Introduction
Electrocardiography (ECG) has been the main heart investigation method for most of the twentieth century and still plays a pivotal role in diagnostics. ECGs can provide a wealth of information about cardiac function and can often show early signs of systemic abnormalities as well. In order to accurately interpret an ECG it is important to first ensure its correct acquisition. The process involves recording small electrical changes on the skin that occur as a result of cardiac muscle depolarisation. Ten electrodes are used to record the heart’s electrical activity in 12 different orientations, which encompass the 12 ‘leads’ of the ECG. It is important therefore to ensure that these 10 electrodes are consistently applied in order to accurately assess for any abnormalities. In addition, account must be made of sources of interference.
Recording the ECG
As posture may affect the appearance of the ECG it is preferably performed in the supine position where practicable. Skin preparation is important to reduce artefacts and may include hair removal and/or skin cleansing.
Limb Electrodes
Moving the limb electrodes away from the distal limbs may affect the ECG appearance and it is therefore preferable that these are placed just proximal to the wrists and ankles in order to produce consistent results. Limb electrodes are often colour coded to aid placement.
Right arm (RA, red) proximal to right wrist
Left arm (LA, yellow) proximal to left wrist
Left leg (LL, green) proximal to left ankle
Right leg (RL, black) proximal to right ankle
Precordial (Chest) Electrodes
Most errors occur in placement of the chest electrodes, especially V1 and V2, which may be placed too high. This can have a significant effect on the resultant ECG. Correct anatomical positioning must be adhered to, with the centre of the electrode aligned with the correct location.
V1 (C1) fourth intercostal space, right sternal edge
V2 (C2) fourth intercostal space, left sternal edge
V3 (C3) midway between V2 and V4
V4 (C4) fifth intercostal space, midclavicular line
V5 (C5) horizontal level with V4, anterior axillary line
V6 (C6) horizontal level with V4, midaxillary line
After placement of V1 and V2, V4 is located in the fifth intercostal space, midclavicular line. V3 is then placed directly in between V2 and V4. V5 and V6 are located at the same horizontal level as V4, perpendicular to the midclavicular line.
Filter Settings and Calibration
Most filter settings are set by default but it is recommended that the low frequency filter is set on or below 0.05 Hz. This filter will account for respiration. If it is set too high, it will distort the ST segment. This may also reduce the accuracy of detecting myocardial ischaemia based on ST segment shifts. The high filter setting should be set on or above 100 Hz. This filter setting should account for the artefact created by muscle tremor. The mains filter (50 Hz) is normally set to ‘Off’.
A standard ECG has a voltage calibration of 10 mm/mV and is recorded with a paper speed of 25 mm/s. This results in 10 second recording. Each small square is equal to 40 ms and each large square (5 small squares) is equal to 200 ms.
The 12 ‘Lead’ ECG
The 12 ‘leads’ of an ECG correspond to 12 vectors along which depolarisation of cardiac tissue is recorded. Each is created by measuring the electrical potential between two points. In each case one of the ten electrodes is the positive pole. There are three bipolar limb leads where another electrode is the negative pole. The negative pole for the unipolar limb leads and the precordial leads is a composite pole (VW) called Wilson’s central terminus, which is created by averaging the potential recorded by RA, LA and LL electrodes:
VW = 1/3 (RA + LA + LL).
The six limb leads view the heart in the coronal (vertical) plane while the six precordial leads view the heart in a perpendicular transverse (horizontal) plane.
Bipolar Limb Leads
Unipolar Augmented Limb Leads
Lead aVR is the potential difference between RA and Vw,
aVR = 3/2 (RA – VW).
Lead aVL is the potential difference between LA and Vw,
aVL = 3/2 (LA – VW).
Lead aVF is the potential difference between LL and Vw,
aVF = 3/2 (LL – VW).
The vectors created by the bipolar and augmented limb leads together form the hexaxial reference system (see Figure 2.1).
Figure 2.1 Hexaxial reference system.
Precordial Leads
For each of the precordial leads the positive pole is the corresponding electrode and the negative pole is VW.
ECG Arrangement
A standard ECG records a 2.5 second tracing of each lead arranged in a grid of four columns and three rows. A marker depicts the change from one lead to the next in each row and can be confused with part of the ECG. The first column contains leads I, II and III, the second column aVR, aVL and aVF, the third column V1, V2 and V3 and the final column contains leads V4, V5 and V6. A fourth row is often provided as a continuous tracing to aid determination of rhythm.
Although each lead records electrical activity of the heart from a different angle, contiguous leads are associated with different anatomical regions.
Inferior leads II, III and aVF
Anterior leads V3 and V4
Septal leads V1 and V2
Lateral leads I, aVL, V5 and V6
Interpretation of the ECG
Waves and Intervals
P Wave
The P wave (see Figure 2.2) represents depolarisation of the right and left atria. In normal sinus rhythm, its origins are from the sinus node, which is normally located posteriorly high in the right atrium. Therefore, the P wave is positive (upright) in all leads except aVR, and can be positive or biphasic in V1. It typically has duration <80 ms.
In right atrial enlargement, the P wave is tall and peaked, and is prolonged and bifid in left atrial enlargement.
PR Interval
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and represents the time taken for atrial depolarisation and conduction through the AV node. A normal PR interval varies between 120 and 200 ms.
If the PR interval is shorter than 120 ms then conduction is bypassing the AV node (see Wolf–Parkinson–White syndrome). Prolongation of the PR interval indicates 1° AV block. Depression of the short isoelectric segment between the end of the P wave and the beginning of the QRS complex can indicate pericarditis.
QRS Complex
Q, R and S waves form the QRS complex, which represents ventricular depolarisation. If the first deflection is negative it is termed the Q wave, otherwise an initial positive deflection is an R wave. A final negative deflection is the S wave. Normally the whole QRS complex is <120 ms in duration. Q waves represent left to right depolarisation of the interventricular septum and are typically seen in left sided leads (I, aVL, V5 and V6). Small Q waves can be seen in most leads except V1, V2 and V3.
Large Q waves are pathological if >40 ms wide, and >2 mm deep or >25% of the height of the QRS complex. This can represent myocardial infarction, cardiomyopathy or electrode malposition.
If the QRS complex is particularly tall this may indicate left ventricular hypertrophy. A number of criteria have been developed to aid diagnosis, none of which are perfect. Most commonly used is the Sokolow–Lyon index:
S wave in V1 + R wave in V5 or V6 (whichever largest) > 3.5 mV.
There is a large S wave in lead V1, which gradually becomes smaller (absent) by lead V6. Its presence or absence is rarely of clinical significance.
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