The ECG in healthy people


The ECG in healthy people

The ECG is frequently used as a screening tool whether from a truly asymptomatic, apparently ‘healthy’, subject (e.g. for an employment medical) or as part of the battery of initial investigations in patients presenting with new symptoms of uncertain significance or cause. ECG findings should always be interpreted in the clinical context in which it was taken. Over-interpretation of normal variations in the ECG may lead to misdiagnosis and risk initiation of unnecessary investigations and inappropriate management. Understanding variations in the ECG that we can expect to find in completely healthy people is therefore a key prerequisite to the accurate interpretation of ECGs that appear ‘abnormal’.

Types of ECG

ECG traces come in many guises including continuous single channel heart monitoring, 3-lead rhythm assessment, and even internal electrograms obtained from implanted devices or during cardiac procedures. The most ‘complete’ external ECG is the traditional 12-lead trace (Fig. 1.1). Accurate lead position (Fig. 1.2) is key as misplaced leads change the appearances of the trace and may lead to misinterpretation. For example, limb lead switches can resemble abnormalities of the cardiac axis (Fig. 1.3), whilst alterations of chest lead position, for example due to displacement in obese patients or by breast tissue, may resemble cardiac rotation with delayed anterior R-wave progression (see Fig. 1.20).


Fig. 1.1 Normal ECG


Fig. 1.2 ECG lead positions for the chest and upper limb leads. Correct positioning of the lower limb leads is also required for a 12-lead ECG (not shown here).

The ‘normal’ ECG

The normal cardiac rhythm

Sinus rhythm is the only normal sustained rhythm (see Fig. 1.1). In young people the R–R interval is reduced (i.e. the heart rate is increased) during inspiration, and this is called sinus arrhythmia (Fig. 1.4). When sinus arrhythmia is marked, it may mimic an atrial arrhythmia. However, in sinus arrhythmia each P–QRS–T complex is normal, and it is only the interval between them that changes.

Sinus arrhythmia becomes less marked with increasing age of the subject, and is lost in conditions such as diabetic autonomic neuropathy due to impairment of vagus nerve function.

The heart rate

There is no such thing as a normal heart rate, and the terms ‘tachycardia’ and ‘bradycardia’ should be used with care. There is no point at which a high heart rate in sinus rhythm has to be called ‘sinus tachycardia’ and there is no lower limit for ‘sinus bradycardia’. Nevertheless, unexpectedly fast or slow rates do need an explanation.

Sinus tachycardia

The ECG in Fig. 1.5 was recorded from a young woman who complained of a fast heart rate. She had no other symptoms, but was anxious. There were no other abnormalities on examination, and her blood count and thyroid function tests were normal.

Box 1.1 shows possible causes of sinus rhythm with a fast heart rate.

The P wave

In sinus rhythm, the P wave is normally upright in all leads except VR. When the QRS complex is predominantly downward in lead VL, the P wave may also be inverted (Fig. 1.10).

In patients with dextrocardia the P wave is inverted in lead I (Fig. 1.11). In practice this is more often seen if the limb leads have been wrongly attached (Fig. 1.3), but dextrocardia can be recognized if leads V5 and V6, which normally ‘look at’ the left ventricle, show a predominantly downward QRS complex.

If the ECG of a patient with dextrocardia is repeated with the limb leads reversed, and the chest leads are placed on the right side of the chest instead of the left, in corresponding positions, the ECG becomes like that of a normal patient (Fig. 1.12).

A notched or bifid P wave (P mitrale) is the hallmark of left atrial hypertrophy, and peaked P waves (P pulmonale) indicate right atrial hypertrophy – but bifid or peaked P waves can also be seen with normal hearts (Fig. 1.13) and are not particularly clinically useful features.

The PR interval

In sinus rhythm, the PR interval is constant and its normal range is 120–200 ms (3–5 small squares of ECG paper) (see Fig. 1.1). In atrial extrasystoles, or ectopic atrial rhythms, the PR interval may be short, and a PR interval of less than 120 ms suggests pre-excitation (see Figs 2.8, 2.9, 2.10).

A PR interval of longer than 220 ms may be due to first degree block, but the ECGs of healthy individuals, especially athletes, may have PR intervals of slightly longer than 220 ms – which can be ignored in the absence of any other indication of heart disease.

The QRS complex

The cardiac axis

There is a fairly wide range of normality in the direction of the cardiac axis. In most people the QRS complex is tallest in lead II, but in leads I and III the QRS complex is also predominantly upright (i.e. the R wave is greater than the S wave) (Fig. 1.14).

The cardiac axis is still perfectly normal when the R wave and S wave are equal in lead I: this is common in tall people (Fig. 1.15).

When the S wave is greater than the R wave in lead I, right axis deviation is present. However, this is very common in perfectly normal people. The ECG in Fig. 1.16 is from a professional footballer.

It is common for the S wave to be greater than the R wave in lead III, and the cardiac axis can still be considered normal when the S wave equals the R wave in lead II (Fig. 1.17). These patterns are common in obese people and during pregnancy.

When the depth of the S wave exceeds the height of the R wave in lead II, left axis deviation is present (see Figs 2.22 and 2.23).

Limb lead switches can sometimes be misinterpreted as abnormalities of cardiac axis (see Fig. 1.3).

Apr 16, 2020 | Posted by in CARDIOLOGY | Comments Off on The ECG in healthy people

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