basics of ecg physiology
TRANSCRIPT
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Limb Leads
Chest Leads
Timing
Calculations
ECG Lead
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ECG leads
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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.
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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.
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Limb leads
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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
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Einthoven’s Triangle
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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)
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Chest Leads
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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
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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
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Gives a 2 dimensional picture of what is going on electrically in the heart
12 Lead ECG Placement
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Rhythm strip
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Calculations of Axis
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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
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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
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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
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Axis trick
Positive in I and II = normal
Positive in I
Negative in II = LAD
Negative in I
Positive in II = RAD
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Timing
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Timing
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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)
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Timing
Are there P waves….?
Normally =0.08 s = 2 small sqrs
Pointy = P pulmonale (RA hypertrophy)
Bifid = P mitrale (LA hypertrophy)
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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
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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
‘
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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)
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Timing
T wave 160ms
Peaked (hyperkalaemia or normal young man)
Inverted/biphasic (ischaemia, previous infarct)
Small (hypokalaemia)
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