Transcript Nurs 259 - Basic EKG Interpretation

Nursing 259 - EKG
Cardiac Anatomy & Physiology
The Conduction System
Arrhythmia Formation
Cardiac Anatomy
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Located beneath the sternum – 2/3 of it
to the left
Rotated towards left so right-sided
chambers are more anterior
Normally size of your fist
Base of heart – upper portion near right
atrium
Apex – lower portion near left ventricle –
just right of 5th intercostal space
midclavicular line
Layers
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Pericardium – outer double-layer encloses
the heart – protects heart from infection,
holds heart in mediastinal space
– Outer parietal layer
– Inner visceral layer – also called epicardium
– Pericardial space between layers contains 10-20
ml of serous fluid to cushion heart and reduce
friction
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Myocardium – middle, thickest layer
responsible for pumping of heart
Endocardium – inner layer that lines
chambers and valves
Chambers
Four chambers
 Right atrium (RA) and ventricle (RV)
 Left atrium (LA) and ventricle (LV)
 Myocardium thinnest in atria
 Thickest in left ventricle as pressure
had to overcome arterial systolic
pressure to eject blood
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Heart Chambers
Blood Flow Through
Heart
Valves
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Aortic and pulmonic valves are semi-lunar
(half-moon shaped)
– Aortic between LV and aorta
– Pulmonic between RV and pulmonary artery
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Triscuspid and Mitral – between atria and
ventricles on right and left sides
– Connected by chordae tendinae to papillary
muscles which controls closure – muscle can be
damaged in MI – note thin fibrous threads
attached to these valves in Heart Chambers
picture – these are chordae tendinae
Conduction System
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SA Node – upper RA – “pacemaker of heart”
– begins normal conduction of heart
Internodal Pathways – throughout RA
Bachmann’s bundle - to LA
AV Node – causes delay in conduction to
allow ventricles to fill with blood
Bundle of His – sends impulse to ventricles
Bundle Branches to RV and LV
Purkinje Fibers – throughout ventricles
Rate of Impulse
Formation
SA Node: 60 - 100
 AV Node: 40 - 60
 Ventricle: 20 – 40
 All have the property of automaticity
 SA node is dominant as it has higher
rate
 AV node and ventricle are latent or
escape pacemakers – if SA node fails
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Coronary Circulation
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Coronary arteries originate off aorta
Receive blood supply during both systole
and diastole, particularly during diastole
Left Coronary Artery: Divides into the left
anterior descending and circumflex
– Left Anterior Descending; Supplies anterior wall
of LV and RV and interventricular septum
Coronary Circulation
– Left Circumflex – supplies SA node in 50% of
population, left atrium, inferior and lateral wall of
LV, AV node in 10% of people, posterior wall in
10% of people
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Right Coronary Artery: Supplies right atrium
and ventricle; inferior wall of the left ventricle;
AV node and posterior wall in 90% of people;
SA Node in 50% of people
Autonomic Nervous
System
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Sympathetic: Increases
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Rate of SA Node
Rate of AV Conduction
Excitability
Force of Contraction
Direct stimulation through nerves and indirectly
through catecholamine (epinephrine release)
– Transplanted heart will still respond to SNS via
epinephrine in blood
Autonomic Nervous
System
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Parasympathetic: Decreases
– Rate of SA Node
– Rate of AV Conduction
– Excitability
– Directly through vagal nerve
– Transplanted heat no longer responds to
vagal stimulation because it is not
attached to vagus nerve
Myocardial Cell
Properties
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Automaticity: Ability to initiate an
impulse – normally only specialized cells of
conduction system have – these cells are
self-excitable
All cells have:
– Excitability: Ability to respond to an impulse
– Conductivity: Ability to transmit an impulse
– Contractility: Ability to respond with pumping
action
Depolarization
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Condition in which the inside of the
cell becomes more positively
charged resulting in cardiac
contraction – usually as a result of
being stimulated by an impulse
Resting State
At rest, cardiac cell is said to be
polarized or resting – inside of cell is
negatively charged (-90 millivolts)
 Called resting potential (RP) or
transmembrane resting potential (TRP)
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Polarized or Resting
State
Depolarized State
Repolarization
Cell returns to resting state with inside
of cell again negatively charged
 Threshold potential (TP) – the level a
cell must recover to in order to be able
to be depolarized again – usually
about –60 to –70 millivolts – allows
fast sodium channels to open
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Repolarization
Action Potential Cycle
Describes the changes in intracardiac
voltage that lead to impulse formation
and conduction and ultimately cardiac
contraction
 Different for cells with automaticity
than cells without that property
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Phase 0
Sharp upstroke
– Sodium freely and rapidly enters cell
– Calcium moves slowly into cell
– Corresponds to depolarization and
beginning of contraction
– Inside of cell becomes more positively
charged “overshoot”
– Inside of cell +20 to +30 millivolts
Calcium
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As the intracellular concentration of
calcium increases, calcium reacts
with contractile elements and
myocardial muscle fibers contract.
After contraction, free calcium ions
are actively pumped out of the cell
and relaxation begins.
Action Potential Cycle
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Phase 1
– Early repolarization
– Sodium channels close
– Potassium flows out (helps to restore negativity
of cell)
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Phase 2
– Plateau phase
– Calcium continues to flow in
– Allows cells to finish contracting
Action Potential Cycle
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Phase 3
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Rapid repolarization
Calcium channels close
Potassium moves out rapidly
Sodium-potassium pump restores potassium back
to cell and sodium to the outside of cell
Phase 4
– Resting phase
– Cell membrane is impermeable to sodium
– Potassium continues to move out
Potentials
Refractory Periods
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Absolute refractory (ARP): Time period
during which the cardiac cell is resistant
to all stimulation – also called effective
refractory period (ERP)
– Between phase 0 to midway between phase 3
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Relative refractory (RRP): Brief period of
relaxation when depolarization can occur
but a stronger stimulus is required –
during second half of phase 3 – also called
vulnerable period
Supernormal Period
At end of phase 3, a weak stimulus
can excite cell (SNP)
 From phase 0 through the end of
phase 3 is called the full recovery time
(FRT) and represents depolarization
and repolarization
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Action Potential of
Automatic Cells
Cells with property of automaticity
have an unstable phase 4
 SA node, AV node, Purkinje fibers
 Allows a steady leak of sodium into
cell until threshold potential is reached
then fast sodium channels open and
cause depolarization
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