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Introduction to Clinical Electrocardiography

Introduction to Clinical Electrocardiography. Gari Clifford, PhD Andrew Reisner, MD Roger Mark, MD PhD. Electrocardiography. The heart is an electrical organ, and its activity can be measured non-invasively Wealth of information related to: The electrical patterns proper

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Introduction to Clinical Electrocardiography

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  1. Introduction to Clinical Electrocardiography Gari Clifford, PhD Andrew Reisner, MD Roger Mark, MD PhD

  2. Electrocardiography • The heart is an electrical organ, and its activity can be measured non-invasively • Wealth of information related to: • The electrical patterns proper • The geometry of the heart tissue • The metabolic state of the heart • Standard tool used in a wide-range of medical evaluations

  3. A heart • Blood circulates, passing near every cell in the body, driven by this pump • …actually, two pumps… • Atria = turbochargers • Myocardium = muscle • Mechanical systole • Electrical systole

  4. To understand the ECG: • Electrophysiology of a single cell • How a wave of electrical current propagates through myocardium • Specific structures of the heart through which the electrical wave travels • How that leads to a measurable signal on the surface of the body

  5. Part I: A little electrophysiology

  6. Once upon a time, there was a cell: K+ K+ 2 K+ 3 Na+ ATPase

  7. a myocyte time Intracellular millivoltage Resting comfortably -90

  8. Depolarizing trigger time Intracellular millivoltage

  9. Na channels open, briefly time Intracellular millivoltage

  10. Mystery current time Intracellular millivoltage In: Na+

  11. Ca++ is in balance with K+ out time Intracellular millivoltage In: Na+

  12. Excitation/Contraction Coupling: Ca++ causes the Troponin Complex (C, I & T) to release inhibition of Actin & Myosin time Intracellular millivoltage In: Na+

  13. More K+ out; Ca++ flow halts Ca++ in; K+ out time Intracellular millivoltage In: Na+

  14. Sodium channels reset In: Ca++; Out: K+ time Intracellular millivoltage In: Na+ Out: K+

  15. Higher resting potential Few sodium channels reset Slower upstroke time Intracellular millivoltage In: Na+

  16. Slow current of Na+ in; note the resting potential is less negative in a pacemaker cell a pacemaker cell time Intracellular millivoltage -55

  17. Threshold voltage a pacemaker cell time Intracellular millivoltage -40

  18. Ca++ flows in time Intracellular millivoltage

  19. . . . and K+ flows out time Intracellular millivoltage

  20. . . . and when it is negative again, a few Na+ channels open time Intracellular millivoltage

  21. How a wave of electrical current propagates through myocardium • Typically, an impulse originating anywhere in the myocardium will propagate throughout the heart • Cells communicate electrically via “gap junctions” • Behaves as a “syncytium” • Think of the “wave” at a football game!

  22. The dipole field due to current flow in a myocardial cell at the advancing front of depolarization. Vm is the transmembrane potential.

  23. Cardiac Electrical Activity

  24. Important specific structures • Sino-atrial node = pacemaker (usually) • Atria • After electrical excitation: contraction • Atrioventricular node (a tactical pause) • Ventricular conducting fibers (freeways) • Ventricular myocardium (surface roads) • After electrical excitation: contraction

  25. The Idealized Spherical Torso with the Centrally Located Cardiac Source (Simple dipole model)

  26. Excitation of the Heart

  27. Excitation of the Heart

  28. Cardiac Electrical Activity

  29. Recording the surface ECG

  30. Clinical Lead Placement • Einthoven Limb Leads:

  31. Precordial leads

  32. 12 Lead ECG

  33. The temporal pattern of the heart vector combined with the geometry of the standard frontal plane limb leads.

  34. Normal features of the electrocardiogram.

  35. Normal sinus rhythm

  36. What has changed?

  37. Sinus bradycardia

  38. Vagal stimulation makes the resting potential MORE NEGATIVE. . . Neurohumeral factors time Intracellular millivoltage

  39. Neurohumeral factors time Intracellular millivoltage . . . and the pacemaker current SLOWER. . .

  40. . . . and raise the THRESHOLD time Intracellular millivoltage

  41. Catecholamines make the resting potential MORE EXCITED. . . time Intracellular millivoltage

  42. . . . and speed the PACEMAKER CURRENT. . . time Intracellular millivoltage

  43. . . . and lower the THRESHOLD FOR DISCHARGE. . . time Intracellular millivoltage

  44. time Intracellular millivoltage Vagal Stimulation:

  45. time Intracellular millivoltage Adrenergic Stim.

  46. Sinus arrhythmia

  47. Atrial premature contractions (see arrowheads)

  48. Arrhythmias - Not firing when you should - Firing when you shouldn't - All of the above (Reentrance)

  49. Firing when you shouldn't • Usually just a spark; rarely sufficient for an explosion • “Leakiness” leads to pacemaker-like current • Early after-depolarization • Late after-depolarization

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