Parasympathetic Stimulation

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Parasympathetic Stimulation

• Vagus nerve
• Primarily innervates atria, but some fibers to
  ventricles also
• Chemical mediator acethycholine
• Effect slows heart rate and AV conduction
• Methods of stimulation Valsalva maneuver,
  carotid sinus pressure

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Sympathetic Stimulation

• Nerves arising in thoracic and lumbar ganglia
• Innervate both atria and ventricles
• Chemical mediator norepinephrine
• Receptor sites alpha, beta

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Effect of alpha Stimulation

• No effect on heart
• Peripheral vasoconstriction

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Effect of beta Stimulation

• Increased rate and conduction
• Increased contractility
• Bronchodilation
• Peripheral vasodilation

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Role of Electrolytes

• Cardiac function, electrical and mechanical,
  influenced by electrolyte imbalances
• Major electrolytes influencing cardiac function
• Na Sodium
• Ca Calcium
• K Potassium

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Role of Electrolytes

• Sodium (Na ) major role in depolarization phase
  of myocardial cells
• Calcium (Ca ) major role in depolarization
  phase of myocardial pacemaker cells and in
  myocardial contractility
• Hypercalcemia increased myocardial contractility
• Hypocalcemia decreased myocardial contractility
  and increased electrical irritability

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Role of Electrolytes

• Potassium (K ) major role in repolarization
  phase
• Hyperkalemia decreased automaticity and
  conduction
• Hypokalemia increased irritability
• Potassium levels are critical to life
• Hyperkalemia Tall peaked T waves

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Electrophysiology

• Electrical properties of the heart
• Automaticity ability to generate an electrical
  impulse without stimulation from another source -
  property of pacemaker cells
• Excitability ability to respond to an electrical
  stimulus -property of all myocardial cells
• Conductivity ability to propagate an impulse
  from cell to cell

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Electrical Conduction System

• Allows electrical impulses to spread through the
  heart six times faster than through muscle alone

• Sequence of normal electrical conduction
• SA node
• Internodal and interatrial tracts
• AV node
• Bundle of His
• Bundle branches
• Purkinje fibers

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Function of electrical conduction structures

• Sinoatrial (SA) node
• Located in right atrium near entrance of superior
  vena cava
• Usually heart's dominant pacemaker

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Internodal and interatrial tracts

• Pathways that carry impulse between SA node and
  AV node and spread it across atrial muscle
• Impulse travel time 0.08 seconds

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Atrioventricular (AV) node

• Part of area called the "AV junctional tissue"
  along with some surrounding tissue and the
  non-branching portion of the Bundle of His
• Responsible for creating slight delay in
  conduction before sending impulse to ventricles
• Impulse travel time 0.08-0.16 seconds
• No pacemaking properties in node itself

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Bundle of His

• Bundle of fibers coming off AV node, located at
  top of interventricular septum
• Considered part of the AV junction
• Makes electrical connection between atria and
  ventricles

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Bundle branches

• Created by bifurcation of Bundle of His into
  right and left branches
• Carry electrical impulse at high velocity to
  interventricular septum and each ventricle
  simultaneously

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Purkinje fibers

• Terminal ends of bundle branches
• Network of fibers helping to spread impulse
  throughout ventricular walls
• Rapid impulse spread through ventricles
  0.08-0.09 seconds

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Depolarization

• Process by which muscle fibers are stimulated to
  contract by the alteration of electrical charge
  of the cell accomplished by changes in
  electrolyte concentrations across the cell
  membrane

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Depolarization at The Cellular Level

• Chemical pumps in cell wall maintain certain
  concentrations of electrolytes within and outside
  the cell
• Resting (polarized) cell normally more
  electrically negative inside cell wall than
  outside ( -90 millivolts (mv) in working cells)

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Depolarization at The Cellular Level

• Electrical stimulation of cell wall changes its
  permeability to sodium (Na)
• Na rushes into cell, causing inside to become
  more positive
• Slower influx of calcium (Ca) also causes cell
  to become positive
• Muscle contraction is response to depolarization
• Depolarization wave is passed from cell to cell
  along the conduction pathway to reach the muscle
  cells

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Spontaneous diastolic depolarization of pacemaker
cells

• Pacemaker cells capable of self-initiated
  depolarization (automaticity)
• Found throughout conduction system except in AV
  node
• During diastole, become less and less negative
  until a certain threshold reached, then rapidly
  and fully depolarize

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Pacemaker Capabilities Rates

• SA node 60-100/minute intrinsic rate
• AV junctional tissue 40-60/minute intrinsic rate
• Ventricles (bundle branches and Purkinje fibers)
  20-40/minute intrinsic rate
• SA node usual pacemaker because it discharges the
  fastest pacemaker cells below SA node normally
  suppressed by it

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Repolarization

• Process by which cells re-establish internal
  negativity and are readied for stimulation return
  to resting or polarized state
• Caused by rapid escape of potassium (K) from the
  cell
• Proper distribution of electrolytes
  re-established by cell wall pumps (Na pumped out
  of cell, potassium pumped back into cell)
• Cell returns to -90mv. internal charge-
  repolarized

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Relationship of ECG to electrical activity

• ECG is record of electrical activity of heart as
  sensed by electrodes on body surface
• Gives information only about electrical activity
  tells us nothing about pump function
• Isoelectric line a flat line on the ECG
  indicating absence of net electrical activity

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P wave

• Rounded wave preceding QRS usually upright
  (positive) in Lead II
• Indicates depolarization of atrial muscle

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QRS complex

• Collective term for three deflections following
  the P wave

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QRS complex

• Wave-first negative deflection after P wave
• R wave-first positive deflection after P wave
• S wave-first negative deflection after R wave

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QRS complex

• All three waves not always present - QRS has many
  shapes
• Indicates depolarization of the ventricular muscle

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T wave

• Rounded wave following QRS complex usually in
  same direction as QRS
• Indicates repolarization of ventricles
• Atrial T wave (atrial repolarization) usually not
  visible buried within QRS complex

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P-R interval

• Distance between beginning of P wave and the
  beginning of QRS complex
• Indicates length of time it takes depolarizatin
  wave to go from atria to ventricles

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S-T segment

• Distance between the S wave of the QRS complex
  and the beginning of the T-wave usually in
  isoelectric line

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Refractory period

• Period of time when cells have been depolarized
  and not yet returned to polarized state
• Heart unable to be stimulated again
• On ECG, includes, QRS complex and T wave

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Absolute refractory period

• Time when stimulation will produce no
  depolarization whatsoever
• From beginning of QRS complex to apex of T wave
• Relative refractory period time when a
  sufficiently strong stimulus may produce
  depolarization
• Corresponds to down slope of T wave

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Nervous control of electrical activity

• Sympathetic (adrenergic) control
• Effects of alpha stimulation no direct effect on
  heart
• Effects of beta stimulation increased rate,
  increased conduction velocity in atria and
  ventricles, increased irritability, (increased
  contractility mechanical effect)

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Parasympathetic (cholinergic) control

• Effects of parasympathetic (vagal) stimulation
• Decreased firing rate of SA node, decreased AV
  conduction, little effect on ventricles

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