ECG concepts:
These three limb lead systems provide clinicians with six distinct viewpoints of the heart, forming the basis of a basic three-lead ECG. During standard assessments, these leads are commonly used to monitor rhythm and detect gross abnormalities. It’s important to note that the right leg electrode does not contribute to any of the leads. Instead, it serves as a ground to stabilise the signal and reduce electrical interference, helping to produce a clearer ECG trace.
The precordial (chest) leads—V1 to V6—offer a transverse (horizontal) view of the heart. Like the augmented limb leads, each of these chest leads is unipolar and uses a single positive electrode. The electrical activity is measured relative to a central point of reference (relative zero potential) assumed to lie at the centre of the heart. This setup allows for precise localisation of cardiac events across different regions of the anterior thorax.
Because these leads can be repositioned across the chest wall, they provide opportunities for additional diagnostic views, such as V4R (right-sided chest lead) and V7–V9 (posterior leads). These variations are particularly valuable when assessing for right ventricular infarction or posterior myocardial infarction, which we will explore in more detail in the advanced ECG section.
Each ECG lead produces a waveform based on the net direction and magnitude of electrical activity it detects. This is why deflections on an ECG trace can be positive or negative, depending on whether the electrical current is moving toward or away from the positive electrode of that lead. For instance, if the wave of depolarisation is moving toward Lead II, the resulting waveform will show a positive deflection. Conversely, if it is moving away from aVR, a negative deflection will be seen in that lead.
The size of these deflections also depends on the strength of the electrical current, which is directly related to myocardial mass. A larger muscle mass produces more action potentials, generating a stronger electrical signal. For example, the left ventricle, with a typical wall thickness of 10–15 mm, generates a standard QRS complex. However, in cases of left ventricular hypertrophy—where the wall thickness may reach 50–60 mm—the resulting electrical activity is significantly greater, often producing a markedly enlarged QRS complex on the ECG.