Action Potential

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Action Potential ECG

• Prof.Lammers
• CVS Module/ Week 2

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Action potential of the cardiac muscles

• The cardiac action potential is made of 3 phases
• Depolarization caused by the opening of Fast Na
  channels slow Ca channels
• Plateau remaining of slow Ca channels open for
  several seconds, drawing large amount of Ca
  inside which prolong depolarization
• Replarization Opening of potassium channels

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The plateau phase

• The plateau phase is explained by
• Opening of the slow Ca channel ? prolong
  depolarization
• After the onset of action potential the
  permeability to potassium channels decreases
  ?prevent rapid return of action potential to
  resting membrane potential.

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Tissues of the heart

• There are three main tissues in the heart
• Atrium and ventricular muscle moderate
  conducting velocity and AP needs to return to
  reach threshold.
• Nodal tissue ( SA, AV, bundle of His) slow
  conducting velocity and could be pace makers
• 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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Conducting system of the heart

• AV bundle that conduct impulses from the atrium
  to the ventricle
• Rt-Lt Bundle branches purkinji fibers that
  conduct 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 by fibrous tissue that surround
  the AV valvular opening.

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ECG waves

• A normal ECG is made of
• P wave that present atrial depolarization
• QRS complex that present ventricular
  depolarization
• T wave that present ventricular repolarization
• PQ interval no electrical signal ( transmission
  of impulses from Atrium to ventricles)
• ST segment present the platue phase. In ischemic
  heart disease, ST segment is elevated due to the
  absence of the platue.

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ECG leads

• Three bipolar leads record impulses from 2
  electrodes located on different sides.
• 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 3rd by summation
  of the 2.

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ECG leads

• Chest leads ( Precordial leads)
• Placing the electrodes on the anterior surface of
  the chest V1? 4th intercostal space ( Right
  sternal border).
• V2 ? 4th intercostal space ( left sternal
  border).
• V3 ? Between V2 and V4.
• V4 ? 5th intercostal space in the mid clavicle
  line.
• V5 ? 5th intercostal space in the anterior
  axillary line.
• V6 ? 5th intercostal space in the mid axillary
  line.

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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.