Bipolar and Unipolar ECG. What’s The Difference? | In In Cardiology | By By Hannah Victoria

Some electrodes are bipolar (standard limb) while the other electrodes are unipolar (augmented limb and precordial / chest). Bipolar and unipolar electrodes are how 10 electrodes can create 12 individual leads and how 4 limb electrodes are responsible for half the tracings on a 12-lead electrocardiogram.

Now take a look at a normal 12-lead. Don’t calculate the PR interval, or march out the QRS complex. Just think about the 12 leads and their respective electrode placements.

On the right we recognize V1-V6 from the ECG placement cheat sheet. But where did I, II, III, aVR, aVL, and aVF come from? Those aren’t on the cheat sheet. And what ever happened with the limb electrodes? What sorcery is this? It’s the magic of bipolar and unipolar electrodes.

Bipolar Limb Leads

These are the multitaskers that form Einthovans triangle. We can see them in action on the far left column of a 12-lead ECG as I, II, and III (Roman numerals). Each of these leads uses one positive electrode and one negative electrode. The positive electrode relies on the negative electrode as a reference point, but depending on the lead you are looking at (I, II or III) rotates (or, multitasks since it happens simultaneously) which electrode has which electrical charge.

Lead I — The positive (+) electrode is LA (left arm) and the negative (-) electrode is RA (right arm). Since the charge is moving left to right, Lead I gives us a view of the left side of the heart.

Lead II — Positive (+) electrode is LL (left leg) and the negative (-) electrode is RA (right arm). Since the charge is moving up at an angle, it gives us a look at the bottom / inferior of the heart looking up toward the right.

Lead III — Positive (+) electrode is still LL (left leg), but the negative (-) electrode is LA (left arm). This gives us a view of the bottom / inferior of the heart looking up toward the left.

Unipolar Augmented Limb Leads

Wait a second. There are bipolar and unipolar limb leads? How does that work? Remember that bipolar leads look at the electrical activity between two electrodes — a positive and a negative. The three unipolar limb leads are positive electrodes that augment the average of the other three limb electrodes to find the center terminal point of the heart. (This is another reason why proper electrode placement is so important. If placement is off, the center terminal is off and the tracing won’t be accurate to what is actually happening in the heart.)

The aV (augmented voltage) limb leads are as follows:

aVR — Positive (+) electrode is RA (right arm). aVR is unique on the 12-lead because it is the only lead that doesn’t contribute a surface view of the heart. However, that doesn’t make it any less valuable as a diagnostic indicator.

aVL — Positive (+) electrode is LA (left arm). aVL looks from the left arm medially and therefore gives us a view of the lateral (left) surface of the heart.

aVF — Positive (+) electrode is LL (left leg). aVF looks from the left leg superiorly which results in giving us a view of the inferior (bottom) surface of the heart. (Hint: I remember aVF placement by associating the F with “foot”)

You’ll notice that in bipolar and unipolar leads RL (right leg) is not actually referenced. That’s because it is a ground lead to help minimize artifact.

Unipolar Precordial Leads

Precordial (also called chest) leads are positive electrodes that measure electrical potential in reference to the center terminal point. These are V1-V6 (V stands for voltage). Unlike the limb leads, the precordial leads are close enough to the heart that they do not require any bonus augmentation.

V1 — Positive (+) electrode is on the right side of the sternum in the 4th intercostal space. Looks at the septum.

V2 — Positive (+) electrode is on the left side of the sternum in the 4th intercostal space. Also looks at the septum.

V3 — Positive (+) electrode is midway between V2 and V4. Looks at the anterior (front) surface of the heart.

V4 — Positive (+) electrode is on the left midclavicular line in the 5th intercostal space. Also looks at the anterior (front) surface of the heart.

V5 — Positive (+) electrode is between V4 and V6 in the 5th intercostal space. Looks at the lateral (back) surface of the heart.

V6 — Positive (+) electrode is on the left midaxillary line in the 5th intercostal space. Also looks at the lateral (back) surface of the heart.

Summary

The bipolar leads (limb) are the multitaskers that give different views by tracing the electrical activity between two (one positive and one negative) electrodes.

The unipolar leads (augmented limb and precordial) are the slow and steady electrodes that stay positive (+) and reference each other to provide an accurate tracing.