An ECG provides a picture of the electrical activity of the heart. It involves recording electrical signals via electrodes placed on the body. A normal ECG contains waves, intervals, segments and complexes that represent different electrical events in the heart during each heartbeat. Key aspects of an ECG include examining the rate, rhythm, axis, intervals and segments to identify normal patterns or abnormalities that may indicate cardiac disease. The standard ECG has 12 leads that measure the heart's electrical activity from different angles.

1. NORMAL ECG

2. CONTENTS
• What is ECG
• ECG leads
• Parts of ECG
• Interpetration
• Rate
• Rhythm
• Axis

3. • What is ECG?
- An electrocardiogram is a picture of the electrical conduction of the
heart
- By examining changes from normal on the ECG, clinicians can identify
a multitude of cardiac disease processes
- There are two ways to learn ECG interpretation:
1. pattern recognition
2. understanding the exact electrical vectors recorded by an ECG as
they relate to cardiac electrophysiology

4. ECG
• The standard ECG has 12 leads
• 6 of the leads are considered “limb leads” because they are placed on the arms and/or legs of the individual.
- lead I
- lead II
- lead III
- aVL lead
- aVR lead
- aVF lead
• Leads I, II, III are called bipolar leads as they measure the potential difference (electrical difference) between two
electrodes
• aVL, aVR, aVF lead are unipolar limb leads/ augmented limb leads; these leads are calculated as a combination of
leads I, II and III
• The other six leads are considered “precordial leads” because they are placed on the torso (precordium).
• The six precordial leads are called leads V1, V2, V3, V4, V5 and V6.

8. Parts of ECG

9. ECG GRID
• First, the standard 12-lead ECG is a 10-second strip. The bottom one
or two lines will be a full “rhythm strip” of a specific lead, spanning
the whole 10 seconds of the ECG
• Other leads will span only about 2.5 seconds

10. • ECG paper has horizontal and
vertical lines at regular
intervals of 1 mm. every 5th
line (5mm) is thickened
• DURATION
- Denoted by vertical lines
- Interval between 2 thin lines
(1mm)= 0.04 s
- 2 thick lines (5mm) = 0.2 s
• AMPLITUDE
- Denoted by horizontal lines
- 1mm = 0.1mV
- 5mm = 0.5mV
• SPEED OF PAPER
- 2 adjustable speed:
- 25mm/sec
- 50mm/sec
- Usually fixed at 25mm/s;
changed to 50mm/s if heart
rate is very high

11. • The standard approach to reading an ECG includes, in this order:
1. Examining rate
2. Examining the rhythm
3. Examining the axis, intervals and segments
4. Examining everything else

12. • A normal ECG contains waves, intervals, segments and
one complex
• Wave: A positive or negative deflection from baseline
that indicates a specific electrical event. The waves on
an ECG include the P wave, Q wave, R wave, S wave, T
wave and U wave.
• Interval: The time between two specific ECG events.
The intervals commonly measured on an ECG include
the PR interval, QRS interval (also called QRS
duration), QT interval and RR interval.
• Segment: The length between two specific points on
an ECG that are supposed to be at the baseline
amplitude (not negative or positive). The segments on
an ECG include the PR segment, ST segment and TP
segment.
• Complex: The combination of multiple waves grouped
together. The only main complex on an ECG is the QRS
complex.
• Point: There is only one point on an ECG termed the J
point, which is where the QRS complex ends and the
ST segment begins.

13. • The waves recorded by limb lead II are considered as the typical waves
Wave/ segment From - to Cause Duration (seconds) Amplitude (mV)
P wave
- Positive wave
- 1st wave in ECG
- Also called atrial
complex
- When SAN creates
action potential that
causes depolarization
of atrial musculature
0.1 0.1 – 0.12
QRS complex
- Initial ventricular
complex
Onset of Q wave to
end of S wave
Ventricular
depolarization
0.08 – 0.10 S = 0.4
Q wave
- Small negative wave
- Depolarization of
basal portion of
interventricular
septum
0.1 – 0.2
R wave
- R wave is the first
upward deflection
after the P wave
Depolarization of the
apical portion of
interventricular
septum and apical
portion of ventricular
muscle
1

14. Wave/ segment From - to Cause Duration (seconds) Amplitude
(mV)
S wave
- Small negative wave
- Depolarization of basal
portion of ventricular muscle
near the atrioventricular ring
0.4
T wave
- The final ventricular complex
- A positive wave
- Repolarization of ventricular
musculature
0.2 0.3
TP segment
- should always be at baseline
- used as a reference to determine
whether the ST segment is
elevated or depressed, as there
are no specific disease conditions
that elevate or depress the TP
segment
from the end of
the T wave to
the beginning of
the P wave
P-R interval Onset of P wave
to onset of Q
wave
Signifies atrial depolarization
and conduction of impulses
through AV node
Shows the duration of
conduction of impulses from
SAN to ventricles through
atrial muscles and AVN
0.18 (0.12- 0.2)
- More that 0.2 signifies
delay in conduction of
impulse from SAN to
ventricles
- Usually delay occurs in AVN
(AV nodal delay)
-

15. Wave/ segment From - to Cause Duration (seconds) Amplitude
(mV)
Q-T interval Onset of Q wave
to end of T wave
the time from the beginning
of the QRS complex,
representing ventricular
depolarization, to the end of
the T wave, resulting from
ventricular repolarization
(electrical activity in
ventricles)
0.4 – 0.42
- Women have a longer QT
interval than men
- Lower heart rates also result
in a longer QT interval
S-T segment End of S wave to
onset of T wave
Isoelectric
- Any deviation from
isoelectric base indicates
pathological condition
- Elevation: MI
- Depression: myocardial
ischaemia and
hypokalemia
0.08

16. Examining rate
• There are two different rates that can be determined on ECGs.
1. The atrial rate: indicated by the frequency of the P waves
2. ventricular rate: indicated by the frequency of the QRS complexes
• Normally, the atrial rate should be the same as the ventricular rate in
the absence of disease
• R-R interval: distance between two consecutive R waves
• HR: number of R waves per unit time

17. 0.2 s
10s

18. Determining rhythm
• The rhythm is either sinus rhythm or not sinus rhythm.
• Sinus rhythm refers to the origination of the electrical activity coming
from SA node
• This results in an upright P wave in lead II on the ECG.
• If there is a P wave before every QRS complex, and it has a sinus
morphology, then normal sinus rhythm, or NSR, is said to be present
• A sinus morphology is an upright P wave in lead II and biphasic (up
and down) P wave in lead V1.

19. • If the P wave has a morphology different from the typical sinus
morphology, it is termed ectopic, meaning coming from somewhere
other than the sinus node

20. Determining axis
• The axis of the ECG is the major direction of the overall electrical
activity of the heart.
• The mean direction of flow of electrical potential at one instance =
instantaneous mean vector/ instantaneous summated vector

21. Degree of instantaneous mean vector at
different limb leads
• When recording ECG in different limb leads, the degree of vector is
altered
• The direction of current flow is always from –ve point to +ve point
• When the electrical potential flows in a horizontal plane from the
right to left side of the heart, the degree of vector is zero

22. Lead I:
Electrical potential travels from right to left in a horizontal plane.
So the degree of vector is considered zero
Lead II:
Vector is from above downwards and slightly towards the left, ie 60
degrees
Lead III:
From above downward and slightly towards right at 120 degrees
aVR:
From below towards upper part of the heart and slightly towards
right at 210 degrees
aVF:
From above downwards at 90 degrees
aVL:
From below towards upper part of the heart and slightly towards
left at -30 degrees/ +330 degrees

23. Calculated vector/ mean QRS vector
• The instantaneous mean vector cannot be determined by the
recording of ECG
• But, another vector can be calculated by measuring the amplitude of
QRS complex from ECG recorded in standard limb leads
• It is called calculated vector/ means QRS vector
= electrical axis of the heart
= cardiac vector

24. • The normal QRS axis should be between -30 and +90
degrees.
• Left axis deviation is defined as the major QRS vector,
falling between -30 and -90 degrees.
• Right axis deviation occurs with the QRS axis and is
between +90 and +180 degrees.
• Indeterminate axis is between +/- 180 and -90 degrees

25. • The fastest non-specific method to determine the QRS axis is to find
the major direction of the QRS complex — positive or negative — in
leads I and aVF.
• Normal QRS axis:
If the QRS complex is upright (positive) in both lead I and lead aVF, then
the axis is normal
• The electrical vector heads towards
the positive of lead I and the
positive of lead aVF, as indicated by
the arrows.
• The QRS axis is thus between these
two arrows, which falls within the
normal range.

26. • Left axis deviation:
- If the QRS is upright in lead I (positive) and downward in lead aVF
(negative), then the axis is between 0 and -90 degrees.
- However, recalling that left axis deviation is defined as between -30
and -90, this scenario is not always technically left axis deviation.
- In this scenario, the QRS axis could fall between 0 and -30, which is
within normal limits. To further distinguish normal from left axis
deviation in this setting, look at lead II. If lead II is downward
(negative), then the axis is more towards -120, and left axis deviation
is present. If the QRS complex in lead II is upright (positive), then the
axis is more towards +60 degrees, and the QRS axis is normal.

28. • Right axis deviation:
- If the QRS is predominantly negative in lead I and positive in lead aVF,
then the axis is rightward (right axis deviation)

29. • Indeterminate axis:
- If the QRS is downward (negative) in lead I and downward (negative)
in lead aVF, then the axis is indeterminate and sometimes referred to
as “northwestern axis.”
- This finding is uncommon and usually from ventricular rhythms;
however, it can also be from paced rhythms, lead misplacement and
certain congenital heart diseases.

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