1. Electrical Activity of the Heart

2. Introduction
Where does the “electro” in electrocardiography come from?

3. Under this condition, the heart cell is said to be polarized

4. Polarization
Imagine two micro-electrodes; one outside the cell, one inside the cell
Difference between the two equals -90 mV inside
The cell is said to be ‘polarized’

5. Action Potential
Depolarization
Repolarization

6. closedgates
opened gates
Skeletal Muscle Fiber
Action Potential in

7. Action Potential
Skeletal
Cardiac

8. Myocyte Action Potentials
Fast and Slow
Fast = non-pacemaker cells
Slow = pacemaker cells (SA and AV node)

9. Ions
Ion
Extra-
Intra- Na
140 10 K 4
135 Ca 2
0.1

10. Action Potential Ion influx Na channels (fast and slow) K channels
Ca channels

11. Inside
Outside
thevirtualheart.org/CAPindex.html

12. Action Potential Phase 0 Stimulation of the myocardial cell
Influx of sodium
The cell becomes depolarize
Inside the cell = +20 mV

13. Action Potential Phase 1 Ions Influx of sodium Efflux of potassium
Partial repolarization
Phase 2
Sodium
Influx of calcium
Plateau

14. Action Potential Phase 3 Ions Efflux of potassium* Influx of calcium
Repolarization (slower process than depolarization)
Phase 4
Interval between repolarization to the next action potential
Pumps restore ionic concentrations

15. Ion 1 2 3 4 Na
influx
pump K
efflux
efflux* Ca

17. Refractory Periods
Absolute refractory period - phase 1 - midway through phase 3
Relative refractory period - midway through phase 3 - end of phase 3

18. SA Node Action Potential
“Funny” currents (phase 4); slow Na channels that initiate spontaneous depolarization
No fast sodium channels
Calcium channels (slow)
Long-lasting, L-type
Transient, T-type
Potassium channels

19. Action Potentials
Fast and Slow

20. Action Potentials
It is important to note that non-pacemaker action potentials can change into pacemaker cells under certain conditions. For example, if a cell becomes hypoxic, the membrane depolarizes, which closes fast Na+ channels. At a membrane potential of about –50 mV, all the fast Na+ channels are inactivated. When this occurs, action potentials can still be elicited; however, the inward current are carried by Ca++ (slow inward channels) exclusively. These action potentials resemble those found in pacemaker cells located in the SA node,and can sometimes display spontaneous depolarization and automaticity. This mechanism may serve as the electrophysiological mechanism behind certain types of ectopic beats and arrhythmias, particularly in ischemic heart disease and following myocardial infarction.

21. Conduction speed varies throughout the heart
Slow - AV node
Fast - Purkinje fibers

23. Action Potential ECG records depolarization and repolarization
Atrial depolarization
Ventricular depolarization
Atrial repolarization
Ventricular repolarization

24. The Body as a Conductor
This is a graphical representation of the geometry and electrical current flow in a model of the human thorax. The model was created from MRI images taken of an actual patient. Shown are segments of the body surface, the heart, and lungs. The colored loops represent the flow of electric current through the thorax for a single instant of time, computed from voltages recorded from the surface of the heart during open chest surgery.

25. Assignment Read “Non-pacemaker Action Potentials”
Read “SA node action potentials”

26. Basic ECG Waves
Chapter 2

27. ECG Complexes

28. ECG Complexes

29. Action Potential & Mechanical Contraction

30. ECG Paper Small boxes = 1 mm Large boxes = 5 mm
Small boxes = 0.04 seconds
Large boxes = 0.20 seconds
5 large boxes = 1.0 second
Paper speed = 25 mm / sec

31. ECG Paper Horizontal measurements in seconds
Example, PR interval = .14 seconds (3.5 small boxes)

32. ECG Paper
Standardization mark
10 mm vertical deflection = 1 mVolt

33. ECG Paper Standardization marks Double if ECG is too small
Half is ECG is too large
Top: Low amplitude complexes in an obese women with hypothyroidism
Bottom: High amplitude complexes in a hypertensive man

34. ECG Description ECG amplitude (voltage) recorded in mm
positive or negative or biphasic

35. ECG Waves Upward wave is described as positive
Downward wave is described as negative
A flat wave is said to be isoelectric
Isoelectric as describes the baseline
A deflection that is partially positive and negative is referred to as biphasic

36. ECG Waves P wave atrial depolarization ≤ 2.5 mm in amplitude
< 0.12 sec in width
PR interval ( sec.)
time of stimulus through atria and AV node
e.g. prolonged interval = first-degree heart block

37. ECG Waves QRS wave Ventricle depolarization
Q wave: when initial deflection is negative
R wave: first positive deflection
S wave: negative deflection after the R wave

38. ECG Waves QRS May contain R wave only May contain QS wave only
Small waves indicated with small letters (q, r, s)
Repeated waves are indicated as ‘prime’

39. ECG Waves
QRS
width usually 0.10 second or less

40. ECG Waves
RR interval
interval between two consecutive QRS complexes

41. ECG Waves J point end of QRS wave and... ...beginning of ST segment
beginning of ventricular repolarization
normally isoelectric (flat)
changes-elevation or depression-may indicate a pathological condition

42. ECG Waves

43. ECG Waves T wave part of ventricular repolarization asymmetrical shape
usually not measured

44. ECG Waves QT interval from beginning of QRS to the end of the T wave
ventricular depolarization & repolarization
length varies with heart rate (table 2.1)
long QT intervals occur with ischemia, infarction, and hemorrhage
short QT intervals occur with certain medications and hypercalcemia

45. ECG Waves QT interval should be less than half the R-R interval
If not, use Rate Corrected QT Interval
normal ≤ 0.44 sec.

46. ECG Waves FYI Long QT interval certain drugs electrolyte distrubances
hypothermia
ischemia
infarction
subarachnoid hemorrhage
Short QT interval
drugs or hypercalcemia

47. ECG Waves U Wave last phase of repolarization
small wave after the T wave
not always seen
significance is not known
prominent U waves are seen with hypokalemia

48. Heart Rate Calculation
divided by the number of small boxes between two R waves
most accurate
take time to calculate
only use with regular rhythms
divided by the number of large boxes between two R waves
quick
not too accurate
only use with regular rhythm
3. Number of large squares w/i RR interval
1 lg sq = 300 bpm
2 lg sq = 150 bpm
3 lg sq = 100 bpm
4 lg sq = bpm
5 lg sq = bpm
6 lg sq = bpm

49. Heart Rate Calculation
For regular rhythm...
Count the number of large boxes between two consecutive QRS complexes. Divide 300 by that number
300 ÷ 4 = 75
Count the small boxes. Divide 1500 by that number
1500 ÷ 20 = 75

51. Heart Rate Calculation
For irregular rhythms…
Count the number of cardiac cycles in 6 seconds and multiple this by 10. (Figure 2.15)

53. The ECG as a Combination of Atrial and Ventricular Parts
Atrial ECG = P wave
Ventricular ECG = QRS-T waves
Normally, sinus node paces the heart and P wave precedes QRS
P-QRS-T
Sometimes, atria and ventricles paced separately (e.g. complete heart block)

54. ECG in Perspective
ECG recording of electrical activity not the mechanical function
ECG is not a direct depiction of abnormalities
ECG does not record all the heart’s electrical activity

55. Questions
End of chapter 2, questions 1-5 and 7.