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The ECG in healthypeople

1

1The normal cardiac rhythm 3

The heart rate 3

Extrasystoles 7

The P wave 7

The PR interval 11

The QRS complex 13

The ST segment 29

The T wave 33

The QT interval 42

The ECG in athletes 43

The ECG in pregnancy 47

The ECG in children 47

Frequency of ECG abnormalities in healthypeople 49

What to do 49

For the purposes of this chapter, we shall assumethat the subject from whom the ECG was recordedis asymptomatic, and that physical examination hasrevealed no abnormalities. We need to consider therange of normality of the ECG, but of course wecannot escape from the fact that not all diseasecauses symptoms or abnormal signs, and a subjectwho appears healthy may not be so and maytherefore have an abnormal ECG. In particular,individuals who present for screening may wellhave symptoms about which they have not consulteda doctor, so it cannot be assumed that an ECGobtained through a screening programme has comefrom a healthy subject.

The range of normality in the ECG is thereforedebatable. We first have to consider the variations inthe ECG that we can expect to find in completelyhealthy people, and then we can think about thesignificance of ECGs that are undoubtedlyabnormal.

2The ECG in healthy people

Sinus arrhythmia becomes less marked withincreasing age of the subject, and is lost inconditions such as diabetic autonomic neuropathydue to impairment of the vagus nerve function.

THE HEART RATE

There is no such thing as a normal heart, and theterms tachycardia and bradycardia should be usedwith care. There is no point at which a high heart

I VR V1 V4

II VL V2 V5

III VF V3 V6

II

Fig. 1.1

THE NORMAL CARDIAC RHYTHM

Sinus rhythm is the only normal sustained rhythm.In young people the RR interval is reduced (that is,the heart rate is increased) during inspiration, andthis is called sinus arrhythmia (Fig. 1.1). When sinusarrhythmia is marked, it may mimic an atrialarrhythmia. However, in sinus arrhythmia eachPQRST complex is normal, and it is only theinterval between them that changes.

other abnormalities on examination, and her bloodcount and thyroid function tests were normal.

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

1The normal cardiac rhythm / The heart rate

3

rate in sinus rhythm has to be called sinustachycardia and there is no upper limit for sinusbradycardia. Nevertheless, unexpectedly fast orslow rates do need an explanation.

SINUS TACHYCARDIA

The ECG in Figure 1.2 was recorded from a youngwoman who complained of a fast heart rate. She hadno other symptoms but was anxious. There were no

Sinus arrhythmia

Note Marked variation in RR interval Constant PR interval Constant shape of P wave and QRS complex

Pain, fright, exercise Hypovolaemia Myocardial infarction Heart failure Pulmonary embolism Obesity Lack of physical fitness Pregnancy Thyrotoxicosis Anaemia Beri-beri CO2 retention Autonomic neuropathy Drugs:

sympathomimetics salbutamol (including by inhalation) caffeine atropine

Box 1.1 Possible causes of sinus rhythm with a fastheart rate


I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.2

I VR V1 V4

II VL V2 V5

III VF V3 V6

II

Fig. 1.3

SINUS BRADYCARDIA

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

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

1The heart rate

5

Sinus tachycardia

Note Normal PQRST waves RR interval 500 ms Heart rate 120/min

Sinus bradycardia

Note Sinus rhythm Rate 44/min One junctional escape beat

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

Box 1.2 Possible causes of sinus rhythm with a slowheart rate

Junctional escape beat


I VR V1 V4

II VL V2 V5

III VF V3 V6

II

Fig. 1.4

I VR V1 V4

II VL V2 V5

III VF V3 V6

II

Fig. 1.5

EXTRASYSTOLES

Supraventricular extrasystoles, either atrial orjunctional (AV nodal), occur commonly in normalpeople and are of no significance (Fig. 1.4). Atrialextrasystoles have an abnormal P wave; in junctionalextrasystoles, either there is no P wave or the P wavemay follow the QRS complex.

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

THE P WAVE

In sinus rhythm, the P wave is normally upright inall leads except VR. When the QRS complex ispredominantly downward in lead VL, the P wavemay also be inverted (Fig. 1.6).

A notched or bifid P wave is the hallmark of leftatrial hypertrophy, and peaked P waves indicateright atrial hypertrophy but bifid or peaked Pwaves can also be seen with normal hearts.

In patients with dextrocardia the P wave isinverted in lead I (Fig. 1.7). In practice this is moreoften seen if the limb leads have been wronglyattached, but dextrocardia can be recognized if leadsV5 and V6, which normally look at the leftventricle, show a predominantly downward QRScomplex.

If the ECG of a patient with dextrocardia isrepeated with the limb leads reversed, and the chestleads are placed on the right side of the chest insteadof the left, in corresponding positions, the ECGbecomes like that of a normal patient (Fig. 1.8).

1Extrasystoles / The P wave

7

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 extrasystole

Note Sinus rhythm, with one ventricular extrasystole Extrasystole has a wide and abnormal QRS complex

and an abnormal T wave

Ventricular extrasystole


I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.6

I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.7

1The P wave

9

Normal ECG

Note In both leads VR and VL the P wave is

inverted, and the QRS complex ispredominantly downward

Inverted P wave in lead VL

Dextrocardia

Note Inverted P wave in lead I No left ventricular complexes

seen in leads V5V6

Inverted P wave anddominant S wave inlead I

Persistent S wave inlead V6

10

The ECG in healthy people

I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.8

I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.9

THE PR INTERVAL

In sinus rhythm, the PR interval is constant and thenormal range is 120200 ms (35 small squares ofECG paper) (Fig. 1.9).

A PR interval of less than 120 ms suggests pre-excitation, and a PR interval of longer than 200 msis due to first degree block. Both of theseabnormalities are seen in normal people, and willbe discussed further in Chapter 2.

1The P wave / The PR interval

11

Dextrocardia, leads reversed

Note P wave in lead I upright QRS complex upright in lead I Typical left ventricular complex in lead V6

Upright P wave andQRS complex in lead I

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

Normal QRScomplex in lead V6

12

The ECG in healthy people

I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.10

I VR V1 V4

II VL V2 V5

III VF V3 V6

Fig. 1.11

THE QRS COMPLEX

THE CARDIAC AXIS

There is a fairly wide range of normality in thedirection of the cardiac axis. In most people theQRS complex is tallest in lead II, but in leads I andIII the QRS complex is also predominantly upright(i.e. the R wave is greater than the S wave)(Fig. 1.10).

The cardiac axis is still perfectly normal when theR wave and S wave are equal in lead I: this iscommon in tall people (Fig. 1.11).

When the S wave is greater than the R wave inlead I, right axis deviation is present. However, thisis very common in perfectly normal people. TheECG in Figure 1.12 is from a professional footballer.

It is common for the S wave to be greater thanthe R wave in lead III, and the cardiac axis can stillbe considered normal when the S wave equals the Rwave in lead II. These patterns are common in fatpeople and during pregnancy (Fig. 1.13).

When the depth of the S wave exceeds the heightof the R wave in lead II, left axis deviation is present(see Fig 2.26).

1The QRS complex

13

Normal ECG

Note QRS complex upright in leads IIII R wave tallest in lead II

Normal ECG

Note This record shows the rightward limit of normality of

the cardiac axis R and S waves equal in lead I

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