# ECG/EKG: The ECG, taken apart

Source: saved session `01a155d5-9447-73b1-84fa-d965eea0359b`, October 19, 2026.

The original detailed write-up is preserved verbatim below. Desktop paths and verification claims refer to that original session, not this import. Educational material only; not a clinical protocol or medical device. Clinical statements, diagnostic thresholds, and treatment recommendations have not been independently verified during import. Original model assumptions and limitations are preserved; independent clinical review is required before teaching or patient-care use.

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I built an interactive EKG figure with the anatomy skill: **Fig 7, "The ECG, taken apart"**. It's in `~/Desktop/02_projects/anatomy-ekg/`. Open `ekg.html` (dark) or `ekg-light.html`. Static previews are `preview-dark.png` and `preview-light.png`.

**The object.** A biomed bench with three things on it: a heart "phantom" (a teaching model), an old-style strip-chart ECG machine, and a printout lying on the bench.

- **Phantom:** a triangular board with electrodes at the three corners of Einthoven's triangle (RA, LA, LL). In the middle is a heart inlay showing the great vessels, chambers and the full conduction path: SA node, Bachmann's bundle, AV node, His bundle, both bundle branches, both left fascicles and the Purkinje fibres. The impulse lights up along that path in time with each beat.
- **The vector arm:** an arm swings from the centre of the heart. It is the heart's electrical vector at that instant: its length is the voltage in millivolts, its angle is the direction the impulse is travelling.
- **The lead ring:** turning the ring points a slotted slider (a clear acrylic "yoke") along the chosen lead's direction. A pin on the arm rides in the slot, so the slider moves by exactly the arm's projection onto that lead. That projection is the lead's voltage. This is the core idea: toward the positive electrode the pen goes up, away it goes down, and when the arm is square to the lead axis the pen barely moves.
- **Recorder:** cables carry the electrode signals to the machine. Its lead switch only subtracts them (II = LL − RA, aVR = RA − ½(LA+LL), and so on), and the pen writes the result on moving 25 mm/s paper. It prints the 1 mV calibration step at the start and labels the paper when you switch leads.
- **Printout:** the current rhythm in all six limb leads at once (I, II, III / aVR, aVL, aVF), plus a lead II rhythm strip.

**Interaction.** When nobody touches it, it tours through the six leads. Keys 1–6 pick a lead, R changes the rhythm, ↑↓ change the rate, S changes paper speed, G changes gain, and space pauses. You can also click the knob, the toggles or the dial.

The eight rhythms are normal sinus, first-degree AV block, LBBB, RBBB, LAFB, inferior STEMI, hyperkalaemia and AF. Each one changes the vector's shape, the timing, and which conduction paths carry the impulse (blocked paths show dotted).

**Below the figure** are four panels:
- **One beat, taken apart:** P, PR, QRS, ST, T and TP, each with what is happening in the heart, normal values, and a cursor that follows the live beat.
- **Six limb leads, one heart:** a hexaxial wheel with the mean axis and the live vector, and six one-beat traces you can click.
- **Measure it:** rate, rhythm, PR, QRS, QT/QTc and axis, with abnormal values highlighted.
- **Reading any ECG:** paper math, a step-by-step reading order, which leads look at which wall and artery, and how the chest leads work.

**The numbers are real.**
- The model is one heart vector in the frontal plane, and the electrodes read its projection.
- Einthoven's law holds exactly (II = I + III).
- Normal axis comes out at +55°; LAFB at −56°.
- In the STEMI preset, ST rises more in III than in II, with mirror-image depression in aVL.
- QT follows the Bazett formula.

It's a teaching model, not for clinical use, and the page says so.

**Checks run:**
- The kit's geometry audit passed with zero problems.
- The line-end check found 0 loose ends in both themes and in several lead positions.
- No console errors.
- I also checked the phone width, reduced-motion mode (shows one still strip), clicks and keys, a slowed-down sheet of a full beat, and 4× close-ups in both themes.

**Limits:**
- Only the frontal plane is modelled. The chest leads (V1–V6) are explained in the notes but aren't on the phantom.
- On a phone the figure reads but is small.

Files are in `~/Desktop/02_projects/anatomy-ekg/`:
- ekg.html
- ekg-light.html
- preview-dark.png
- preview-light.png
- build.mjs (rebuild with `node build.mjs`, `--light`, `--audit`)