ECG Simplified – Part 4

ECG Simplified – Part 4

Most commonly recorded ECG is a 12 lead ECG. Twelve leads in a standard ECG are as follows:
Standard Limb Leads: I, II, III
Augmented Limb Leads: aVR, aVL, aVF
Chest Leads: V1, V2, V3, V4, V5, V6
Thus, a standard 12 lead ECG does not include right chest leads known as V3R, V4R, V5R etc. But in most cases 12 lead ECG includes a long rhythm strip – either lead II or V1 or both, for facilitating rhythm analysis.
If the ECG machine has only a single recording channel, 12 leads are recorded sequentially. More sophisticated recorders have facility to record 3 or even 12 channels simultaneously. Simultaneous multichannel recording can record all leads of a single beat and is better for analysis of complex heart rhythm abnormalities.
Electrode combination for limb leads are as follows:
Lead I: Left arm positive, Right arm negative
Lead II: Right arm negative, Left foot positive
Lead III: Left arm negative, Left foot positive
Unipolar limb leads are derived using the limb lead electrode potentials. All the three limb leads are internally connected to a central terminal – Wilson’s central terminal, using fixed value resistors. Usual limb lead recordings are called bipolar recordings with one limb serving as positive electrode and another as negative electrode as illustrated above.
Unipolar recordings are obtained by taking the limb electrode as positive and central terminal as negative. Unipolar limb leads are called VR, VL and VF depending on whether the positive electrode is at right arm, left arm or left leg.
Voltages of these recordings can be augmented by disconnecting the connection to the corresponding limb electrode from the central terminal, causing almost 50% increase in the amplitude of the recordings. The augmented limb leads thus generated are called aVR, aVL and aVF. Augmented limb leads were described by Goldberger in 1942.
While monitoring ECG in an intensive care unit, the limb electrodes are placed on the body at a point close to the origin of the limbs. This is to reduce movement artefacts in the tracing. Placing the leads on the body also prevents tethering of the person and allows free movement of the limbs. Same method is followed while recording ECG during an exercise test.
Most modern monitors have the option to use chest leads in addition to limb leads. When chest lead monitoring is needed, additional electrodes are placed on the chest. Chest lead monitoring is useful in picking up ST segment changes. ST segment changes occur when there is a decrease in blood supply to heart muscle. This may be associated with chest pain.
Location of the leads on the chest depends on the type of patient. While locations on the front of the chest are available in chest pain patients, a surgical patient may require different locations depending on the location of operation and dressings. Leads placed in the food pipe may be used in some rare situations to decipher difficult heart rhythm abnormalities. These are known as esophageal leads.
Mason-Likar modification of 12 lead ECG is most popular during treadmill exercise test. It can also be used in the electrophysiology laboratory along with electrodes within the heart. In this lead system, limb electrodes are placed on the nearest location on the body to prevent artefacts due to limb movement during exercise.
Chest electrodes are placed in the conventional positions. Difference in pattern of the modified 12 lead ECG mandates caution while trying to interpret Q waves and other abnormalities on a Mason-Likar modification. This system is mainly meant for assessment of ST segment deviations and heart rhythm abnormalities during exercise test.
Lewis lead is a modified lead to enhance the amplitude of P waves and thereby enable better arrhythmia analysis. Right arm and left arm leads are repositioned on the chest as shown in the image. This lead system is useful in detecting P waves during a wide QRS tachycardia and helps in differentiating between ventricular and supraventricular tachycardia. Supraventricular tachycardias are abnormal rhythms originating above the ventricles.
Atrial repolarization wave or Ta wave is usually not evident in the ECG as it has a low amplitude of 100 – 200 microvolts and is usually hidden in the QRS complex. It can also extend into the ST segment causing ST segment depression, especially during an exercise test and cause a false positive response. A representative image of the Ta wave is shown here.
A modified limb lead system for detection of atrial repolarization on surface ECG has been devised by Sivaraman J and colleagues. This lead system has been shown to be useful in detecting atrial repolarization on surface ECG, more so in case of complete heart block, where most of the atrial repolarization activity is not masked by ventricular activity.