PATIENT DETAILS AND ECG CALIBRATION

Always record and check the patient’s name and date of birth, as well as the date and time, on an ECG. Also check the calibration; ECGs are usually recorded at a paper speed of 25 mm/s and calibrated so that a signal of 1 mV = 10 mm.

THE NORMAL ECG

Figure 19.1 shows a normal 12-lead ECG, and the terminology and limits of normality for the key waves and intervals are shown in Figure 19.2.

Fig. 19.1 A normal 12-lead ECG. Note how the R wave becomes gradually move positive from V₁ to V₆.

Fig. 19.2 The components of the ECG correspond to the depolarisation and repolarisation. The upper limit of the normal range for each interval is given in brackets.

P WAVE

The P wave is caused by atrial depolarisation, and is normally <0.2mV (2 small squares) in amplitude and <0.12s (3 small squares) in duration.

PR INTERVAL

The PR interval is from the onset of the P wave to the onset of the QRS complex, and is normally 0.12–0.2s in duration (3–5 small squares).

QRS COMPLEX

The QRS complex is caused by ventricular depolarisation.

Non-pathological small Q waves are often seen in leads I, aVL, III, V₅ and V₆.

QRS duration: Normally <0.1s (2.5 small squares). A broad QRS complex (>0.12s) can occur with delayed ventricular depolarisation (e.g. bundle branch block) or if the impulse is generated from a focus outside the bundle branches (e.g. ventricular ectopic or ventricular tachycardia).

QRS amplitude: Affected by LV mass, axis and patient build. Large-amplitude complexes can be normal in thin patients, while small complexes can be seen in patients with thick chest walls, emphysema, pericardial effusions or hypothermia.

ST SEGMENT

The ST segment is usually isoelectric.

T WAVE

T waves are caused by ventricular repolarisation. They can be inverted in lead III, V₁ or V₂ in normal individuals. T wave abnormalities are common and non-specific.

QT INTERVAL

The QT interval is from the beginning of the QRS complex to the end of the T wave. It varies with heart rate and should be corrected for this:

The QT_C is normally 0.38–0.46s. Prolonged QT can predispose to VT and may be caused by:

U WAVE

U waves follow the T wave and are not always present. Prominent U waves can be seen with bradycardia, hypokalaemia and some drugs.

RATE

Heart rate is calculated from the R–R interval. At a standard paper speed of 25 mm/s, the number of large squares between each R wave divided into 300 indicates the heart rate in beats per minute (bpm). For example, 2 large squares between R waves = 150 bpm, 3 large squares = 100 bpm, and so on. If the heart rate is irregular, take an average over 5 R–R intervals.

RHYTHM

This is best assessed in lead II where the P waves are usually clearly seen. Firstly, note if the rhythm (R–R interval) is regular or irregular. Then look for P waves and establish their relationship to the QRS complexes. In sinus rhythm:

CARDIAC AXIS

This is the mean direction of ventricular depolarisation in the coronal plane. It is most accurately estimated from the limb leads using the hexaxial system (Fig. 19.3). Firstly, identify the equiphasic lead (where the positive and negative deflections of the QRS complex are roughly equal). Then find the lead at 90° to this on the hexaxial diagram.

Fig. 19.3 The normal appearance of the ECG from different leads in the coronal plane.

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