THE NORMAL CARDIAC RHYTHM

Sinus rhythm is the only truly normal rhythm. ‘Sinus bradycardia’ is sometimes said to be present when the heart rate is below 60/min, and ‘sinus tachycardia’ is sometimes used for heart rates above 100/min ( Box 5.1), but these terms are really not helpful, and it is far more useful to describe a patient as having ‘sinus rhythm at x/min’ ( Fig. 5.2).

THE P WAVE

Tall P waves may be due to right atrial hypertrophy, and are significant if there is evidence of right ventricular hypertrophy as well. Tall P waves alone may indicate tricuspid stenosis, but this is rare. If the patient is well and has no abnormal physical signs, ‘tall’ P waves are probably within the normal limits.

For an example mitral stenosis, see p. 293

Bifid P waves in the absence of signs of associated left ventricular hypertrophy can indicate mitral stenosis (now fairly rare), but a bifid and not particularly prolonged P wave is often seen in the anterior leads of normal ECGs. Figure 5.7 shows the ECG of an asymptomatic patient with a clinically normal heart.

The P waves of atrial extrasystoles tend to be abnormally shaped compared to the P waves of the sinus beats of the same patient ( Fig. 5.4).

For more on hyperkalaemia, see p. 331

P waves cannot always be seen in all leads, but if there is a total absence of P waves the rhythm is probably not sinus and may be sinus arrest, junctional escape or atrial fibrillation; or the patient may have hyperkalaemia.

CONDUCTION

The upper limit of the PR interval in a normal ECG is usually taken as 220 ms, a longer PR interval indicating first degree heart block. However, the ECGs of healthy individuals, especially athletes, not uncommonly have PR intervals slightly longer than 220 ms, and these can be ignored in the absence of any other indication of heart disease.

The ECG in Figure 5.8 was recorded from a healthy, asymptomatic individual at a screening examination. Nevertheless, PR interval prolongation to this extent is probably evidence of disease of the conducting tissue.

Second degree heart block of the Mobitz 1 (Wenckebach) type may be seen in athletes, but otherwise second and third degree block are indications of heart disease.

A QRS complex that is predominantly downward (that is, the S wave is greater than the R wave) in leads II and III indicates left axis deviation, and a downward QRS complex in lead II, still within the normal limit of 120 ms, indicates left anterior hemiblock ( Fig. 5.9).

Right axis deviation is indicated if the QRS complexes are predominantly downward in lead I. It is common in healthy subjects, particularly if they are tall, as with the ECG in Figure 5.10, and under these circumstances is unimportant unless there is some other evidence of right ventricular hypertrophy, or the patient has had a myocardial infarction, raising the possibility of left posterior hemiblock.

THE QRS COMPLEX

Depolarization of the whole ventricular muscle mass should occur within 120 ms, so this represents the maximum width of the normal QRS complex. Any widening indicates conduction delay or failure within the bundle branch system, pre-excitation (see below), or a ventricular origin of depolarization – any of which may be seen in healthy subjects.

Left bundle branch block is always a sign of heart disease. Right bundle branch block with a QRS complex duration greater than 120 ms is sometimes seen in healthy subjects, but should be taken as a warning of things like an atrial septal defect. Partial (incomplete) right bundle branch block (RSR¹ pattern in lead V₁, but with a QRS complex duration less than 120 ms; Fig. 5.11) is very common and of no significance ( Box 5.2).

The height of the QRS complex is related to the thickness of heart muscle, but is actually a poor indicator of ventricular hypertrophy.

Right ventricular hypertrophy causes a dominant R wave in lead Vj, but unless there are other significant ECG signs (right axis deviation, or T wave inversion in leads V₂-V₃), this can be a normal variant ( Fig. 5.12).

One ECG feature of left ventricular hypertrophy is the increased height of the QRS complex in the leads that ‘look at’ the left ventricle ( Fig. 5.13), the generally accepted upper limit of normality being a QRS complex height of 25 mm in lead V₅ or V₆. The Sokolow-Lyon criteria define left ventricular hypertrophy as being present when the sum of the R wave height in lead V₅ or V₆ plus the depth of the S wave in lead V₁ exceeds 35 mm. In fact these criteria are unreliable, and a QRS complex height greater than 25 mm is often seen in fit young men. Left ventricular hypertrophy can only be diagnosed with confidence when tall QRS complexes are associated with inverted T waves in the lateral leads (see Ch. 4). This is sometimes referred to as a ‘strain’ pattern, but this term is essentially meaningless.

If the QRS complexes seem too small to be consistent with the clinical findings, check the calibration of the ECG recorder. If this is correct, possible explanations of small QRS complexes are obesity, emphysema and pericardial effusion.

For more on left ventricular hypertrophy, see pp. 295–302

Q waves are the hallmark of the fully developed changes of an ST segment elevation myocardial infarction, but they also result from septal depolarization (see p. 17). Narrow Q waves in the inferior and lateral leads ( Fig. 5.14), and sometimes even quite deep ones, may also be perfectly normal.

A Q wave in lead III but not in lead VF is likely to be normal, even when associated with an inverted T wave ( Fig. 5.15). These features often disappear if the ECG is repeated with the subject taking and holding in a deep breath.

THE ST SEGMENT

If the ST segment is raised following an S wave it is described as ‘high take-off’, and is a normal variant ( Fig. 5.16); this pattern is typically seen in the anterior leads. It is important to differentiate this from the raised ST segment of an ST segment elevation myocardial infarction.

Horizontal ST segment depression is a sign of ischaemia (see Ch. 4), but minor degrees of depression, often downward-sloping, are seen in the ECGs of normal people, and are best described as ‘nonspecific’ ( Fig. 5.17).

THE T WAVE

The T wave is almost always inverted in lead VR, usually in lead V₁ and sometimes in lead III. Occasionally, a normal heart may be associated with an inverted T wave in lead V₂, and in black people there may be T wave inversion in leads V₃ and V₄ as well ( Fig. 5.18). This can lead to an incorrect diagnosis of a non-ST segment elevation myocardial infarction (non-Q wave infarction).

Tall and peaked T waves ( Fig. 5.19) are sometimes seen in the early stages of a myocardial infarction, when they may be described as ‘hyperacute’ changes. Peaked T waves are also associated with hyperkalaemia, but in fact some of the tallest and most peaked T waves are seen in perfectly normal ECGs.

U WAVES

Flat U waves following flat T waves, together with a prolonged QT interval, may be a sign of hypokalaemia. However, the best examples of prominent U waves come from normal people ( Fig. 5.20).

For more on hypokalaemia, see pp. 331–334

THE ECG IN ATHLETES

Athletes’ ECGs can show a wide variety of changes that could be considered ‘abnormal’ (see Box 5.3).