The electrical discharge for each cardiac cycle normally starts in a special area of the right atrium called the ‘sinoatrial (SA) node’ ( Fig. 1.1). Depolarization then spreads through the atrial muscle fibres. There is a delay while depolarization spreads through another special area in the atrium, the ‘atrioventricular node’ (also called the ‘AV node’, or sometimes just ‘the node’). Thereafter, the depolarization wave travels very rapidly down specialized conduction tissue, the ‘bundle of His’, which divides in the septum between the ventricles into right and left bundle branches. The left bundle branch itself divides into two. Within the mass of ventricular muscle, conduction spreads somewhat more slowly, through specialized tissue called ‘Purkinje fibres’.

Fig. 1.1 The wiring diagram of the heart

THE RHYTHM OF THE HEART

As we shall see later, electrical activation of the heart can sometimes begin in places other than the SA node. The word ‘rhythm’ is used to refer to the part of the heart which is controlling the activation sequence. The normal heart rhythm, with electrical activation beginning in the SA node, is called ‘sinus rhythm’.

THE DIFFERENT PARTS OF THE ECG

The muscle mass of the atria is small compared with that of the ventricles, and so the electrical change accompanying the contraction of the atria is small. Contraction of the atria is associated with the ECG wave called ‘P’ ( Fig. 1.2). The ventricular mass is large, and so there is a large deflection of the ECG when the ventricles are depolarized: this is called the ‘QRS’ complex. The ‘T’ wave of the ECG is associated with the return of the ventricular mass to its resting electrical state (‘repolarization’).

Fig. 1.2 Shape of the normal ECG, including a U wave

The letters P, Q, R, S and T were selected in the early days of ECG history, and were chosen arbitrarily. The P Q, R, S and T deflections are all called waves; the Q, R and S waves together make up a complex; and the interval between the S wave and the beginning of the T wave is called the ST ‘segment’.

In some ECGs an extra wave can be seen on the end of the T wave, and this is called a U wave. Its origin is uncertain, though it may represent repolarization of the papillary muscles. If a U wave follows a normally shaped T wave, it can be assumed to be normal. If it follows a flattened T wave, it may be pathological (see Ch. 4).

The different parts of the QRS complex are labelled as shown in Figure 1.3. If the first deflection is downward, it is called a Q wave ( Fig. 1.3a). An upward deflection is called an R wave, regardless of whether it is preceded by a Q wave or not ( Figs 1.3b and 1.3c). Any deflection below the baseline following an R wave is called an S wave, regardless of whether there is a preceding Q wave ( Figs 1.3d and 1.3e).

Fig. 1.3 Parts of the QRS complex
(a) Q wave. (b, c) R waves. (d, e) S waves

TIMES AND SPEEDS

ECG machines record changes in electrical activity by drawing a trace on a moving paper strip. ECG machines run at a standard rate of 25 mm/s and use paper with standard-sized squares. Each large square (5 mm) represents 0.2 second (s), i.e. 200 milliseconds (ms) ( Fig. 1.4). Therefore, there are five large squares per second, and 300 per minute. So an ECG event, such as a QRS complex, occurring once per large square is occurring at a rate of 300/min. The heart rate can be calculated rapidly by remembering the sequence in Table 1.1.

Table 1.1

Relationship between the number of large squares between successive R waves and the heart rate

R-R interval (large squares)	Heart rate (beats/min)
1	300
2	150
3	100
4	75
5	60
6	50

Fig. 1.4 Relationship between the squares on ECG paper and time. Here, there is one QRS complex per second, so the heart rate is 60 beats/min

Just as the length of paper between R waves gives the heart rate, so the distance between the different parts of the P-QRS-T complex shows the time taken for conduction of the electrical discharge to spread through the different parts of the heart.

The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex, and it is the time taken for excitation to spread from the SA node, through the atrial muscle and the AV node, down the bundle of His and into the ventricular muscle. Logically, it should be called the PQ interval, but common usage is ‘PR interval’ ( Fig. 1.5).

Fig. 1.5 The components of the ECG complex

The normal PR interval is 120–220 ms, represented by 3–5 small squares. Most of this time is taken up by delay in the AV node ( Fig. 1.6).

Fig. 1.6 Normal PR interval and QRS complex

If the PR interval is very short, either the atria have been depolarized from close to the AV node, or there is abnormally fast conduction from the atria to the ventricles.

The duration of the QRS complex shows how long excitation takes to spread through the ventricles. The QRS complex duration is normally 120 ms (represented by three small squares) or less, but any abnormality of conduction takes longer, and causes widened QRS complexes ( Fig. 1.7). Remember that the QRS complex represents depolarization, not contraction, of the ventricles – contraction is proceeding during the ECG’s ST segment.

Fig. 1.7 Normal PR interval and prolonged QRS complex

The QT interval varies with the heart rate. It is prolonged in patients with some electrolyte abnormalities, and more importantly it is prolonged by some drugs. A prolonged QT interval (greater than 450 ms) may lead to ventricular tachycardia.

CALIBRATION

A limited amount of information is given by the height of the P waves, QRS complexes and T waves, provided the machine is properly calibrated. A standard signal of 1 millivolt (mV) should move the stylus vertically 1 cm (two large squares) ( Fig. 1.8), and this ‘calibration’ signal should be included with every record.

Fig. 1.8 Calibration of the ECG recording

THE ECG – ELECTRICAL PICTURES

The word ‘lead’ sometimes causes confusion. Sometimes it is used to mean the pieces of wire that connect the patient to the ECG recorder. Properly, a lead is an electrical picture of the heart.

The electrical signal from the heart is detected at the surface of the body through electrodes, which are joined to the ECG recorder by wires. One electrode is attached to each limb, and six to the front of the chest.

The ECG recorder compares the electrical activity detected in the different electrodes, and the electrical picture so obtained is called a ‘lead’. The different comparisons ‘look at’ the heart from different directions. For example, when the recorder is set to ‘lead I’ it is comparing the electrical events detected by the electrodes attached to the right and left arms. Each lead gives a different view of the electrical activity of the heart, and so a different ECG pattern. Strictly, each ECG pattern should be called ‘lead …’, but often the word ‘lead’ is omitted.

The ECG is made up of 12 characteristic views of the heart, six obtained from the ‘limb’ leads (I, II, III, VR, VL, VF) and six from the ‘chest’ leads (Vi-V₆). It is not necessary to remember how the leads (or views of the heart) are derived by the recorder, but for those who like to know how it works, see Table 1.2. The electrode attached to the right leg is used as an earth, and does not contribute to any lead.

Table 1.2

ECG leads

Lead	Comparison of electrical activity
1	LAandRA
II	LLandRA
III	LL and LA
VR	RA and average of (LA + LL)
VL	LA and average of (RA + LL)
VF	LL and average of (LA + RA)
v₁	V₁ and average of (LA + RA + LL)
v₂	V₂ and average of (LA + RA + LL)
v₃	V₃ and average of (LA + RA + LL)
v₄	V₄ and average of (LA + RA + LL)
v₅	V₅ and average of (LA + RA + LL)
v₆	V₆ and average of (LA + RA + LL)

Key: LA, left arm; RA, right arm; LL, left leg.

THE 12-LEAD ECG

ECG interpretation is easy if you remember the directions from which the various leads look at the heart. The six ‘standard’ leads, which are recorded from the electrodes attached to the limbs, can be thought of as looking at the heart in a vertical plane (i.e. from the sides or the feet) ( Fig. 1.9).

Fig. 1.9 The ECG patterns recorded by the six ‘standard’ leads

Leads I, II and VL look at the left lateral surface of the heart, leads III and VF at the inferior surface, and lead VR looks at the right atrium.

The six V leads (V₁-V₆) look at the heart in a horizontal plane, from the front and the left side. Thus, leads V₁ and V₂ look at the right ventricle, V₃ and V₄ look at the septum between the ventricles and the anterior wall of the left ventricle, and V₅ and V₆ look at the anterior and lateral walls of the left ventricle ( Fig. 1.10).

Fig. 1.10 The relationship between the six chest leads and the heart

As with the limb leads, the chest leads each show a different ECG pattern ( Fig. 1.11). In each lead the pattern is characteristic, being similar in individuals who have normal hearts.

Fig. 1.11 The ECG patterns recorded by the chest leads

The cardiac rhythm is identified from whichever lead shows the P wave most clearly – usually lead II. When a single lead is recorded simply to show the rhythm, it is called a ‘rhythm strip’, but it is important not to make any diagnosis from a single lead, other than identifying the cardiac rhythm.

THE SHAPE OF THE QRS COMPLEX

We now need to consider why the ECG has a characteristic appearance in each lead.

THE QRS COMPLEX IN THE LIMB LEADS

The ECG machine is arranged so that when a depolarization wave spreads towards a lead the stylus moves upwards, and when it spreads away from the lead the stylus moves downwards.

Depolarization spreads through the heart in many directions at once, but the shape of the QRS complex shows the average direction in which the wave of depolarization is spreading through the ventricles ( Fig. 1.12).

Fig. 1.12 Depolarization and the shape of the QRS complex
Depolarization (a) moving towards the lead, causing a predominantly upward QRS complex; (b) moving away from the lead, causing a predominantly downward QRS complex; and (c) at right angles to the lead, generating equal R and S waves

If the QRS complex is predominantly upward, or positive (i.e. the R wave is greater than the S wave), the depolarization is moving towards that lead ( Fig. 1.12a). If predominantly downward, or negative (the S wave is greater than the R wave), the depolarization is moving away from that lead ( Fig. 1.12b). When the depolarization wave is moving at right angles to the lead, the R and S waves are of equal size ( Fig. 1.12c). Q waves, when present, have a special significance, which we shall discuss later.

THE CARDIAC AXIS

Leads VR and II look at the heart from opposite directions. When seen from the front, the depolarization wave normally spreads through the ventricles from 11 o’clock to 5 o’clock, so the deflections in lead VR are normally mainly downward (negative) and in lead II mainly upward (positive) ( Fig. 1.13).

Fig. 1.13 The cardiac axis

The average direction of spread of the depolarization wave through the ventricles as seen from the front is called the ‘cardiac axis’. It is useful to decide whether this axis is in a normal direction or not. The direction of the axis can be derived most easily from the QRS complex in leads I, II and III.

A normal 11 o’clock-5 o’clock axis means that the depolarizing wave is spreading towards leads I, II and III, and is therefore associated with a predominantly upward deflection in all these leads; the deflection will be greater in lead II than in I or III ( Fig. 1.14).

Fig. 1.14 The normal axis

When the R and S waves of the QRS complex are equal, the cardiac axis is at right angles to that lead.

If the right ventricle becomes hypertrophied, it has more effect on the QRS complex than the left ventricle, and the average depolarization wave – the axis – will swing towards the right. The deflection in lead I becomes negative (predominantly downward) because depolarization is spreading away from it, and the deflection in lead III becomes more positive (predominantly upward) because depolarization is spreading towards it ( Fig. 1.15). This is called ‘right axis deviation’. It is associated mainly with pulmonary conditions that put a strain on the right side of the heart, and with congenital heart disorders.

Fig. 1.15 Right axis deviation

When the left ventricle becomes hypertrophied, it exerts more influence on the QRS complex than the right ventricle. Hence, the axis may swing to the left, and the QRS complex becomes predominantly negative in lead III ( Fig. 1.16). ‘Left axis deviation’ is not significant until the QRS complex deflection is also predominantly negative in lead II. Although left axis deviation can be due to excess influence of an enlarged left ventricle, in fact this axis change is usually due to a conduction defect rather than to increased bulk of the left ventricular muscle (see Ch. 2).

Fig. 1.16 Left axis deviation

The cardiac axis is sometimes measured in degrees ( Fig. 1.17), though this is not clinically particularly useful. Lead I is taken as looking at the heart from 0°; lead II from + 60°; lead VF from + 90°; and lead III from + 120°. Leads VL and VR look from − 30° and − 150°, respectively.

Fig. 1.17 The cardiac axis and lead angles

The normal cardiac axis is in the range − 30° to + 90°. If in lead II the S wave is greater than the R wave, the axis must be more than 90° away from lead II. In other words, it must be at a greater angle than − 30°, and closer to the vertical (see Figs 1.16 and 1.17), and left axis deviation is present. Similarly, if the size of the R wave equals that of the S wave in lead I, the axis is at right angles to lead I or at + 90°. This is the limit of normality towards the ‘right’. If the S wave is greater than the R wave in lead I, the axis is at an angle of greater than + 90°, and right axis deviation is present ( Fig. 1.15).

MAKING A RECORDING – PRACTICAL POINTS

Now that you know what an ECG should look like, and why it looks the way it does, we need to think about the practical side of making a recording. Some, but not all, ECG recorders produce a ‘rhythm strip’, which is a continuous record, usually of lead II. This is particularly useful when the rhythm is not normal. The next series of ECGs were all recorded from a healthy subject whose ‘ideal’ ECG is shown in Figure 1.22.

Fig. 1.22 A good record of a normal ECG
Note

• The upper three traces show the six limb leads (I, II, III, VR, VL, VF) and then the six chest leads

• The bottom trace is a ‘rhythm strip’, entirely recorded from lead II (i.e. no lead changes)

• The trace is clear, with P waves, QRS complexes and T waves visible in all leads

It is really important to make sure that the electrode marked LA is indeed attached to the left arm, RA to the right arm and so on. If the limb electrodes are wrongly attached, the 12-lead ECG will look very odd ( Fig. 1.23). It is possible to interpret the ECG, but it is easier to recognize that there has been a mistake, and to repeat the recording.

Fig. 1.23 The effect of reversing the electrodes attached to the left and right arms
Note

• Compare with Figure 1.22, correctly recorded from the same patient

• Inverted P waves in lead I

• Abnormal QRS complexes and T waves in lead I

• Upright T waves in lead VR are most unusual

Reversal of the leg electrodes does not make much difference to the ECG.

The chest electrodes need to be accurately positioned, so that abnormal patterns in the V leads can be identified, and so that records taken on different occasions can be compared. Identify the second rib interspace by feeling for the sternal angle – this is the point where the manubrium and the body of the sternum meet, and there is usually a palpable ridge where the body of the sternum begins, angling downwards in comparison to the manubrium. The second rib is attached to the sternum at the angle, and the second rib space is just below this. Having identified the second space, feel downwards for the third and then the fourth rib spaces, over which the electrodes for V₁ and V₂ are attached, to the right and left of the sternum, respectively.

The other electrodes are then placed as shown in Figure 1.24, with V₄ in the midclavicular line (the imaginary vertical line starting from the midpoint of the clavicle); V₅ in the anterior axillary line (the line starting from the fold of skin that marks the front of the armpit); and V₆ in the midaxillary line.