Lead II

The disc is the front of the chest, seen face on: the patient’s left is on your right, the feet are at the bottom. The pale curves are the path traced by the tip of the heart’s electrical vector during one beat: a small loop for the atria, P; a large one for the ventricles, QRS; a slow one for their recovery, T. The needle is that vector at this instant.

Each line through the centre is a lead. A lead does not see the heart; it sees one direction. What it writes is the needle’s shadow on its own line: up when the needle leans toward its labelled end, down when it leans away, nothing when the needle stands square across it. Six directions, six shadows of one needle.

Textbooks often draw a single arrow, the mean electrical axis, and read the leads off it. Slide the instrument from loop to arrow and watch what that throws away. The arrow keeps the same mean, so every lead keeps the same net area. But inside each wave all six now write one curve, only taller, shorter or upside down.

A still arrow gives six sizes. The six different shapes of a real tracing, the small q, the tall R, the s that follows, come from the path the arrow travels in time, and from nothing else.

Two identities hold for any heart, simply from the way the leads are wired: I + III = II, and aVR + aVL + aVF = 0, at every instant. The line under the dial does the first sum as the beat runs. What Einthoven’s triangle adds is the geometry: put the heart at its centre and lead I looks along 0°, II along +60°, III along +120°; aVR, aVL and aVF along −150°, −30° and +90°.

The augmented leads read √3⁄2 of what a limb lead would read along the same direction: the dial shows the whole shadow, the aVR, aVL and aVF traces write that fraction of it. The axis printed on the dial is the direction of the mean vector; the net QRS area in I against the net area in aVF, scaled back by 2⁄√3, gives the same angle.

Each wave is a few activation fronts added together: two for the atria, right and then left; four for the ventricles, the septum first, then the bulk of both walls in two halves, then the base; two for recovery. Each front is a direction with a rise and a fall in time. The loop is not drawn by hand. It is where their sum points, one millisecond at a time.

At 75 beats a minute the intervals and the heights sit inside the usual adult ranges. It is a model of the geometry, made to be taken apart, not the recording of a heart.

It is not a diagnostic tool, and it takes nobody’s data: there is no field here for a tracing, a number or a symptom, on purpose. The loops it draws are the ones you make with your hands. A question about a real heart belongs with a doctor who can examine it.