12-Lead Layout

I start with axis when I want to rant about the layout of 12-leads.  Which probably seems weird.  Bear with me.

One of the components of 12-lead interpretation is to determine the frontal plane axis.  Axis is the heart’s mean (average) direction of depolarization.  It is similar to stadium lighting: In a bank of stadium lights, there is an individual light that points at the 20-yard line, another points at the 35-yard sideline, another points at the endzone.  But where is the bank of lights pointing?  At the field.  The mean direction of the lights is toward the field.  Ventricular depolarization is similar.  There are waves of depolarization moving in multiple directions – left to right across the septum, along the left anterior fascicle of the left bundle branch out to the left lateral wall, along papillary muscles in the right ventricle, everywhere.  But where is the mean (average) direction of depolarization?  What direction do all of those individual waves average out to?

Cardiac axis is described by a circle.  Zero degrees lies horizontally to the patient’s left.  Ninety degrees is straight down and negative 90 degrees is straight up.  Horizontally to the patient’s right is 180°.  Each ECG lead lies along a bearing.  Lead I is at 0°, Lead aVF is at +90°, and so on.  It is normal for people to have a QRS axis that lies somewhere between -30° and +120°. 

Look at the axis wheel, showing where each ECG lead lies along the circle. 

(By Bron766, via Wikimedia Commons)

Lead aVL is at -30°, Lead I is at 0°, and Leads II, aVF, and III are at 60°, 90°, and 120° respectively.  Lead aVR is at 210° on this wheel, but is also at -150° in other layouts (210° and -150° are the same - it just depends on what you want to call it).

The quickest way to figure an ECG's axis is to use Leads I and aVF.  If Lead I is upright, the axis lies on the patient's left.  If Lead I is negative, the axis is on the patient's right.  The axis is in the inferior hemisphere if aVF is upright and in the superior hemisphere if aVF is downward.  When you combine the two, the axis is normal, left, right, or right shoulder.

Onto 12-lead layouts. 

The layout of a 12-lead is pretty well set.  It came about because the first ECG, created by Willem Einthoven, showed what would become Leads I, II, and III.  Later the augmented leads were added.  So a 12-lead layout commonly looks something like this*:

The bipolar frontal leads (I, II, and III) are in a column, and the augmented leads are in the next column.  The leads jump all around.  The first column has leads at 0°, 60°, and 120°.  Next come leads at -150°, -30°, and 90°.  It jumps all over the axis wheel.  If we did the same thing on the precordial leads, it would go V2, V4, and V6 followed by V3, V1, and V5.  Think about how hard that would be to read in an intuitive way.  I think it should look like this:

I suck at Photoshop.  I'm a paramedic, not a Photoshopper.  So you get this crappy, taped-together version.  But you get the point I'm trying to make, right?

Do you see what changed?  I think the layout should follow the axis wheel, with the addition of negative aVR.  (The monitor’s computer can print the opposite of each aVR deflection as easily as it can print aVR.)  So the first column shows aVL, I, and -aVR.  The next column has II, aVF, and III. 

There would be several advantages to this.  The three inferior leads are all together in a single column.  The two high lateral leads (aVL and I) are grouped together.  Leads I and aVF (which are used to quickly calculate the axis quadrant) are side by side in the middle.  Reversing all waveforms on Lead aVR means the lead becomes less useless and actually fits into the gap between I and II.  You can quickly glance at the 12-lead, see which leads are biggest, and know where the axis is. 

Why isn’t the 12-lead laid out like this?  Tradition, it seems.  Nobody has ever told me a good reason.

*This is my 12-lead.  This is the first one that has had a 1° AV Block. It's odd - being able to run an ECG on yourself means that you can kind of track your ECG changes as you move through life.  When I was 25, I had a bunch of ST elevation from benign early repolarization and a pretty wild sinus arrhythmia.  Now that I'm 40, the BER has gone away and I have a 1° AV Block.  Sucks getting old.  At least there are no pathologic Qs, bundle branch blocks, or hypertrophies.  I can still pull a resting heart rate in the 60s, too...