The document provides a comprehensive overview of the electrical conduction system of the heart and how to interpret ECG strips. It outlines the components of a normal ECG, including the P wave, QRS complex, and T wave, and details an 8-step procedure for ECG reading. Key metrics like heart rate, rhythm, PR interval, QRS interval, T wave, QT interval, and ST segment are described with normal ranges and interpretation tips.

1. Electrical conduction of the heart
and
Ecg interpretation
Prepared by: ROMMEL LUIS C. ISRAEL III
BY ROMMEL LUIS C. ISRAEL III 1

2. Review of the heart
• Watch this
Video:
• https://www.youtube.com/w
atch?v=oHMmtqKgs50
BY ROMMEL LUIS C. ISRAEL III 2

3. Conduction system
• Watch this
video:
• https://www.youtube.c
om/watch?v=RYZ4da
FwMa8
BY ROMMEL LUIS C. ISRAEL III 3

4. Electrical conduction
BY ROMMEL LUIS C. ISRAEL III 4

5. Understanding an Ecg strip
• This is a 6 seconds ECG strip that
is usually taken and read to look
for electric abnormalities in the
heart.
• It is important to learn the times
indicated by each box to be able to
Each box is 0.2
secs
Black marks at the top
indicate 3 secs intervals
The smallest
boxes are 0.04
secs
BY ROMMEL LUIS C. ISRAEL III 5

6. A normal ecg
• P wave is the first
upward (positive)
deflection
waveform.
• It indicates atrial
depolarization
• QRS complex follows the P
wave.
• Normally, it begins with a
downward deflection, Q; a
larger upwards deflection, R;
and then a downwards S
wave.
• The QRS complex
represents ventricular
depolarization and
contraction.
• T wave is
normally a
modest
upwards
waveform.
• It representing
ventricular
repolarization
• U wave indicates the
recovery of the
Purkinje conduction
fibers.
• This wave component
may not be
observable.
BY ROMMEL LUIS C. ISRAEL III 6

7. Reading ecg
How to read an ECG strip?
An 8 step procedure is
followed:
1. Rate
2. Rhythm
3. P Wave
4. PR Interval
5. QRS Interval
6. T Wave
7. QT Interval
8. ST Segment
BY ROMMEL LUIS C. ISRAEL III 7

8. rate
Counting R waves:
Count the number of R
waves in a 6 second rhythm
strip, then multiply by 10.
Or
Count the number of R
waves in a 10 second
rhythm strip, then multiply by
6.
Counting boxes: 300 - 150 - 100
- 75 - 60 - 50
Count the number of large boxes
to the next R wave. If the second
R wave is 1 large box away the
rate is 300, 2 boxes - 150, 3
boxes - 100, 4 boxes - 75, etc.
Normal: 60 – 100 bmp
BY ROMMEL LUIS C. ISRAEL III 8

9. rhythm
• For ventricular rhythm, examine the R-R intervals.
Are they regular?
• For atrial rhythm, observe the P-P intervals. Are they
regular?
P-P Interval R-R Interval
BY ROMMEL LUIS C. ISRAEL III 9

10. P Wave
How to determine the rhythm?
• Are P waves present?
• Is there a P wave before every QRS complex and a QRS
complex after every P wave?
• Do P waves look alike? (smooth, rounded and upright)
P waves: atrial
rhythm
R waves: ventricular
rhythm
BY ROMMEL LUIS C. ISRAEL III 10

11. Example
• Are P waves present? YES ( )
• Is there a P wave before every QRS complex and a QRS
complex after every P wave? YES and YES ( )
• Do P waves look alike? YES
PR Interval P wave
P waves and
QRS
BY ROMMEL LUIS C. ISRAEL III 11

12. Pr interval
• Does the PR-Interval fall
within the norm of 0.12-
0.20 seconds?
• Is the PR-Interval constant
across the ECG tracing?
P-R interval: measure the interval
from where the P wave begins
until the beginning of the QRS
complex.
Normal PR interval: 0.12 to 0.20 s
BY ROMMEL LUIS C. ISRAEL III 12

13. Qrs Interval
The QRS complex indicates
ventricular depolarization that
triggers contraction of the
ventricles.
• Does the QRS interval fall within
the range of 0.08-0.12 seconds?
• Are the QRS complexes similar in
appearance across the ECG
tracing?
QRS interval: measure the QRS
interval from the end of the PR
interval to the end of the S
wave.
Normal QRS interval is 0.06 to
BY ROMMEL LUIS C. ISRAEL III 13

14. T wave
• The T wave indicates the
repolarization of the
ventricles.
• T waves which have a
downward (negative)
deflection or of T waves
with tall, pointed peaks
should be looked into.
• The U-wave is a small
upright, rounded bump.
When observed, it follows
the T-wave.
BY ROMMEL LUIS C. ISRAEL III 14

15. Qt interval
• The QT interval represents
the time of ventricular
activity (both
depolarization and
repolarization).
• QT interval: measured
from the beginning of the
QRS complex to the end
of the T wave.
• Normally, the QT interval
is 0.36 to 0.44 seconds (9-
11 boxes).
BY ROMMEL LUIS C. ISRAEL III 15

16. St segment
• The ST segment
represents the early part of
ventricular repolarization.
• The ST segment is the line
that from the end of the
QRS complex to beginning
of the T wave.
• Normally the ST segment
is flat, being neither
positive or negative
relative to the baseline.
BY ROMMEL LUIS C. ISRAEL III 16

17. Example
What is the heart rate?
60
90
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18. example
What is the rhythm? Regular or
Irregular?
Regular
Irregular
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19. example
How are the P waves?
P wave shape
varies
Normal
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20. example
How are the PR intervals?
Long PR interval
Normal
BY ROMMEL LUIS C. ISRAEL III 20

21. example
How are the QRS intervals?
Wide QRS, bizarre
appearance
No QRS, bizarre rhythm
BY ROMMEL LUIS C. ISRAEL III 21

22. example
How are the QT intervals?
QT intervals are short
Normal
BY ROMMEL LUIS C. ISRAEL III 22