Skin
Dry, hairless, cleanClip hair, wipe sweat or lotion with alcohol, let it dry, and rub lightly with gauze to abrade the surface. Most "artifact" is a bad electrode contact, not a bad patient.
Ten electrodes, twelve leads
A 12-lead ECG uses ten electrodes: four on the limbs and six across the chest. The machine combines them into twelve leads, twelve different electrical viewpoints of the same heartbeat. A lead is only as good as its electrode, so placement is the first skill. Tap any electrode on the torso, or a step in the list, to see exactly where it goes and how to find the landmark.
Tap an electrodelimb pads shown at the edge of the torso; place them on the limbs
Placement, in the order you should do it
Finding the fourth intercostal space
Every chest lead keys off V1, and V1 keys off the fourth intercostal space. Do not guess from the nipple line: it moves with body habitus and tells you nothing in women. Walk the sternum instead.
- Find the sternal notch, the dip at the top of the sternum between the collarbones.
- Slide down to the ridge about 4–5 cm below it. That ridge is the sternal angle (angle of Louis), where the manubrium meets the body of the sternum. It is palpable in nearly everyone.
- Move your finger just lateral to the ridge. The rib you feel there is the 2nd rib; the space immediately below it is the 2nd intercostal space.
- Walk down: 3rd rib, 3rd space, 4th rib, 4th space. V1 goes here at the right sternal border, V2 directly across at the left sternal border.
- Drop one more space and come out to the midclavicular line for V4. Then V3 splits the distance between V2 and V4, and V5 and V6 stay level with V4 as you track around the side.
Why it matters: every space you miss shifts V1–V3 about 2.5 cm. One space too high and the machine prints a tracing that can mimic an anterior infarct, a Brugada pattern, or hide a real anterior MI.
Skin prep and the limb question
Limb leads: limbs
Wrists and ankles for a diagnostic 12-leadAnywhere on the limb is electrically the same; distal is traditional, proximal (upper arm, thigh) is acceptable when the hands or feet are unavailable.
Limb leads: torso
Monitoring onlyMoving arm and leg electrodes onto the chest (the Mason-Likar position) shifts the frontal axis rightward and changes Q and R waves in the inferior and lateral leads. Fine for rhythm monitoring, not for comparison with a hospital 12-lead.
Women
V4–V6 under the breastLift the breast and place the electrodes on the chest wall beneath it, at the correct level. Placing them on breast tissue attenuates the signal and moves the leads.
Right leg (green)
Ground onlyRL contributes no signal; it is the reference that removes electrical noise. Any limb will do, but it must be attached.
Consistency
Mark the spotsIf you will be comparing serial ECGs, mark electrode sites with a skin pen. Small shifts between tracings create "changes" that are not in the patient.
Common placement errors and what they do to the tracing
V1 and V2 too high (2nd or 3rd space)
The most common error. Produces incomplete RBBB pattern, poor R-wave progression, inverted T in V1–V2 or a Brugada look-alike. Can mimic or hide anterior and septal infarction.
LA and RA swapped
Lead I flips upside down, aVR looks like a normal aVL, and lead II becomes lead III. If lead I is negative and aVR looks "too normal," check the arm cables before calling it dextrocardia.
V5 and V6 too low or too forward
Drifting down the ribs or placing V6 on the anterior chest changes R-wave size and ST baseline in the lateral leads and defeats serial comparison.
Limb electrodes on the chest
Mason-Likar placement is normal for a monitor, but it shifts the axis right, can create or hide inferior Q waves, and must be noted if the tracing goes to a cardiologist for comparison.
V2 and V3 on breast tissue
Dampened voltages and misplaced leads. Lift the breast and place on the chest wall.
Leg electrode on the arm, arm on the leg
RL/LL swap does nothing (RL is ground). An arm/leg swap in the other direction can turn a limb lead nearly flat; a flat-line lead I, II or III with a good chest tracing is a cable error.
What each lead is looking at
Think of each lead as a camera pointed at the heart from a fixed spot. Electricity moving toward the camera draws an upward deflection; moving away draws a downward one. Leads that point at the same wall from nearby spots are contiguous and see the same injury, which is why ST elevation has to show up in two contiguous leads before it counts. The grid below is the standard printout layout; its colors group the leads by the wall they face. Tap a lead to see its view on the heart and the artery that feeds that wall.
Inferior
Lateral
Septal
Anterior
aVR: the cavity
Right ventricle (V4R)
Posterior (V7–V9)
12-lead printout layout
tap a leadLeft ventricle, short axis, viewed from the apexRV sits to the right of the septum here, not drawn
Pick a lead
Frontal plane: the six limb leadsnormal axis −30° to +90°
Horizontal plane: the six chest leadscross-section, viewed from below
How the limb leads are built
Bipolar: I, II, III
Two electrodes, one looking at the otherI = LA − RA (looks left). II = LL − RA (down and left, the long axis of the heart, which is why it is the default rhythm lead). III = LL − LA (down and right). Together they form Einthoven's triangle.
Augmented: aVR, aVL, aVF
One electrode against the average of the other twoaVR points at the right arm (−150°), aVL at the left arm (−30°), aVF at the feet (+90°). They fill the 60° gaps between the bipolar leads so the frontal plane is sampled every 30°.
Precordial: V1–V6
Each chest electrode against the central terminalThey sweep from the right of the septum (V1) around the apex (V4) to the left lateral wall (V6), sampling the horizontal plane. The normal R wave grows from small in V1 to tall in V5/V6 as the camera turns to face the left ventricle: that is R-wave progression.
Why aVR is upside-down
It looks into the cavityEvery other lead sees depolarization coming toward it. aVR, on the right shoulder, sees it moving away, so P, QRS and T are normally negative. ST elevation in aVR with widespread depression elsewhere points to left main or proximal LAD disease, or global ischemia.
Walls, leads, arteries, and reciprocal changes
| Wall | Leads that see it | Usual artery | Reciprocal depression in | Worth knowing |
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Reciprocal change is the same injury seen from the opposite side of the heart: ST elevation in one wall, ST depression in the leads facing away. Its presence makes a STEMI call far more specific. Coronary anatomy varies: about 85% of people are right-dominant, so the inferior wall is usually RCA, but in the rest it is the circumflex.
Real 12-leads, from normal to acute STEMI
Every tracing here is a real patient, read by a cardiologist. Pick a diagnosis on the left to see what to look for, then page through the examples with the territory highlighted. When the pattern is familiar, go to the Quiz tab and turn on 12-leads: the same tracings come back unlabeled. Printed at 25 mm/s and 10 mm/mV, the standard 3×4 layout with a lead II rhythm strip.
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Cardiologist's report from the database
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Reports are reproduced as written in the database (English, German or Swedish). ST levels shown are measured automatically at J+50 ms against the PR segment and are a study aid, not a machine interpretation.
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