How can action potentials be recorded with extracellular electrodes?

1. How can action potentials be recorded with extracellular electrodes?

2. Cells are negative inside and positive outside. No potential differences are seen in the extracellular fluid.

3. Extracellular voltage differences are created when some of the cells are depolarized and the rest are still polarized. Notice that the right side is positive

4. When all of the cells are depolarized all cells are negative outside and the voltage differences disappear again

5. As the first cells to depolarize repolarize, a voltage difference reappears again but is opposite to that seen in A2

6. A dipole is a potential difference between two points that are a finite distance apart in space - + It is a Vector quantity having both magnitude and direction 1.5 V

7. Volume conductor refers to a 3-dimensional conductor like a tank of salt water or a patient’s chest

8. Isopotential (same voltage) lines are perpendicular to the current flow If a dipole is placed in a volume conductor, lines of current flow from the +pole to the –pole.

9. The isoelectric plane is formed by a set of very important isopotential lines. It forms halfway between the two poles of the dipole. It is also always perpendicular to the dipole. Voltage in the isoelectric plane will always be zero.

10. A dipole will occur only when part of the heart is depolarized and part is still polarized. The voltage field set up by the dipole within the volume conductor of the thorax can be measured on the body surface

11. A wave of depolarization over the heart will have positive voltage ahead of the wave and negative voltage behind it

12. Because the dipole has direction as well as magnitude, the potentials observed on the body surface should be a function of where we attach the electrodes.

13. The ECG machine is a differential voltage-sensing meter with two inputs.

14. A wave of depolarization over the heart will have positive voltage ahead of the wave and negative voltage behind it 0 - +

15. A wave of depolarization over the heart will have positive voltage ahead of the wave and negative voltage behind it 0 - +

16. VOLTS Voltage calibration 0.1 mV/mm(each box = 0.1 mv) Time calibration 25 mm/sec(each box = 0.04 s or 40 ms) 1 mV 1 Sec TIME

17. capacitor Unlike the meter in the previous picture the ECG amplifier is AC-coupled, i.e. it cannot measure a steady voltage, only rapid changes in voltage Meter Input Time

18. The chest is represented by Einthoven’s triangle, an equilateral triangle with the shoulders at two of the apices and the pubis at the third apex

19. Volume conductor due to the electrolytes in the body fluids. Despite the obvious shape differences between most patient’s chest and an equilateral triangle assuming the geometry of an equilateral triangle works amazingly well in calculating electrical vectors .

20. Note the angle convention used for ECG

21. The 3 limb leads in the frontal plane

22. I = LA (+) – RA (0°) II = LL (+) - RA (60°) III = LL (+) - LA (120°) _ + _ _ + +

23. A zero-voltage reference electrode can be made by simply connecting the wires from the two arms and the left leg together _ +

24. The positive lead from the amplifier can then be placed anywhere on the body as an exploring or unipolar electrode _ + “V” lead

25. Typically the exploring electrode is placed on specific locations on the chest to record the leads V1-V6.

26. Augmented V leads RA(+) – (LA + LL+RA) 3

27. aVR = RA(+) – (LA + LL) (-150°) 2 Augmented V leads

28. Augmented V leads aVL = LA (+) – RA + LL (-30°)

29. Augmented V leads aVF = LL(+) – RA + LA (90°)

30. 6 leads in the frontal plane

31. Transpose the leads to get the “star” diagram

32. The dipole is constantly changing as the wave of depolarization marches over the heart

33. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

34. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

35. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

36. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

37. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

38. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

39. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

40. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

41. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

42. Plotting the instantaneous vector over a complete QRS complex describes a vector loop

43. Plotting the average deflection in each lead yields the mean electrical axis of the ECG

44. Lets plot the QRS deflection vectorially to determine the mean QRS vector

45. Draw the line (blue) at right angles to the lead (red). The geometric method

46. Drawing a vector is easy using I and aVF

47. It is also easy with any two leads. Just remember, draw the perpendicular to the lead.

49. aVL is positive In a 6-lead configuration you can almost always find an isolelectric lead

50. End first hour