parasympathetic stimulation
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Ken
Parasympathetic Stimulation
Vagus nervePrimarily innervates atria, but some fibers
to ventricles alsoChemical mediator: acethycholineEffect: slows heart rate and AV conductionMethods of stimulation: Valsalva
maneuver, carotid sinus pressure
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Sympathetic Stimulation
Nerves arising in thoracic and lumbar ganglia
Innervate both atria and ventriclesChemical mediator: norepinephrineReceptor 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 contractilityBronchodilationPeripheral
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 contractilityHypocalcemia: 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 irritabilityPotassium levels are critical to lifeHyperkalemia = 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) nodeLocated in right
atrium near entrance of superior vena cava
Usually heart's dominant pacemaker
sa
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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 secondsNo 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 junctionMakes 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 branchesNetwork of fibers helping to spread
impulse throughout ventricular wallsRapid 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 rateAV junctional tissue: 40-60/minute
intrinsic rateVentricles (bundle branches and Purkinje
fibers): 20-40/minute intrinsic rateSA 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 waveRelative 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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