09: Cardiac Investigations: Electrocardiography
12 lead ECG
Why use 12 lead?
A 12 lead ECG is an important tool in clinical decision making and can provide a wealth of information, for example, diagnosis of evolving MI; identify potentially life threatening arrhythmias; long term effects of sustained hypertension; or acute effects of a pulmonary embolus. In contrast to cardiac monitoring, the 12 lead ECG shows the 3-dimensional electrical activity of the heart recorded from 12 different leads or viewpoints. It provides the whole picture. This is achieved by using Bipolar (I, II & III) and Unipolar leads (augmented leads & precordial chest leads).
ECG electrodes are placed on specific areas of the body in predetermined positions so that different pictures of the heart's electrical activity can be recorded accurately and replicated between recordings.
The bipolar limb leads show the electrical differences between the positive and negative electrodes placed on the limbs as far away from the heart as possible. These 3 leads are equidistant from the heart and create an equilateral triangle (Einthovens triangle) which the heart sits in the centre of.
- Lead 1 shows the electrical difference between the left arm (positive electrode) and the right arm (negative electrode)
- Lead 2 shows the electrical difference between right arm (negative electrode) and left foot (positive electrode)
- Lead 3 shows the electrical difference between the left arm (negative electrode) and left foot (positive electrode)
12 Lead ECG in detail
Page 1: Standard Bipolar Leads
The bipolar limb leads show the difference between the positive and negative electrodes placed on the limbs as far away from the heart as possible. These 3 leads create a triangle (Einthovens triangle) and are equidistant form the heart, forming an equilateral triangle in which the heart sits in the centre.
Page 2: Augmented Unipolar Leads
These are Augmented Voltage Right (aVR), Augmented Voltage Left (aVL) and Augmented Voltage Foot (aVF) leads. aVR will always be negative if the limb leads are placed correctly
The augmented leads (AVR, AVL and AVF) generate low electrical signals and are, therefore, augmented by the ECG machine. Therefore, AVR appears to be looking in at the heart from the postilions of the right shoulder, AVL the left shoulder and AVF from the left foot. As AVR looks into the cavity of the heart it will see all electrical activity ( P, QRS and T waves) moving away from it and, therefore, these deflections are usually negative on a 12 lead ECG.
Page 3: Precordial leads
The precordial leads look at specific areas of the the right and left ventricles. On a 12 lead ECG, chest leads C1-6 are annotated as V1-6 on the ECG recording:
- V1 Fourth intercostal space to the right of the sternum.
- V2 Fourth intercostal space to the left of the sternum.
- V3 Directly between V2 & V4.
- V4 Fifth intercostal space, midclavicular line.
- V5 Level with V4 at left anterior axillary line.
- V6 Level with V5 at midaxillary line (midpoint of the armpit).
Page 4: Lead views of the heart
|I Lateral||aVR||V1 Septal||V4 Anterior|
|II Inferior||aVL Lateral||V2 Septal||V5 Lateral|
|III Inferior||aVF Inferior||V3 Anterior||V6 Lateral|
Acute MI = ST elevation >2mm in V1-V3 and >1mm in all other leads in >2 contiguous leads1.
Infarction can present as Q wave1.
ST depression with elevated troponin
Page 5: Wave of depolarisation
The ECG morphology depends on whether the signal is towards, away from or parallel to the recording electrode
Page 6: Vertical and Horizontal Planes
The Normal 12 lead ECG
When looking at an ECG its important to know what is normal.
Within the first 6 leads (I,II,III, avR, avL and avF) all, with the exception of aVR, should be upright.
In the precordial or chest leads the r wave progresses from small to tall from V1 to V6
The polarity or direction of the ST segment and T wave are generally the same as the preceding QRS complex, for example, if the QRS complex is positive or upright then the T wave will also be positive.
All ST segments should remain on the isoelectric line or baseline.
Normal 12 Lead ECG interactive
Slide 1: Normal ECG
Slide 2: R wave progression through the chest leads
- R wave amplitude should increase from V1 to V6
- Transitional Zone V4 usually at the apex
Slide 3: Cardiac (QRS) axis: the overall direction of current flow which usually points leftward and inferiorly lying between 0 and 90 (some will extend to -30).
See Common Cardiac Investigations: The Electrocardiogram (ECG) for a video from Glasgow University on indications, equipment, placement, preparation and procedure and recording an ECG.