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Nursing 259 - EKG

Nursing 259 - EKG. Cardiac Anatomy & Physiology The Conduction System Arrhythmia Formation. Cardiac Anatomy. 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

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Nursing 259 - EKG

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  1. Nursing 259 - EKG Cardiac Anatomy & Physiology The Conduction System Arrhythmia Formation

  2. Cardiac Anatomy • 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

  3. Layers • 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 • Myocardium – middle, thickest layer responsible for pumping of heart • Endocardium – inner layer that lines chambers and valves

  4. 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

  5. Heart Chambers

  6. Blood Flow Through Heart

  7. Valves • Aortic and pulmonic valves are semi-lunar (half-moon shaped) • Aortic between LV and aorta • Pulmonic between RV and pulmonary artery • 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

  8. Conduction System • 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

  9. 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

  10. Coronary Circulation • 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

  11. 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 • 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

  12. Autonomic Nervous System • Sympathetic: Increases • 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

  13. Autonomic Nervous System • 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

  14. Myocardial Cell Properties • 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

  15. Depolarization • 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

  16. 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)

  17. Polarized or Resting State

  18. Depolarized State

  19. 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

  20. Repolarization

  21. 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

  22. 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

  23. Calcium • 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.

  24. Action Potential Cycle • Phase 1 • Early repolarization • Sodium channels close • Potassium flows out (helps to restore negativity of cell) • Phase 2 • Plateau phase • Calcium continues to flow in • Allows cells to finish contracting

  25. Action Potential Cycle • Phase 3 • 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

  26. Potentials

  27. Refractory Periods • 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 • 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

  28. 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

  29. 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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