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A systematic approach to ECG interpretation

Created: 28/8/2006
Updated: 10/8/2006
Like the physical examination, it is desirable to follow a standardised sequence of steps in order to avoid missing subtle abnormalities in the ECG tracing, some of which may have clinical importance. The following order may be helpful:

1) Documentation: Any ECG record should include the name of the patient and the date and time it was recorded.

2) Calibration signal: The amplifier gain is normally adjusted so that a 1 millivolt signal through the ECG amplifier results in a vertical deflection of 10 mm (two large ECG squares). All voltage measurements on the ECG depend entirely on the accuracy of this calibration signal. (The paper speed is 25 mm/s, which amounts to 0.04 s per small box on the horizontal axis).

3) Recording quality: Look for any baseline drift (which makes ST segment analysis impossible), skeletal muscle interference (seen as sharp, irregular, spiky waves throughout the recording - e.g. during shivering) or mains frequency interference (seen as regular sine wave oscillation with a frequency of 50 Hz).

4) Measurements:

a) Heart rate: Can be calculated easily from the ECG paper itself.

Because ECG paper moves at a standardised 25 mm/s, the vertical lines can be used to measure time. There is a 0.20 s interval between two of the large lines. Therefore, if you count the number of heart beats (QRS complexes) in between 30 large boxes (6 s) and multiply by 10, you have beats per minute. Conveniently, ECG paper usually has special markings every 3 s, so you do not have to count 30 large boxes.

There is, however, an easier and quicker way to estimate the heart rate. As seen in the diagram below, when QRS complexes are one box apart, the rate is 300 bpm. Two boxes apart... 150 bpm, etc. So if you memorise these simple numbers, you can estimate the heart rate at a glance!

Estimation of heart rate

Rule of 300 - Provided that the rhythm is regular, heart rate can be estimated by dividing 300 by the number of large boxes in the R-R interval.

i.e. Heart Rate = 300/ no. of large boxes in the R-R interval

If the rhythm is irregular, a longer ECG strip should be obtained and the number of QRS complexes on a strip of determined duration must be counted.

If the number of QRS complexes on a strip of 60 large squares (which is the number of complexes in 12 s, because each large square is 0.2 s in duration) is multiplied by 5, then the rate per minute is deduced.

b) PR interval: 0.12 - 0.20 s
c) QRS duration: 0.06 - 0.10 s
d) Corrected QT interval (QTc): normal QTc = 0.40 s

Bazett's formula: QTc = QT / RR1/2 (in seconds) 

e) Frontal plane QRS Axis: +90° to -30° (in the adult)

5) Rhythm: Normal sinus rhythm or any abnormalities present

6) Morphological information: gives information about the physical condition of the heart. This information is contained in the P waves, QRS complexes, ST segments and T waves. The waves should be studied in both the limb and precordial leads, as discussed above.

7) Comparison with previous ECG: The present ECG should be compared with any previous ones in the patient’s notes, to see if any significant changes have occurred. These changes may have important implications for clinical management decisions.

Note: Horizontal and vertical heart:

(1) Mean frontal plane QRS axis is between 0° to -30°: heart is said to be horizontal

(2) Mean frontal plane QRS axis is between +60° to +90°, the heart is said to be vertical.

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