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Action Potential & ECG
Prof.LammersCVS Module/ Week 2
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The plateau phase
The plateau phase is explained by:
1. Opening of the slow Ca channel prolong
depolarization
2. After the onset of action potential the
permeability to potassium channels
decreasesprevent rapid return of actionpotential to resting
membrane potential.
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Tissues of the heart
There are three main tissues in the heart:
1. Atrium and ventricular muscle: moderate
conducting velocity and AP needs to return toreach
threshold.
2. Nodal tissue ( SA, AV, bundle of His): slow
conducting velocity and could be pace makers
3. Purkinji fibers: has the fastest conducting velocity.
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Conducting system of the heart
The conducting system of the heart is responsible for
generating rhythmical impulses through the heart to
cause rhythmical contrition. It is made of: SA node: where the
normal impulse is generated as
they have the capacity of self-excitation
AV node: in which impulses is delayed before it
reaches the ventricles, to allow it to fill.
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AV bundle: that conduct impulses from the atrium to
the ventricle
Rt-Lt Bundle branches & purkinji fibers: thatconduct
impulses to all parts of the ventricles.
* AV bundle is the only way in which impulses could
be conducted from the atrium to the ventricles.
Because the atria is separated from the ventricles byfibrous
tissue that surround the AV valvular opening.
Conducting system of the heart
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ECG waves
A normal ECG is made of: P wave: that present atrial
depolarization
QRS complex: that presentventricular depolarization
T wave: that present ventricularrepolarization
PQ interval: no electrical signal( transmission of impulses
fromAtrium to ventricles)
ST segment: present the platue
phase. In ischemic heart disease,ST segment is elevated due
tothe absence of the platue.
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ECG leads Three bipolar leads: record impulses
from 2 electrodes located on differentsides.
Lead I: (+) Left limb (-) right limb. Lead II: (+) left foot (-)
right limb.
Lead III: (+) left foot (-) left limb.
These 3 leads for Einthowens triangle in
which the heart is located in the middle. Einthowens law: if
electrical potential of
2 leads was known, we can know the 3rdby summation of the 2.
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Augmented Voltage Leads
Unipolar leads because there is 1 positive electrode
in combination with the ground that is considered.
AVL: Left arm. AVR: Right arm.
AVF: Left foot.
