ECG in depth — 12-lead recording, electrode placement and artifacts.
How to ensure a technically correct ECG that the doctor can trust
Introduction
The ECG recording is one of the most common tools in your everyday work as a hospital technical assistant, and the simple curve on the screen is based on over a hundred years of electrophysiological research. But behind the action — placing electrodes on a patient and hitting 'record' — lies a precise standard for placement that determines whether the doctor can trust the result.
From Einthoven's triangle to the modern 12-lead system
The classic 12-lead ECG builds on the work of Dutch physician Willem Einthoven, who in the early 1900s described the heart's electrical axis as a triangle between the right arm, left arm and left leg. Electrodes on arms and legs give the three classic Einthoven leads (I, II, III) plus the amplified Goldberger leads (aVR, aVL, aVF). In addition there are six chest leads (V1-V6) measuring the heart's electrical activity from six points around the chest based on the so-called Wilson central terminal principle. Together the 12 leads give a spatial picture of the heart's electrical activity seen from many angles.
| Drainage group | Electrodes | What the derivative tells |
|---|---|---|
| Bipolar limb leads (I, II, III) | Right arm left arm left leg | The heart's electrical axis seen from the side |
| Enhanced leads (aVR aVL aVF) | Same limb electrodes, calculated amplified | Supplements the Axle View from Three Additional Angles |
| Chest leads (V1-V6) | Six electrodes at fixed points on the chest | The heart's activity seen 'from the inside out' in the horizontal plane |
How to place the chest electrodes correctly
- 01V1: 4th intercostal space, directly to the right of the breastbone (sternum).
- 02V2: 4th intercostal space, directly to the left of sternum.
- 03V4: 5th intercostal space, in the midclavicular line (opposite the midpoint of the collarbone).
- 04V3: midway between V2 and V4.
- 05V5: same height as V4, in the anterior axillary line.
- 06V6: same height as V4 and V5, in the mid-axillary line.
Preparation of skin — the prerequisite for a clean signal
A large part of poor ECG recordings is not due to the machine, but to the skin under the electrode. Skin with sebum, sweat, hair or skin cells gives high contact resistance and a noisy signal. Therefore, you clean the skin with an alcohol wipe or similar, and you shave a small area if necessary where there is a lot of hair. The electrodes must sit on clean, dry skin and be pressed firmly so the gel gets full contact. Between each patient you follow the department's hygiene routines, which are based on Statens Serum Institut's National Infection Control Guidelines (NIR) on hand hygiene and patient-near equipment.
Common artifacts – and how you avoid them
- 01Muscle tremor: patient tenses or shakes — ask patient to relax and support arms and legs if needed.
- 02Loose electrode or dry gel: gives a 'broken' or flat signal — replace the electrode.
- 03Mains hum (50 Hz noise): often caused by electrical interference — check grounding and keep cables away from power cables.
- 04Electrode misplacement: gives a curve image that doesn't match the patient's anatomy — always double-check placement if the curve looks unusual.
“A 12-lead recording is only as good as the poorest electrode.”