ECG · Section 1
What the machine is measuring, what one beat is made of, and which of the twelve views looks at which part of the heart. Start here, then read real rhythms in the rhythm library.
What a lead measures
A lead is not a wire. It is a direction of measurement — the voltage between two points on the body, plotted over time.
A wave of depolarisation travelling towards a lead's positive electrode writes an upward deflection. Travelling away from it writes a downward one.
In a normal heart the impulse spreads down and to the left, towards the apex. Lead II sits down and to the left, so it sees the wave coming and everything is upright — which is why II is the conventional rhythm strip. aVR sits up and to the right, sees the same wave going away, and draws it inverted.
10 → 12
Ten electrodes — one on each limb and six across the chest — produce twelve views. The limb electrodes are combined in different pairings to make six frontal-plane leads, and each chest electrode adds one horizontal-plane view of its own. The right leg electrode is not a view at all: it is a reference used to cancel noise.
The parts of a beat
Drawn by the same engine that runs the rhythm library and the scenario monitor. The grid is magnified so the parts are big enough to point at, but it keeps its calibration — a small square is still 40 ms across and 0.1 mV tall, so squares counted here count the same way they do on a real strip.
The parts of one complex
Choose a part to mark it on the trace.
25 mm/s · 10 mm/mV — small square 40 ms / 0.10 mV, large square 200 ms / 0.5 mV. Rate = 300 ÷ large squares R-to-R.
Where the twelve views come from
Three groups, derived three different ways from the same ten electrodes.
Limb leads (bipolar)
I · II · IIIThe voltage between two limb electrodes: I is left arm minus right arm, II is left leg minus right arm, III is left leg minus left arm.
Plane: Frontal — top-to-bottom and side-to-side, as if looking at the patient face on.
Einthoven's triangle. Because they share electrodes they are not independent: I + III = II always, which is a quick sanity check that the limb leads are on correctly.
Augmented limb leads
aVR · aVL · aVFOne limb electrode compared against the average of the other two, giving a view from that limb towards the heart's centre. No extra wires — the same three electrodes, combined differently.
Plane: Frontal, filling in the gaps between I, II and III.
aVR looks at the heart from the right shoulder, so a normal depolarisation travels away from it and every wave is inverted. An upright QRS in aVR is a finding, not a normal ECG.
Chest leads (precordial)
V1 · V2 · V3 · V4 · V5 · V6Each chest electrode compared against the average of the limb electrodes, giving a view from that spot on the chest wall inwards.
Plane: Horizontal — a slice through the chest, as if looking up at the patient from below.
They run right to left across the chest, so the QRS should progress with them: mostly negative in V1, becoming mostly positive by V5–V6. Failure of that progression is itself a finding.
Which lead looks at which wall
This is the layout a 12-lead actually prints in. It does not group leads by wall, which is exactly why reading one takes practice — a wall means picking leads out of different columns.
No wall selected — choose one below to light up its leads.
What each lead is looking at
Choose a wall to ring its leads on the 12-lead above.
Walls a 12-lead cannot see
- Posterior wall (V7–V9) — Seen on a standard 12-lead only as a mirror image in V1–V3: tall R waves with horizontal ST depression. The direct leads (V7–V9) take the same 1 mm threshold as any other lead outside V1–V3.
- Right ventricle (V4R) — Involved in up to 40% of inferior infarcts. It is preload-dependent, so finding it changes whether nitrates are safe.
Which leads confirm each other
Every criterion for calling a STEMI is phrased as two or more contiguous leads. Contiguous does not mean printed side by side — on a real 12-lead it almost never does.
Two or more contiguous leads. Contiguous means two views next to each other by angle or by chest position — not that they are printed next to each other, and not simply that they share a wall.
One lead showing ST elevation is weak evidence: normal variants, a poorly stuck electrode and simple noise all do it in a single view. A genuine injury is a patch of muscle, and an adjacent view is what confirms the patch rather than the artefact. Requiring two contiguous leads is what turns a suspicious squiggle into a territory.
Pick a lead, see what confirms it
The same 3×4 layout a machine prints.
Choose any lead to ring the leads that would confirm a finding in it.
Limb leads: by angle
They are directions, not places, so “next to” means the next direction round the frontal plane — every one 30° from the last. In print order they look scrambled; sorted by angle it is a single chain, and contiguous means adjacent in that chain — not simply “the same wall”, which the inferior wall below shows is not the same thing.
-aVR is dashed because no machine prints it: it is aVR turned upside down. It is drawn anyway because it fills the otherwise unexplained 60° gap between I and II — and it sits exactly on the boundary between the lateral leads and the inferior ones, which is why I and II are not contiguous despite being neighbours in the list.
Chest leads: by position
These are places on the chest wall, marching right to left. Neighbours in the numbering are neighbours on the patient, so contiguity is simply the numbering — and it crosses the wall boundaries freely.
right sternal bordermid-axillary line
Elevation in V1 and V6 together is two leads and is not a STEMI pattern — those are opposite ends of the chest with everything in between normal, which no single occluded artery produces. It is a reason to check the electrodes.
The groups, in full
- II · III · aVFInferior wallfrontal plane
- V1 · V2Septumhorizontal plane
- V3 · V4Anterior wallhorizontal plane
- I · aVLLateral wall — high (limb leads)frontal plane
- V5 · V6Lateral wall — apical (chest leads)horizontal plane
The lateral wall is one artery but two groups, because I and aVL are a direction and V5 and V6 are a position. They are the same wall seen from two planes — not two adjacent views of the same muscle, which is what contiguity means.
A group listed together is one wall, not automatically a mutually contiguous set — the inferior wall has three limb leads, and II and III are not directly contiguous with each other, only each with aVF between them. Pick either one above to see it.
Why the printed layout fights you
The top row of a printed 12-lead is I, aVR, V1, V4. No two of those four are contiguous with each other — and the middle row, II aVL V2 V5, is no better.
Reading a 12-lead across, the way the eye wants to, compares views that have nothing to do with one another. Every real pattern on the sheet is read down a column, or by picking leads out of two columns at once — which is why finding an inferior STEMI on paper is a skill and not a glance.
One exception: III and aVF are printed next to each other in the bottom row and are genuinely contiguous — both inferior. It is the only place on the sheet where reading across happens to work, and it is a coincidence of the layout rather than a pattern. II, the third inferior lead, is a row above them.
How much elevation counts
Measured at the J point, in two contiguous leads.
- V1–V32 mm
Normal early repolarisation reaches into this range in the septal and anterior chest leads, so a 1 mm rule here would call healthy chests infarcted.
- Everywhere else — including V4R, V8, V91 mm (one small square)
The right-sided and posterior leads gained by repositioning take the same 1 mm as every standard limb or lateral lead. They are not read against a lower bar.
Follow your own service's directive where it differs.
Putting the electrodes on
Ten electrodes, and each one in the wrong place rewrites several leads at once. A misplaced electrode does not produce an obviously broken ECG — it produces a plausible one, which is why this is worth drilling rather than looking up. Wrong answers are named: you get told which real-world error you just made and what it would have done to the trace.
Place the ten electrodes
Drag one onto the body, or tap an electrode and then a spot.
Beyond the standard ten
- Right-sided leads (V4R (or V1R–V6R)) — V4R mirrors V4 onto the right chest — 5th space, right midclavicular line. Worth doing on every inferior STEMI, because a right ventricular infarct is preload-dependent and changes whether nitrates are safe.
- Posterior leads (V7–V9) — Continued round the back at V6's level: posterior axillary line, tip of the scapula, and beside the spine. The threshold for elevation there is the ordinary 1 mm, the same as any lead outside V1–V3.
Section 2
ECG rhythm library
Twenty-five rhythms on the same paper, with ectopy, ST elevation, measurements and a printable report.
Simulation — for education only. Not a medical device. Not for diagnostic or clinical use.