IILL − RA25 mm/s · 10 mm/mV72
Heart vector arm and pin: length is millivolts, angle is where the impulse is headingSlotted yoke on the lead ring: slides by the vector's shadow on the lead axis, the lead's voltagePen and paper: that voltage, written at 25 mm/s; the printout holds all six limb leads at onceThe heart's electrical activity, at any instant, adds up to one vector in the frontal plane. The arm on the phantom is that vector: it swings toward the atria's left-downward spread (P), flicks right with the septum, swings hard left-down through the ventricular free wall (QRS), rests at zero through the plateau (ST) and returns softer for repolarisation (T). A lead is nothing but a direction: the yoke, turned onto the lead's axis by the ring, slides by the vector's shadow on it, and that shadow is the voltage the recorder writes. Toward the positive electrode the pen goes up, away it goes down, and square to the axis it barely moves.
Lead Paper Gain
Heart

Normal sinus rhythm.

One beat, taken apart

Every wave is one stage of a single impulse that starts in the SA node and ends with the ventricles recharged. The moving line is where the phantom is in the beat right now.

P waveThe SA node fires and the impulse spreads through the right atrium, then across Bachmann's bundle to the left. The arm swings left and down, toward +55°.under 120 ms, under 2.5 mm · upright in I, II, aVF
PR segmentThe AV node holds the impulse about a tenth of a second so the atria can finish emptying into the ventricles, then passes it to the His bundle. Too few cells to register: the arm sits at zero.PR interval (start of P to start of QRS) 120 to 200 ms
QRSThe impulse races down the right and left bundle branches into the Purkinje fibres. The septum fires first, left to right (a small q in I and aVL), then the thick left free wall swings the arm hard toward the apex (R), and the base fires last (s).under 120 ms · the atria repolarise unseen inside it
ST segmentEvery ventricular cell sits in its plateau at the same voltage, so there is nothing to see. A shift here means injured muscle leaking current.flat, at the level of the TP baseline
T waveThe ventricles recharge. The outer wall recovers first, so recovery runs outside-in, the opposite way to activation, with the opposite sign: the two flips cancel and T points the same way as QRS.QT (start of QRS to end of T) shortens as the rate rises
TP baselineElectrical rest while the ventricles fill. This is true zero, the line you measure ST against.shortens as the rate rises; gone above about 150 /min

Six limb leads, one heart

A lead is a direction. Each one reads the shadow of the same heart vector on its own axis: impulse heading toward the lead's positive pole, pen up; heading away, pen down; square to it, a small up-and-down. That is why one beat looks different in every lead, and why the leads can find the axis.

Einthoven's law holds exactly: II = I + III, and aVR + aVL + aVF = 0. The augmented leads compare one limb with the average of the other two and read √3⁄2 of the projection. The shaded sector is the normal axis; the grey arrow is this rhythm's mean QRS axis; the bold line is the heart vector right now.

Measure it

Reading any ECG

The paper

At 25 mm/s and 10 mm/mV, one small box is 1 mm: 0.04 s across and 0.1 mV up. One big box is 5 mm: 0.2 s and 0.5 mV. Five big boxes are one second. The square step at the start of the strip is the 1 mV calibration: it should be two big boxes tall. Change the gain or speed above and watch the same beat grow or stretch.

In order, every time

  • Rate: 300 ÷ big boxes between two R waves (1500 ÷ small boxes). Irregular: count QRS in a 6 s strip × 10.
  • Rhythm: a P before every QRS, a QRS after every P, the same PR each time, P upright in II: sinus.
  • Axis: QRS up in I and up in aVF is normal. Up in I, down in aVF: check II; down in II too is left axis deviation.
  • Intervals: PR 120 to 200 ms, QRS under 120 ms, QTc under about 450 ms (men) or 470 ms (women).
  • Waves: P shape; pathological Q waves (wider than 40 ms or deeper than a quarter of the R); QRS voltage; R-wave growth V1 to V6; ST shifts; T inversions.
  • Compare with the last ECG. New is what matters.

Where each lead looks

LeadsWallUsual artery
II, III, aVFInferiorRight coronary (most), circumflex
I, aVL, V5, V6LateralCircumflex, diagonal branches of the LAD
V1, V2SeptalLAD (septal branches)
V3, V4AnteriorLAD
aVRLooks into the cavityUp with ST down everywhere else: left main or three-vessel ischaemia

The other six: chest leads

V1 to V6 are single electrodes across the chest, each read against Wilson's central terminal, the average of RA, LA and LL. They work exactly like the phantom, but in the horizontal plane: V1 sits over the right ventricle and sees the vector heading away (mostly down), V6 sits over the left ventricle and sees it coming (mostly up), and the R wave grows from V1 to V6. Limb leads plus chest leads are the 12-lead ECG: the same vector seen from twelve directions in two planes.

What the model is

A teaching model, not for clinical use. The heart is one dipole in the frontal plane, a sum of Gaussian lobes for P, the septal, free-wall and basal parts of QRS, an ST injury current and an asymmetric T; the electrodes read Φ = (v · r) ⁄ √3 at the corners of an equilateral Einthoven triangle, and the recorder only subtracts them. QT follows Bazett. Rhythm presets change the lobes, the timing and which conduction paths carry the impulse.