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Disorders of Cardiac Function

Disorders of Cardiac Function. Introduction. The Heart as Two Pumps. The Heart as Two Pumps. The heart is really two pumps in tandem The right heart sends blood to the lungs The left heart gets blood back from the lungs and sends the blood to the systemic circulation

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Disorders of Cardiac Function

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  1. Disorders of Cardiac Function

  2. Introduction

  3. The Heart as Two Pumps

  4. The Heart as Two Pumps • The heart is really two pumps in tandem • The right heart sends blood to the lungs • The left heart gets blood back from the lungs and sends the blood to the systemic circulation • This is a bigger job because the systemic circulation is larger and has more gravity

  5. Global Tissue Oxygenation

  6. Global Tissue OxygenationMade Ridiculously Simple 100% Venous Oxygen Delivery SvO2 = 75% 25% Arterial Oxygen Delivery Oxygen Consumption

  7. Global Tissue OxygenationSimple Description

  8. Global Tissue OxygenationSimple Description • Each Hb molecule can carry four oxygen molecules • The hemoglobin in the blood picks up oxygen in the lungs • The hemoglobin sends the oxygen in the blood through the arteries to the tissues • The tissues do not extract 100% of the oxygen from the hemoglobin • 25% of oxygen is in the tissues, 75% in the veins • The Hb then goes back to the loading station

  9. Global Tissue OxygenationDetailed Description

  10. Global Tissue OxygenationDetailed Description • The lungs load each hemoglobin with 4 oxygen molecules. • Oxygen content is 20% of total volume. • At the tissue level, Oxygen extraction is a ratio of oxygen consumed (VO2 = 250 mL/min) to the amount delivered (DO2) = 25% • Thus 75% of oxygen delivered is returned to the venous side, i.e. normal SvO2 = 75%. • Oxygen consumption (VO2) is a function of cardiac output and the difference between arterial (Hb x SaO2 x 13.4) and venous oxygen content (Hb x SvO2 x 13.4). • Given the same CO and Hb, VO2 is analogous to the difference between arterial and venous oxygenation. • For example, 1 Hb will deliver 4 oxygen molecules to the tissue -> 1 oxygen molecule is consumed (VO2) by the tissue + 3 oxygen molecules are returned to the venous outflow.

  11. Coronary CirculationDescription

  12. Coronary CirculationDescription • The arteries and veins in the heart perfuse the heart with oxygen • The coronary arteries come off of the aorta at the place of the aortic valve • Left and right coronary arteries • Left almost immediately branches into the circumflex and the left anterior descending artery • Nurses the left side of the heart • Right • Both nourish the septum • Blood then goes into the capillaries and then the veins of the heart • Large vein that delivers the blood back to the heart is the coronary sinus

  13. Coronary Circulation

  14. Cardiac Conduction System

  15. Cardiac Conduction System • Conduction system stimulates the myocardium to contract and pump blood • Conduction system usually controls the rhythm of the heart (unless the person has a pacemaker) • Heart has two conduction systems • One controls atrial activity • One that controls ventricular activity

  16. Anatomy of the Conduction System

  17. Anatomy of the Conduction System SA Node AV Node Bundle of His Bundle branches Purkinje fibers Porth, 2007, Essentials of Pathophysiology, 2nd ed., Lippincott, p. 331.

  18. SA Node

  19. SA Node • Pacemaker of the heart • Impulses originate here • Located in posterior wall RA • Fires at 60 -100 bpm • Responsible for the heart rate in the normal person • Impulse causes atrial contraction

  20. AV Node

  21. AV Node • Connects the atria and ventricles, provides one way conduction • Would beat independently • Fires at 40 -60 bpm • Can assume pacemaker function if SA fails to discharge • There is a pause here • The speed of conduction in the AV node is influenced by the SNS (beta-1)

  22. Purkinjie Fibers

  23. Purkinjie Fibers • Supplies the ventricles • Supplies the impulse to the cardiac muscle • Large fibers, rapid conduction for swift and efficient ejection of blood from heart • Large fibers – fast conduction • Small fibers – slow conduction • Fire 15-40 bpm • Only occurs if there is no input from the other areas • Assume pacemaker of ventricles if AV fails • HR reflects intrinsic firing of these structures

  24. Action Potentials (AP)

  25. Action Potentials (AP) • Stimulus • The only intrinsic conduction in the heart is in the SA node • Any other conduction comes from depolarization of the muscle  excitable tissues (muscle and conduction system)  evokes an AP characterized by a sudden change in voltage resulting from transient depolarization and then repolarization. • AP’s are electrical currents involving the movement/flow of electrically charged ions at level of cell membrane. • AP’s are conducted throughout the heart, responsible for initiating each cardiac contraction.

  26. Types of Action Potentials

  27. SLOW SA & AV Nodes FAST Purkinje Fiber & Muscle

  28. Types of Membrane Ion Channels that Contribute to Voltage Changes during the AP

  29. Types of Membrane Ion Channels that Contribute to Voltage Changes during the AP • Fast Na+ channels • Rapid depolarization of muscles • Important in cardiac APs and Purkinje fibers • Slow Na+ channels • Pacemaker activity (SA, AV) • Potassium channels • Speedy repolarization

  30. Three Phases of Action Potentials

  31. Three Phases of Action Potentials • Resting • Depolarization • Repolarization

  32. Resting Phase

  33. Resting Phase • Membrane is relatively permeable to K+, but much less so to Na+ • Inside is negative, outside is positive

  34. Cardiac Muscle Cell Firing • Cells begin with a negative charge: resting membrane potential • Calcium leaklets Ca2+ diffuse in, making the cell more positive Threshold potential Resting membrane potential Calcium leak

  35. Depolarization Phase

  36. Depolarization Phase • Cell membrane becomes permeable to Na+ • Na+ enters cell, inside the cell is more +

  37. Cardiac Muscle Cell Firing (cont.) • At threshold potential, more Na+ channels open • Na+ rushes in, making the cell very positive: depolarization • Action potential: the cell responds (e.g. by contracting) Action potential Threshold potential Resting membrane potential Calcium leak

  38. Plateau Phase

  39. Cardiac Muscle Cell Firing (cont.) • K+ channels open • K+ diffuses out, making the cell negative again (starting to repolarize), but Ca2+ channels are still allowing Ca2+ to enter • The cell remains positive: plateau Action potential PLATEAU Threshold potential Calcium leak

  40. Repolarization Phase

  41. Repolarization Phase • Outward flow of positive charges, mainly K+ • Inside the cell is more negative • Assisted by Na+-K+ pump • Relatively slow method of repolarization • Potassium ions made a bigger, faster difference

  42. Cardiac Muscle Cell Firing (cont.) • During plateau, the muscle contracts strongly • Then the Ca2+ channels shut and it repolarizes • The potassium channels opened a while ago so the potassium comes out, leading to repolarization Action potential PLATEAU Threshold potential Calcium leak

  43. Cardiac Action Potentials

  44. Cardiac Action Potentials • Unlike nerve cells, cardiac cells have five phases in their action potential • Phase 4 – the resting membrane potential. • Phase 0 – there is rapid depolarization • The QRS complex corresponds to this section • Phase 1 – there is a short repolarization (only observed in ventricular muscle) • Occurs right in the end of depolarization • Only observed in ventricular muscle • Phase 2 – the membrane potential remains depolarized in a plateau • When calcium is entering the cell, so further repolarization is prevented (because cell is more positive) • Phase 3 – the membrane potential becomes repolarized. • The T wave corresponds to the repolarization

  45. Cardiac Muscle Action Potential 5 Phases Unlike nerve cells, cardiac cells have 5 phases in their action potential. Phase 0: Upstroke, rapid depolarization Phase 1: Early, short repolarization Seen only in ventricular muscle Phase 2: Plateau phase; membrane potential remains depolarized Phase 3: Final rapid repolarization Phase 4: Resting, diastolic repolarization

  46. Cardiac Muscle Cell Contraction

  47. Cardiac Muscle Cell Contraction • During Phase 2, the plateau, calcium ion enters the muscle cell, causing it to contract strongly. • The strength of contraction is directly proportional to the number of calcium ions that enter the cell. • Calcium channel opening is controlled by voltage (the calcium channels only open when the membrane is at a certain voltage) and by beta1 receptors in the ventricular myocardium.

  48. Importance of Actions Potentials

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