The EKG

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The EKG

• Physiology in action

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Objectives

• understand basic cardiac anatomy
• understand how cellular action potentials give
  rise to a signal that can be recorded with
  extracellular electrodes
• understand the path for action potential
  propagation through the heart
• understand the origin of the main phases of
  electrocardiogram (EKG)

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The Heart
is a pump
has electrical activity (action potentials)
generates electrical current that can be measured
on the skin surface (the EKG)
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Currents and Voltages

• At rest, Vm is constant
• No current flowing
• Inside of cell is at constant potential
• Outside of cell is at constant potential

0 mV
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Currents and Voltages

• During AP upstroke, Vm is NOT constant
• Current IS flowing
• Inside of cell is NOT at constant potential
• Outside of cell is NOT at constant potential

An action potential propagating toward the
positive ECG lead produces a positive signal
Some positive potential
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More Currents and Voltages
------------------------
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More Currents and Voltages
Repolarization spreading toward the positive ECG
lead produces a negative response
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The EKG

• Can record a reflection of cardiac electrical
  activity on the skin- EKG
• The magnitude and polarity of the signal depends
  on
• what the heart is doing electrically
• depolarizing
• repolarizing
• whatever
• the position and orientation of the recording
  electrodes

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Cardiac Anatomy
Superior
vena cava
Pulmonary veins
Atrioventricular (AV) node
Sinoatrial (SA)A node
Left atrium
Atrial muscle
Mitral valve
Tricuspid valve
Purkinje fibers
Ventricluar
muscle
Descending aorta
Inferior vena cava
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Flow of Cardiac Electrical Activity
SA node
AV node (slow)
Purkinje fiber conducting system
Ventricular muscle
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Conduction in the Heart
0.12-0.2 s
approx. 0.44 s
SA
node
Atria
AV
node
Purkinje
Ventricle
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The Normal EKG
Right Arm
Lead II
Left Leg
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Action Potentials in the Heart
AV
Purkinje
Ventricle