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).

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.

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.

Box 1.1

Possible Causes of Sinus Rhythm With a Fast Heart Rate

• • Pain, fright, exercise
  
• • Hypovolaemia
  
• • Myocardial infarction
  
• • Heart failure
  
• • Pulmonary embolism
  
• • Obesity
  
• • Lack of physical fitness
  
• • Pregnancy
  
• • Thyrotoxicosis
  
• • Anaemia
  
• • CO₂ retention
  
• • Autonomic neuropathy
  
• • Drugs:
  
  
  
  • • sympathomimetics
    
  • • salbutamol (including by inhalation)
    
  • • caffeine
    
  • • atropine

Sinus bradycardia

The ECG in Fig. 1.6 was recorded from a young professional footballer. His heart rate was 44 bpm, and at one point the sinus rate became so slow that a junctional escape beat appeared.

The possible causes of sinus rhythm with a slow heart rate are summarized in Box 1.2.

Box 1.2

Possible Causes of Sinus Rhythm With a Slow Heart Rate

• • Physical fitness
  
• • Vasovagal attacks
  
• • Sick sinus syndrome
  
• • Acute myocardial infarction, especially inferior
  
• • Hypothyroidism
  
• • Hypothermia
  
• • Obstructive jaundice
  
• • Raised intracranial pressure
  
• • Drugs:
  
  
  
  • • beta-blockers (including eye drops for glaucoma)
    
  • • calcium channel blockers (e.g. verapamil)
    
  • • digoxin
    
  • • ibabradine

Extrasystoles

Supraventricular extrasystoles, either atrial or junctional (atrioventricular [AV] nodal), occur commonly in normal people and are of no significance. Atrial extrasystoles (Fig. 1.7) have an abnormal P wave; in junctional extrasystoles either there is no P wave or the P wave may follow the QRS complex.

In healthy people, normal sinus rhythm may be replaced by what are, in effect, repeated atrial extrasystoles. This is sometimes called an ‘ectopic atrial rhythm’ and it is of no particular significance (Fig. 1.8).

Ventricular extrasystoles are also commonly seen in normal ECGs (Fig. 1.9). Ventricular extrasystoles are almost universal, but when very frequent they indicate populations at increased risk and may merit further investigation (see Ch. 4, p. 101).

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 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).

The size of R and S waves in the chest leads

In lead V₁ there should be a small R wave and a deep S wave, and the balance between the two should change progressively from V₁ to V₆. In lead V₆ there should be a tall R wave and no S wave (Fig. 1.18).

Typically the ‘transition point’, when the R and S waves are equal, is seen in lead V₃ or V₄ but there is quite a lot of variation. Fig. 1.19 shows an ECG in which the transition point is somewhere between leads V₃ and V₄.