1. Reading ECGs
Dr Sherwan R Shal
2007
Exclusive
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2. YOU should have the following information:
• Patients IDs e.g. name,
• Date (in years, months, days, hours and
minutes).
• Place where ECG was taken.
• ALL written or printed on the ECG paper
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3. You should be able to..
Differentiate normal from abnormal ECG.
And at this level:
1. Heart rate.
2. Rhythm.
3. Time duration.
4. Voltage amplitude.
5. Heart axis, deviations to right or left.
6. Overall wave forms and configuration.
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4. • ECG is recorded on a paper which is divided by
horizontal and vertical lines forming squares.
• A large square is divide into 25 small squares.
• One large square is (5 X 5) square mm; i.e.
5mm length and 5mm width.
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5. • Each large square is
divide into 25 small 1 mm
squares. 5 raws and
5 columns.
• Each small square is 5 mm
1X1 square mm; i.e.
1mm in length and
1mm width.
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6. • The normal speed of the ECG paper is 25mm/second.
• This is equal to 25 small squares per second.
• This also means 5 large squares per second.
Speed =
25 mm/second
25 small squares
1 2 3 4 5
5 large squares/second
One large square
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7. • Horizontal line represents time.
• So, one small square equals to 1/25 second or equals to 0.04
second.
• And, one large square equals to 1/5 second or 0.2 second.
5 large squares 25 small squares
= one second = one second
One large square one small square
= 0.2 second = 0.04 second
5 4 3 2 1
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8. • Vertical line represents voltage.
• Traditionally, every 1o small squares (or 2 large squares) up or down is 1mV.
• So, one small square is 1/10 or 0.1 mV.
• And, one large square is ½ mV or )0.5 mV.
One large
square
0.5 mV
One small square
2 0.1 mV
Two large
Squares 1
1 mV
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9. 13:55 9

10. Measurements
1. Measurements
Heart Rate: 60 - 100 bpm
PR Interval: 0.12 - 0.20 sec (= 3 to 5 small sq.)
QRS Duration: 0.06 - 0.10 sec (= 1.5 to 2.5 small sq.)
QT Interval (QTc < 0.40 sec) (= ≤ 10 small sq.)
QT < 0.42 @ 60 bpm
Corrected QT interval = QT interval /√ (RR) interval
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11. 13:55 11

12. Measurements
2. Rhythm: Normal sinus rhythm:
The P waves in leads I and II must be upright (positive) if
the rhythm is coming from the sinus node.
3. Conduction:
Normal Sino-atrial (SA), Atrio-ventricular (AV), and Intraventricular
(IV) conduction
Both the PR interval and QRS duration should be within the limits
specified above.
P Wave P duration < 0.12 sec P amplitude < 2.5 mm
QRS Complex QRS duration < 0.10 sec
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13. Normal Voltages in the Electrocardiogram
 The voltage of the QRS complex usually is 1.0 to 1.5
millivolt from the top of the R wave to the bottom
of the S wave;
 The voltage of the P wave is between 0.1 and 0.3
millivolt;
 The voltage of T wave is between 0.2 and 0.3
millivolt.
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14. P-Q or P-R Interval
 The time between the beginning of the P wave and the
beginning of the QRS complex is the interval between the
beginning of electrical excitation of the atria and the
beginning of excitation of the ventricles.
 This period is called the P-Q interval.
 The normal P-Q interval is about 0.16 second. (Often this
interval is called the P-R interval because the Q wave is likely
to be absent.)
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15. Heart Rate
• Heart rate is measured in beats/minute.
• ECG paper speed is in mm/second, so we
should change it to mm/minutes.
• 25 mm/second equals to 60X25 mm/second
• = 1500 mm/min. (1500 small sq/min.)
• = 1500/5 =300 large sq. /min.
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16. Heart Rate
• One heart beat is one cardiac cycle.
• One cardiac cycle starts from one wave e.g. R
wave , to the next R wave (or from p to next p)
• Count the number of small sq. between two
successive R waves,
• OR the number of large sq. between two RR.
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17. Heart Rate
= 1500 / No. of small sq. bet. 2 RR waves
= 300/ No. of large sq. bet. 2 RR waves .
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18. 13:55 18

19. Applications
RR interval
Voltage (mV)
P QRS
T
Q
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Time (seconds) 19

20. Applications
Heart rate
P duration 1 2 3 4 5 6 7 8 9
P Voltage
QRS duration
QT duration
P QRS
T time T
T amplitude Q
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21. Regular Rhythm
Regular if
the
distance
between
two
successive
R waves R
regular and
all other
wave are
found and
sinus
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22. Irregular Rhythm
Irregular if
the
distance
between
two
successive
R waves R
is irregular
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23. Physiological Causes of axis deviation
• When the diaphragm ascends up with
expiration, the heart becomes more
horizontal (i.e. deviates to the left)
• Inspiration, on the other hand, makes the
diaphragm to descends down, and the heart
will be hanged or vertical (i.e. right axis)
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24. Physiological Causes of axis deviation
• Diaphragm moves down with inspiration,
standing (because abdominal contents
descends) this happen also with tall persons.
• Diaphragm moves up with expiration, lying
down, or in fatty person and any cause of
increased intraabdominal mass (like
pregnancy)
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25. AXIS DEVIATION
Heart
Diaphragm

26. The axis of the heart
Normal cardiac axis is about 58o
It is close to lead II
It might shift to the Left
left or to the right
Right
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27. Left Axis
Axis is to the left of
hexagonal reference
system.
Angle is near zero and
less
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28. Left Axis
Axis is to the left of
hexagonal reference
system.
Angle is near zero and
less
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29. Right Axis
Axis is to the right of
hexagonal reference
system.
Angle is near 90 and
more
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30. Right Axis
Axis is to the right of
hexagonal reference
system.
Angle is near 90 and
more
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31. Left and Right axis
I
I
III
III
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32. Left axis deviation Right axis deviation
I I
II II
III III
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33. Normal
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34. Test Your Knowledge
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35. Test Your Knowledge
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36. Test Your Knowledge
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