Electrical Activity of the Heart

Electrical Activity of the Heart

1. Electrical Activity
of the Heart
By
SATHISKUMAR G
([email protected])

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
D
e
p
o
la
riz
a
tio
n
R
e
p
o
la
riz
a
tio
n

6. A
c
t
io
n

P
o
t
e
n
t
ia
l
in

S
k
e
le
t
a
l
M
u
s
c
le

F
ib
e
r
closed
gates
opene
d gates

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
✦Ions
✦Sodium
✦Efflux of potassium
✦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 0 1 2 3 4
Na influxinflux pump
K effluxeffluxefflux*pump
Ca influxinfluxpump

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

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 (0.12 - 0.20 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
✦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
✦Long QT interval
✦certain drugs
✦electrolyte distrubances
✦hypothermia
✦ischemia
✦infarction
✦subarachnoid hemorrhage
✦Short QT interval
✦drugs or hypercalcemia
FYI

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
1. 1500 divided by the number
of small boxes between two R
waves
• most accurate
• take time to calculate
• only use with regular rhythms
2. 300 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 = 75 bpm
5 lg sq = 60 bpm
6 lg sq = 50 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

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

51. 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)

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

53. T
H
A
N
K
Y
O
U