Basics of ecg physiology

2. Limb Leads
Chest Leads
Timing
Calculations
ECG Lead

3. ECG leads

4. ECG leads
Lead systems allow you to look at the
heart from different angles. Each
different angle is called a lead.
Each lead has a positive and negative
pole attached to the surface of the
skin, which can then be used to
measure the spread of electrical
activity within the heart.

5. ECG leads
Upward deflection on the ECG- is produced
when electrical impulses travel towards a
positive electrode.
Downward deflection on the ECG- is produced
when electrical impulses travel towards a
negative electrode.
Flat line (isoelectric line)- is produced when
there is no electrical spread through the heart,
or if the electrical forces are equal.

6. Limb leads

7. Limb leads
• Lead 1
• Negative right shoulder
• Positive left shoulder
• Lead 2
• Negative right shoulder
• Positive left lower chest
• Lead 3
• Negative left shoulder
• Positive left lower chest
Electrical
current
moving
from
negative
electrode
to positive
electrode

8. Einthoven’s Triangle

9. They are called the augmented limb leads because they are
augmented (or amplified) through a modification of
Wilson’s Central Terminal (WCT). The modification was
necessary because otherwise the complexes would have
been too small
aVR – positive electrode
right shoulder
aVL– positive electrode left
shoulder
aVF – positive electrode left
lower chest (foot)

15. Chest Leads

16. Chest Leads
Unlike limb leads that measure electrical
activity in the vertical plane, the
precordial leads measure activity in the
horizontal plane. Each of the 6
electrodes are set as positive

19. V1 = right ventricle and far left
ventricle
V2 = right ventricle and AV
node
V3 = anterior left ventricle
V4 = anterior left ventricle
V5 = lateral left ventricle
V6 = lateral left ventricle

21. Gives a 2 dimensional picture of
what is going on electrically in the
heart
12 Lead
ECG
Placement

23. Rhythm strip

24. Calculations of Axis

26. Normal Cardiac Axis
In healthy individuals you would expect the normal 11 o’clock
to 5 o’clock spread
Therefore the spread of depolarisation would be
heading towards leads I,II & III
As a result you would see a positive deflection in all of these
leads
With lead II been the most positive (it’s at 5 o’clock)
You would expect to see the most negative deflection in aVR
This is due to aVR looking at the heart in
the opposite direction to lead II

28. Right axis deviation
Right axis deviation (RAD) is usually caused by right
ventricular hypertrophy.
In right axis
deviation the direction of depolarisation is distorted to
the right (1-7 o’clock)
Extra heart muscle causes a stronger signal to be
generated by the right side of the heart
This causes deflection in lead I to
become negative & deflection in lead II & III to be more
+ve
RAD is associated with pulmonary conditions as they
put strain on the right side of the heart

30. Left axis deviation
In left axis deviation (LAD) the
general direction of depolarisation becomes distorted
to the left
This causes the deflection in lead III to
become negative
It is only considered significant if
the deflection of Lead II also becomes negative
LAD is usually caused by conduction defects & not by
increased mass of the left ventricle

32. Axis trick
Positive in I and II
= normal
Positive in I
Negative in II =
LAD
Negative in I
Positive in II = RAD

33. Timing

34. Timing

35. Timing
Rate
R-R interval
Is it regular?
What is the heart rate?
300, 150, 100, 75, 60, 50
300 / (# of large boxes)
1500 / (# of small boxes)

36. Timing
Are there P waves….?
Normally =0.08 s = 2 small sqrs
Pointy = P pulmonale (RA
hypertrophy)
Bifid = P mitrale (LA hypertrophy)

37. PR interval
Start of P wave to
start of QRS
Normal = 0.12-0.2s
Too short – can
mean WPW
Too long –means
AV block (heart
block) - 1st/2nd/3rd
degree

38. QRS complex
Should be <0.12s duration
>0.12s = BBB (either LBBB or RBBB)
QRS amplitude
R in V5 or V6 < 2.6 mV
Increased amplitude indicates cardiac hypertrophy
‘

39. Timing
ST segment connects the QRS complex and the T
wave and has a duration of 0.08 to 0.12 secR-R interval
ST depression
Downsloping or horizontal = abnormal
Ischaemia (coronary stenosis)
ST elevation
Infarction (coronary occlusion)
Pericarditis (widespread)

40. Timing
T wave 160ms
Peaked (hyperkalaemia or normal young
man)
Inverted/biphasic (ischaemia, previous
infarct)
Small (hypokalaemia)