistance.

The dramatic changes at birth, with the removal of the low resistance placental circulation, the fall in pulmonary resistance as the lungs open and the functional (later permanent) closure of the PDA and foramen ovale, require significant physiological adaptation. Most of these changes occur rapidly over the first few hours and days of life but continue throughout the early childhood period and more gradually into adulthood. The left ventricle grows rapidly. By 1 month of age the LV/RV ratio has changed to 1.5:1 from the birth ratio 0.8:1, by 6 months 2:1 and then slowly to the adult ratio 2.5:1.

With these rapidly changing haemodynamics, one can expect the interpretation of the neonatal ECG to be the most challenging. There are many normal variables with a wide overlap from normal to abnormal. The less than 35 week gestation premature infant will have a different ECG from the full term infant. The normal infant ECG changes rapidly over the first few weeks of life and it is not until 3 years of age that it begins to resemble that of an adult. Significant differences, however, persist. Unless the interpreter is aware of these, the young patient is in danger of becoming an “electrocardiographic casualty” like several normal children I have seen with the ECG diagnosis of “acute myocardial infarction.”

In order to interpret paediatric electrocardiograms, the age of the patient and a table of normal values are essential. Davignon et al published the most recent values for a normal population. 2141 white children were divided into 12 different age groups, 7 within the first year of life.

Important Normal Variants

T wave inversion:

Infants older than 48 hours of age should have inverted T waves in the right praecordial leads. These findings persist throughout childhood with inversion to V4 being accepted as normal. There is a progressive change to an upright T wave across the praecordial leads from left to right as the child grows older. Until 8 years of age an upright T wave in V1 is considered a sign of right ventricular hypertrophy. Many children will show persistence of an inverted T wave in V1 until their late teens.

RSR’ complex

7% normal children under 5 years of age plus a few older children, will show an RSR’ complex in the right praecordial leads. To be considered normal the width of the QRS should be no more than 10msec longer than normal and the R’ voltage in V1 should be less than 15mm in infants under 1 year and less than 10mm over 1 year.

Elevated J point

Early repolarization is commonly seen in adolescence with an elevated J point, most obvious in the mid praecordial leads. This is a completely normal finding and must be distinguished from pathological elevation of the ST segments.

Reading and Interpreting Electrocardiograms

Unless the patient's age is known, the paediatric ECG cannot be interpreted. With the age in mind, the tracing may then be read objectively but to be interpreted fully additional clinical information is required, including:

Indication

Clinical diagnosis: cardiac and other

Medications: cardiovascular drugs, others eg. cisapride, tricyclics

Electrolytes

The electrocardiogram should be read systematically:

Heart rate

P wave axis

Rhythm

QRS axis

Intervals PR, QRS, QT/QTc

P wave amplitude and duration

QRS amplitude, R/S ratio, Q waves

ST segments and T wave

Normal tracings

Examples of normal tracings for the different age groups follow, preceded by a general description of the characteristics of that age group.

Example: 1day old infant

Example: 2 day old infant

Example: 3 week old infant

Example: 2 month old infant

Example: 3 month old infant

Example: 8 month old infant

Example: 23 month old female

Example: 2 year old female

Example: 6 year old female

Example: 15 year old male