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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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