The ECG in Healthy People

For the purposes of this chapter, we shall assume that the subject from whom the ECG was recorded is asymptomatic, and that physical examination has revealed no abnormalities. We need to consider the range of normality of the ECG, but of course we cannot escape from the fact that not all disease causes symptoms or abnormal signs, and a subject who appears healthy may not be so and may therefore have an abnormal ECG. In particular, individuals who present for screening may well have symptoms about which they have not consulted a doctor, so it cannot be assumed that an ECG obtained through a screening programme has come from a healthy subject.

The range of normality in the ECG is therefore debatable. We first have to consider the variations in the ECG that we can expect to find in completely healthy people, and then we can think about the significance of ECGs that are undoubtedly ‘abnormal’.

THE ‘NORMAL’ ECG

THE NORMAL CARDIAC RHYTHM

Sinus rhythm is the only normal sustained rhythm. In young people the R-R interval is reduced (i.e. the heartrate is increased) during inspiration, and this is called sinus arrhythmia ( Fig. 1.1). 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.

Fig. 1.1 Sinus arrhythmia
Note

• Marked variation in R-R interval

• Constant PR interval

• Constant shape of P wave and QRS comple

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 the 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 upper limit for ‘sinus bradycardia’. Nevertheless, unexpectedly fast or slow rates do need an explanation.

The ECG in Figure 1.2 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.

Fig. 1.2 Sinus tachycardia
Note

• Normal P-QRS-T waves

• R-R interval 500 ms

• Heart rate 120/min

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

• Autonomic neuropathy

• Drugs:

– sympathomimetics

– salbutamol (including by inhalation)

– caffeine

– atropine

SINUS BRADYCARDIA

The ECG in Figure 1.3 was recorded from a young professional footballer. His heart rate was 44/min, and at one point the sinus rate became so slow that a junctional escape beat appeared.

Fig. 1.3 Sinus bradycardia
Note

• Sinus rhythm

• Rate 44/min

• One junctional escape beat

Junctional escape beat

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)

– verapamil

– digoxin

EXTRASYSTOLES

Supraventricular extrasystoles, either atrial or junctional (AV nodal), occur commonly in normal people and are of no significance. Atrial extrasystoles ( Fig. 1.4) have an abnormal P wave; in junctional extra-systoles either there is no P wave or the P wave may follow the QRS complex.

Fig. 1.4 Supraventricular extrasystole
Note

• In supraventricular extrasystoles the QRS complex and the T wave are the same as in the sinus beat

• The fourth beat has an abnormal P wave and therefore an atrial origin

Early abnormal P wave

Ventricular extrasystoles are also commonly seen in normal ECGs ( Fig. 1.5).

Fig. 1.5 Ventricular extrasystole
Note

• Sinus rhythm, with one ventricular extrasystole

• Extrasystole has a wide and abnormal QRS complex and an abnormal T wave

Ventricular extrasystole

In healthy people, normal sinus rhythm may be replaced by what are, in effect, repeated atrial extra-systoles. This is sometimes called an ‘ectopic atrial rhythm’ and it is of no particular significance ( Fig. 1.6).

Fig. 1.6 Normal variant: ectopic atrial rhythm
Note

• Sinus rhythm

• Inverted P wave in leads II-III, VF, V₄-V₆

• Constant PR interval

Inverted P waves in lead II

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. 1.7).

Fig. 1.7 Normal ECG
Note

• In both leads VR and VL the P wave is inverted, and the QRS complex is predominantly downward

Inverted P wave in lead VL

In patients with dextrocardia the P wave is inverted in lead I ( Fig. 1.8). In practice this is more often seen if the limb leads have been wrongly attached, but dextrocardia can be recognized if leads V₅ and V₆, which normally ‘look at’ the left ventricle, show a predominantly downward QRS complex.

Fig. 1.8 Dextrocardia
Note

• Inverted P wave in lead I

• No left ventricular complexes seen in leads V₅-V₆

Inverted P wave and dominant S wave in lead I

Persistent S wave in lead V₆

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. 1.9).

Fig. 1.9 Dextrocardia, leads reversed
Note

• Same patient as in Figure 1.8

• P wave in lead I upright

• QRS complex upright in lead I

• Typical left ventricular complex in lead V₆

Upright P wave and QRS complex in lead I

Normal QRS complex in lead V₆

A notched or bifid P wave is the hallmark of left atrial hypertrophy, and peaked P waves indicate right atrial hypertrophy – but bifid or peaked P waves can also be seen with normal hearts ( Fig. 1.10).

Fig. 1.10 Normal ECG
Note

• Sinus rhythm

• Bifid P waves, best seen in leads V₂-V₄

• Peaked T waves and U waves, best seen in leads V₂-V₃ – normal variants

Bifid P wave in lead V₃

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) ( Fig. 1.11). 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.

Fig. 1.11 Normal ECG
Note

• PR interval 170 ms

• PR interval constant in all leads

• Notched P wave in lead V₅ is often normal

PR interval 170 ms

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.

PR interval ‘abnormalities’ will be discussed further in the context of normal people in Chapter 2.

THE QRS COMPLE

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. 1.12).

Fig. 1.12 Normal ECG
Note

• QRS complex upright in leads I-III

• R wave tallest in lead II

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. 1.13).

Fig 1.13 Normal ECG
Note

• This record shows the ‘rightward’ limit of normality of the cardiac axis

• R and S waves equal in lead I

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 Figure 1.14 is from a professional footballer.

Fig 1.14 ?Normal ECG
Note

• Right axis deviation: S wave greater than R wave in lead I

• Upright QRS complexes in leads II-III

Dominant S wave in lead I

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. 1.15). These patterns are common in fat people and during pregnancy.

Fig 1.15 Normal ECG
Note

• This shows the ‘leftward limit’ of normality of the cardiac axis

• S wave equals R wave in lead II

• S wave greater than R wave in lead III

S wave = R wave in lead II

S wave > R wave in lead III

When the depth of the S wave exceeds the height of the R wave in lead II, left axis deviation is present (see Figs 2.25 and 2.26).

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. 1.16).

Fig 1.16 Normal ECG
Note

• Lead V₁ shows a predominantly downward complex, with the S wave greater than the R wave

• Lead V₆ shows an upright complex, with a dominant R wave and a tiny S wave

S wave > R wave in lead V₁

Dominant R wave in lead V₆

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. Figure 1.17 shows an ECG in which the transition point is somewhere between leads V₃ and V₄.

Fig 1.17 Normal ECG
Note

• In lead V₃ there is a dominant S wave

• In lead V₄ there is a dominant R wave

• The transition point is between leads V₃ and V₄

Figure 1.18 shows an ECG with a transition point between leads V₄ and V₅, and Figure 1.19 shows an ECG with a transition point between leads V₂ and V₃.