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)