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Slide 1

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The contraction of muscles is associated with electrical changes called
`depolarisation', and these changes can be detected by electrodes attached
to the surface of the body
When the electrical changes associated with cardiac muscle contraction
are inscribed on a ruled strip of paper, they provide an electrocardiogram
that is a permanent record of cardiac activity
In order to understand the ECG trace, it is necessary to consider the electrical
properties of cardiac muscle…read more

Slide 2

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During diastole, when cardiac muscle cells are at rest, they display an
unequal distribution of charge across the membrane
At rest, cardiac muscle cells are internally negative and externally positive
Cardiac Muscle Cell
This charge difference creates a small voltage across the membrane, which
can be detected by electrodes across the chest wall
When cardiac muscle cells are at rest and therefore internally negative and
externally positive, they are described as being POLARISED…read more

Slide 3

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When a stimulus from the SA node spreads along the cardiac muscle cell
membranes, there is a reversal of the charge distribution
Stimulus from
SA node
Cardiac Muscle Cell
The muscle cell is now internally positive and externally negative and the
cell is described as being DEPOLARISED
Depolarisation stimulates the cardiac muscle to contract
Depolarisation results in a voltage change across the membrane and this is
detected by electrodes applied to the chest…read more

Slide 4

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Cardiac muscle cells form an interconnected network
The stimulus originating from the SA node spreads from cell to cell
creating a wave of depolarisation throughout the muscle network
Depolarised Cardiac Muscle Cell
Depolarisation results in muscle contraction and thus
a wave of contraction spreads throughout the network…read more

Slide 5

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As the stimulus dies away, the muscle cells return to their
This event is termed REPOLARISATION
The ECG electrodes detect the waves of depolarisation and
repolarisation occurring during the cardiac cycle and the ECG trace
is a record of these events…read more

Slide 6

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T wave
P wave
The ECG trace for each heartbeat displays a P wave, a QRS wave or complex
and a T wave…read more

Slide 7

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Slide 8

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