Introduction to ECG Leads

Electrocardiogram (ECG) leads are configurations of electrodes that record the heart’s electrical activity from different orientations. In a standard 12-lead ECG, leads help provide a complete electrical view of the heart—both in the frontal and transverse planes.

ECG leads are broadly classified into two types:

• Bipolar Leads: Measure the voltage difference between two specific electrodes.

• Unipolar Leads: Measure the electrical potential at one electrode referenced against a composite reference formed by averaging other electrodes.

Types of Leads in Human ECG

1. Standard Limb Leads (I, II, III) — Bipolar Leads:
   These leads measure the voltage difference between two limbs’ electrodes (right arm, left arm, left leg). For example, Lead I measures the potential between left arm and right arm electrodes. These are bipolar as they compare two active electrodes.

2. Augmented Limb Leads (aVR, aVL, aVF) — Unipolar Leads:
   These leads measure voltage at one limb electrode referenced to a composite (central terminal) derived from the other two limb electrodes. They are unipolar.

3. Precordial Leads (V1-V6) — Unipolar Leads:
   Chest leads record the potential from a single chest electrode referenced to the central terminal and are unipolar.

Analyzing the Options

Conclusion

The lead that is NOT unipolar in the options given is:

(3) Standard limb leads