The ECG - lecture.ppt for medical students

basic ecg interpretation

1. The Electrocardiogram
Basics of Interpretation

2. •To understand :
-What is an ECG?
-How is an ECG recorded?
-Why is an ECG recorded ?
Review a sample of common ECGs
Objectives

3. Normal sequence
of cardiac
depolarization
-a review-

5. ECG: What is it?
Surface recordingof the algebraic
sum of the action potentials of
cardiac fibres
Human body is a volume conductor
Action potentials fluctuate and are
recorded by an Electrocardiograph
machine on moving paper strip

6. Willem Einthoven
Nobel prize in 1924 for electrocardiogram
(discovered in 1903)

7. 12 conventional ECG leads -record the difference in
potential between electrodes placed on the surface of the
body -6 limb and 6 chest leads)
Limb leads record the potentials transmitted onto the
frontal plane
Types of limb leads
a) Standard Bipolar leads-I, II and III
Measure the difference in potential at 2 extremities.
Lead I -right arm and left arm voltages
Lead II –right arm and left leg
Lead III –left arm and left leg
b)augmented unipolar leads
aVR –right arm
aVL–left arm
aVF–Left foot
ECG: 12 Leads

8. ECG: Lead Reference System I

9. Precordial Leads

10. 2004 Anna Story 10
Precordial Leads

11. ECG: Lead Reference system II

12. Einthoven’s Triangle
‘The Traffic Lights’
RED YELLOW
GREENBLACK

13. ECG activity: Two cell model
Positive
deflection
Negative
deflection
isoelectric
When wave of depolarization moves towards a
positive electrode, the deflection is positive.

14. P wave –Depolarization of Atria
QRS complex –Depolarization of ventricular muscle
T wave –Repolarization of ventricular muscle
*repolarization of atria lies under QRS
The Normal ECG Record

15. ECG: Interpretation
Sequence of analysis
1.Heart Rhythm
2.Heart Rate
3.Intervals (PR, QRS, ST)
4.Mean QRS axis
5.Abnormalities of the P wave
6.Abnormalities of the QRS
7.ST and T wave abnormalities
8.Chamber -enlargement

16. ECG: Getting your bearings! I
Identity?
Correct lead
Placement?
Leads II
and AVR
Ignore Auto
Report

17. ECG: Getting your bearings! II
R-wave
Progression

18. Vectors determine lead groupings
•Inferior –II, III, and aVF
•Lateral –I, aVL, v5, and v6
•Anterior –v1, v2, v3 and v4
Contiguous Leads
Lead Groupings

19. 1. Heart Rhythm
Is there Sinus Rhythm?
Normal P waves preceding every
QRS complex
Is the Rhythm Regular?
Confirm by checking P-P and R-R
intervals
Yes, Yes, proceed!

20. 2. Heart Rate
Use Basic Rules
Boxes–small boxes or Large boxes
1500/# of small boxes between R-R
300/# of Large boxes between R-R
Remember paper Speed –25 or 50
mm/sec
Time
60 sec/ time interval between R-R (sec)
Normal range 60 –100 bpm

21. 2. Heart Rate 2

22. 3. Intervals and segments I

23. 3. Intervals and segments II

24. Easily remember normal values:
PR interval: ≤ 200 msec
QRS duration ≤ 100 msec
QT interval ≤ 400 msec
1000 msec = 1 sec
3. Intervals and segments III

25. 4. Mean QRS axis I
Gives information about:
Orientation of the heart in the chest
Chamber enlargement/ wall
hypertrophy

26. 4. Mean QRS axis II

27. Mean QRS Electrical Axis
Represents the average of instantaneous
forces generated during the sequence of
ventricular depolarization
Normal values is between -30
0
to +110
0
Axis can be determined by plotting the
QRS complexes of different leads on an
Axial Reference diagram

28. Determination of QRS Axis
Tri-Axial Reference system

29. Determination of QRS Axis

30. Mean QRS Axis -Calculations

31. 4. Mean QRS axis III
Inspect Bipolarand Augmented Leads
HEXAAXIAL reference system (Figure)
Use Equiphasic Lead OR Dominant R
wave
Equiphasic /Isoelectric Lead–Positive
and Negative Deflections are equal in
amplitude = Absolute vector is ZERO
Rule I: Mean QRS axis is perpendicular to
Equiphasic lead
Dominant R-wave
Rule 2: The Mean QRS axis is in the
direction of the Dominant R-wave

32. 4. Mean QRS axis IV
Dominant R+ve
Lead II
Mean QRS Axis
Approx: +60º

33. 4. Mean QRS axis V
Several Leads have Equiphasic
Deflections
Mean QRS axis is:
Indeterminate
Extreme
In NW territory

34. The Quadrant Approach
QRS up in I and up in aVF=
Normal

35. ECG
Practice questions
1.Comment on the rhythm
2.Calculate the heart rate, PR interval, QRS duration
and QT interval, for each comment whether normal or
abnormal
3.Determine the QRS axis> is it normal