The effects of other conditions on the ECG
The ECG is not a good method for investigating or diagnosing any condition that is not primarily cardiac. However, some generalized diseases do affect the ECG: it is important to recognize this, and not assume that a patient has heart disease simply because their ECG seems abnormal.
Artefacts in ECG recordings
The effects of abnormal muscle movement
Although ECG recorders are designed to be especially sensitive to the electrical frequencies of cardiac muscle contraction, the ECG will also record the contraction of skeletal muscles. The most common pattern of ‘ECG abnormality’ is a high-frequency oscillation due to general muscular tension in a patient who is not properly relaxed.
Sustained involuntary tremors, such as those associated with Parkinsonism (Fig. 8.1) cause rhythmic ECG abnormalities that may be confused with cardiac arrhythmias.
Hypothermia
Hypothermia causes shivering, and therefore artefacts due to muscular activity. However, there can be other changes in the ECG, and the characteristic ECG feature of hypothermia is the ‘]’ wave. This is a small hump seen at the end of the QRS complex (Fig. 8.2).
The ECG in Fig. 8.2 was recorded from a 76-year-old woman who was admitted to hospital with a temperature of 30°C after lying for a prolonged period in a freezing house, after a fall. She initially had a heart rate of 26 bpm, and the rhythm was atrial flutter. J waves can be seen in the lateral chest leads. On re-warming, she began to shiver, and, despite the muscle artefact, her heart can be seen to have reverted to sinus rhythm with first degree block. J waves are still visible (Fig. 8.3). When her temperature had returned to normal, the PR interval normalized, and the J waves disappeared (Fig. 8.4).
The ECG in congenital heart disease
The ECG provides a limited amount of help in the diagnosis of congenital heart disease by showing which chambers of the heart are enlarged. It is important to remember (see Ch. 1, p. 52) that at birth the ECG of a normal infant shows a pattern of ‘right ventricular hypertrophy’, and this gradually disappears during the first 2 years of life.
If the infant pattern persists beyond the age of 2 years, right ventricular hypertrophy is indeed present. If there is a left ventricular, or normal adult, pattern before this age, then left ventricular hypertrophy is probably present. In older children, the criteria for left and right ventricular hypertrophy are the same as in adults.
Box 8.1 lists some common congenital disorders and the associated ECG appearances.
The ECG in Fig. 8.5 shows all the features of severe right ventricular hypertrophy: it came from a boy with severe pulmonary stenosis.
The ECG in Fig. 8.6 shows left ventricular hypertrophy, and was recorded in an 8-year-old with severe aortic stenosis.
The ECG in Fig. 8.7 shows right ventricular hypertrophy, and came from a young woman who had had a partial correction of Fallot’s tetralogy 20 years previously.
The ECG in Fig. 8.8 suggests right atrial hypertrophy and shows right bundle branch block (RBBB). It came from a teenager with Ebstein’s anomaly and an atrial septal defect.
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