ECG is recording of the electrical activity of the heart.

  Normally the intracellular potential is negative and the extracellular potential is positive.

      When electrically stimulated, the intracellular potential becomes positive.

  The recorded electrical activity is formed by the heart muscle (myocardial) cells and Purkinje cells.

  The recorded electrical acitivity is directly related to the total mass of the cells.

  Since the Purkinje cells constitute only a very small part of the heart's total mass, the ECG mainly

      represents the electrical activity of the heart muscle (myocardial) cells in the same cardiac chamber.

  In a healthy adult, the left ventricular mass is more than the right ventricular mass. Therefore,

      the QRS complex and the T wave represents
LEFT ventricular depolarization and repolarization,


Why do we use more than one electrode for ECG recording?

  The heart's electrical activity is a three dimensional event.

  Several electrodes, each located differently, observe the heart from different angles.

      If the electrical activity is recorded isoelectric at a certain electrode it will not be observable.

      A differently located electrode may record that electrical activity, thus making it observable.

Color coding of the limb lead electrodes

  Color coding is used for quick and correct selection of the electrodes.

  Different colors are preferred in Europe and U.S.A.

  In Europe: red for right arm, yellow for left arm, black for right foot, green for left foot.

  In U.S.A.: white for right arm, black for left arm, green for right foot, red for left foot.

Some ECG machines designate chest electrodes with the letter " V ",

some others designate chest electrodes with the letter "
C ".

Chest electrodes are placed as follows:

  V1: 4th intercostal space, at the right sternal border.

  V2: 4th intercostal space, at the left sternal border.

  V3: between leads V2 and V4.

  V4: At the 5th intercostal space on the midclavicular line.

  V5: At the 5th intercostal space on the anterior axillary line.

  V6: At the 5th intercostal space on the midaxillary line.

Common misplacement errors of the chest electrodes

  Placing the leads V1 and V2 at the 2nd intercostal spaces.

  Placing the lead V3 at the right border of the sternum: You can realize this misplacement error by

      observing that the R wave amplitude increases from lead V1 to lead V2, then decreases at lead V3

      and re-increases at lead V4.

Correct electrode placement is crucial for the serial ECG recordings of a patient during hospitalization.

Differently placed electrodes may result in the erroneous appearance of newly developed ST segment

depression or elevation.

The recording principle of electrical activity is as follows:

  The electrical activity is inscribed

positive if it is approaching the electrode

negative if it is going away from the electrode

isoelectric if it moves perpendicular to the electrode.

Sweep speed of the ECG

  The sweep speed of standard ECG is 25 mm/second.

  When the sweep speed is 25 mm/second,

      one small square on the paper equals to 40 milisecond (0.04 sec)

      1 large square equals to 200 milisecond (0.2 second)

      5 large squares equal to 1 second.

  If long strip recording is needed for rhythm monitoring, 5 or 10 mm/second sweep speed is preferred.

      By this way, long rhythm recording will be possible, permitting the observation of infrequent arrhythmias.

  If tachycardia is present, 50 mm/second sweep speed is preferred to see the details.

      This is especially useful in infants and small children.

Figure 1. The sweep speed of a standard ECG is 25 mm/second.

Figure 2a. Correct placement of chest electrodes is shown above.
Intercostal spaces are palpated.
V1 electrode is placed to the right side of the sternum, at the 4th intercostal space.
Blue vertical line shows
the midclavicular line.

Figure 2b. Above is an example of
incorrect placement of chest electrodes.
V1 and V2 electrodes should not be placed on the 2nd intercostal spaces.

Figure 2c. Above is another example of
incorrect placement of chest electrodes.

Figure 2d. Above is another example of
incorrect placement of chest electrodes.
Electrodes from V1 to V4 are placed like a zigzag.
You can suspect this type of malplacement when the R wave amplitude in V1 is bigger than V2,
and the one in V3 is smaller than the one in V4.

ECG 1a. The ECG above belongs to a man who was admitted to the hospital at the 6th hour of acute anterior myocardial
infarction. During recording of the initial ECG, the C1 and C2 electrodes were misplaced at the 2nd intercostal spaces.

Click here for a more detailed ECG

ECG 1b. The ECG was recorded again after placing the leads C1 and C2 to the 4th intercostal spaces (correct placement).
Since the electrodes C1 and C2 are closer to the ventricles now, the amplitude of the S wave increased and the ST segment
is elevated more. This case clearly shows that in patients hospitalized for management of acute coronary syndromes,
the detection of recently developed ST segment deviations rely heavily on correct electrode placement.

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ECG 2a. The ECG above belongs to a 43 years-old asymptomatic woman. There are ventricular couplets and triplets.
However, this is a 10-second recording only. It does not give us an idea about the frequency of the ventricular beats.

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ECG 2b. The above 6-channel rhythm strip was recorded at a calibration of 10 mm/mV but at a sweep speed of 5mm/second.
The above recording shows approximately 1 minute rhythm, which is 6 times longer than the tracing in ECG 2a.
By this way, a longer recording can be accommodated on the standard ECG paper, allowing us to see how frequent
the ventricular couplets and triplets are.

Click here for a more detailed ECG

ECG 3. Above is a one-channel rhythm tracing recorded at a normal speed and calibration (10 mm/mV, 25 mm/sec).
Some ECG machine producers provide this type of printing.
You can select this format to have an idea on the frequency of extrasystoles.
By this way, you can see 60 seconds of rhythm on a single page without sacrificing from the image quality.

Click here for a more detailed ECG