How to Understand 12 Lead ECG

Written by donna macciochi
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How to Understand 12 Lead ECG
Electrocardiogram printout (ecg image by Zbigniew Nowak from

An electrocardiogram measures electrical activity in the heart through electrodes attached to the skin on the arms, feet and chest. The signal is fed to an ECG machine through 12 leads clipped to each electrode. Each lead is marked with its corresponding position -- RA, RF, LA, LF, V1-6. The signal is converted to a graph or series of waves, which is printed out. An ECG is used to diagnose cardiac disorders such as a heart attack, heart block or abnormal rhythm. Carlo Matteucci, professor of physics at the University of Pisa, discovered that the heart contains electric currents in 1838. It was not until 1895, however, that the first ECG was recorded. ECGs are commonplace today; most health care providers can take an ECG and understand its meaning.

Skill level:

Things you need

  • ECG machine
  • ECG paper
  • 12 electrodes

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  1. 1

    Remove the patient's jewellery and make sure she is resting comfortably on the couch. Reassure the patient that the procedure is painless.

  2. 2

    Attach an electrode to the left and right arms and feet -- RA, RF, LA, LF.

    How to Understand 12 Lead ECG
    Electrodes (electrode sèche image by YvesBonnet from
  3. 3

    Feel down the patient's rib cage; you will feel a rib, then a space. Count four spaces down and attach an electrodes -- V1 and V2 -- over this space on either side of the breastbone.

  4. 4

    Attach an electrode over the fifth space in line with the centre of the left collarbone -- V4.

  5. 5

    Attach the next electrode, V3, halfway between V2 and V4.

  6. 6

    Attach electrode V5 on a horizontal plane next to V4, in line with the start of the left armpit. V6 goes next to V5 on a horizontal plane in line with the centre of the left armpit.

  7. 7

    Attach each marked lead to the corresponding electrode.

  8. 8

    Set the filter speed on the ECG machine to 25mm per second. Ask the patient to remain still and breathe normally.

  9. 9

    Press "Start" or "Acquire" to take the ECG.

  10. 10

    Observe the printout. Look for the P, Q, R, S and T waves. P is the first bump in each sequence, Q is the left-hand base of the spike, R is the peak of the spike, S is the right-hand base of the spike and T is the second bump. Interpretation involves checking the rate of, size of and time between each wave, as well as checking for absent or repeated waves.

    How to Understand 12 Lead ECG
    P, Q, R, S and T waves (ecg image by Zbigniew Nowak from
  11. 11

    Look for common cardiac conditions. An absence of P waves suggests atrial fibrillation, an elevated ST segment is often seen in a heart attack and a long PR interval corresponds to heart block.

Tips and warnings

  • Clip or shave a male patient's hairy chest to ensure the electrodes are securely attached to the skin.
  • MedlinePlus has an information leaflet for patients outlining the procedure.
  • Only trained practitioners should take and read ECGs.

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