Understanding 12 Lead EKGs A Practical Approach Brenda M' Beasley, RN, BS, EMT Paramedic Michael C'

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Understanding 12 Lead EKGsA Practical
ApproachBrenda M. Beasley, RN, BS, EMT-
ParamedicMichael C. West, RN, MS, EMT- Paramedic

• Chapter 3
• Basic Electrophysiology

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Basic Electrophysiology

• Objectives
• State the two basic myocardial cell groups
• Describe the function of each myocardial cell
  group
• Discuss the four primary properties of cardiac
  cells
• List the three major electrolytes that affect
  cardiac function

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Basic Electrophysiology

• Objectives continued
• Describe the movement of ions
• Describe cardiac depolarization
• Describe cardiac repolarization
• Define refractory period
• Describe the absolute refractory period
• Describe the relative refractory period

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Basic Cell Groups

• The Myocardial Working Cells
• responsible for generating the physical
  contraction of heart cells
• muscular layer of atrial walls and thicker
  muscular layer of ventricular walls
• primary functions are both contraction and
  relaxation
• physical contraction of myocardial tissue
  actually generates blood flow

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Basic Cell Groups

• Specialized Pacemaker Cells
• do not have the ability to contract
• Responsible for controlling rate and rhythm by
  coordinating regular depolarization
• found in electrical conduction system of the
  heart
• primary function is generation and conduction of
  electrical impulses

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Basic Cell Groups

• Threshold - point at which a stimulus will
  produce a cell response
• all or none phenomenon - stimulus is strong
  enough for cardiac cells to reach threshold, ALL
  cells will respond to this stimulus or none will
  respond

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Primary Cardiac Cell Characteristics

• Possess four primary cell characteristics
• 1. Contractility - mechanical function is
  also referred to as rhythmicity, is the ability
  of cardiac cells to shorten and cause cardiac
  muscle contraction in response to an electrical
  stimulus
• 2. Automaticity - electrical function
  ability of cardiac pacemaker cells to
  spontaneously generate own electrical impulses
  without external stimulation

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Primary Cardiac Cell Characteristics

• 3. Excitability - Electrical Function
  ability of cardiac cells to respond to electrical
  stimulus is also referred to as irritability.
• 4. Conductivity - Electrical Function
  ability of cardiac cells to receive an
  electrical stimulus and then to transmit the
  stimulus to other cardiac cells, they function
  collectively as a unit

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Major Electrolytes That Affect Cardiac Function

• Myocardial cells bathed in electrolyte solutions
• Electrolyte - substance or compound whose
  molecules dissociate into charged components, or
  ions
• Produce charged ions cation and
• - charged ions anion when placed in water

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Three Major Cations that affect Cardiac Function

• Potassium K - performs major function in
  cardiac depolarization and repolarization
• Sodium Na - performs vital part in
  depolarization of myocardium
• Calcium Ca - important function in
  depolarization and myocardial contraction

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Movement of Ions

• Ionic difference on two sides of cell
• Potassium ion -concentration greater inside cell
• Sodium ion -concentration greater outside cell
• Sodium-potassium exchange pump - active
  transport, potassium / sodium moved in and out of
  cell through cell membrane
• During polarized , or resting state, inside of
  cell electrically negative relative to outside of
  cell

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Cardiac Depolarization

• When impulse develops and spreads throughout the
  myocardium, changes occur in heart muscle fibers
• Cardiac Depolarization - sodium ions rush into
  cell, changing interior charge to after cell
  stimulated
• Cardiac Repolarization - sodium ions return to
  outside of cell, potassium returns to inside of
  cell.

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Ion Shifts Fig. 3-1

• Ion shifts during depolarization and
  repolarization

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Terms to Remember

• Resting Membrane Potential - state of cardiac
  cell - inside negative, outside positive

• Action Potential - change in polarity produces
  change in cell electrical charge caused by
  stimulation of myocardial cells

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Terms to Remember

• Syncytium - cardiac muscle cell groups that are
  connected together and function as a unit

• Polarized State - resting state of cardiac cell,
  inside of cell is electrically negative relative
  to outside

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Refractory Periods

• Attempts to ensure muscle is totally relaxed
  before another action potential or depolarization
  can be initiated
• Atrial muscle 0.15 sec.
• Ventricular muscle 0.25 to 0.3 sec.
• Period of rest is referred to as
• Cardiac Repolarization

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Two Stages of Repolarization

• Absolute Refractory Period - cardiac cell unable
  to respond to new electrical stimulus, cannot
  spontaneously depolarize

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Two Stages of Repolarization

• Relative Refractory Period - repolarization is
  almost complete, cardiac cell can be stimulated
  to contract prematurely if stimulus is stronger
  then normal