1. ECG Basics

2. What is an ECG?
The electrocardiogram (ECG) is a representation of the electrical events of the cardiac cycle.
Each event has a distinctive waveform, the study of which can lead to greater insight into a patient’s cardiac pathophysiology.

3. Electrocardiogram (ECG/EKG)
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5. ECG Leads
Leads are electrodes which measure the difference in electrical potential.
They can be of two types:
1. Two different points on the body (bipolar leads)
2. One point on the body and a zero electrical potential.

6. EKG Leads The standard EKG has 12 leads: 3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from which it looks at the heart.

9. Standard Limb Leads

10. Types of ECG Recordings
Bipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground)
Lead I records between right arm & left arm
Lead II: right arm & left leg
Lead III: left arm & left leg
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12. aVR, aVL, aVF (augmented limb leads)
Summary of Leads
Limb Leads
Precordial Leads
Bipolar
I, II, III
(standard limb leads) -
Unipolar
aVR, aVL, aVF (augmented limb leads)
V1-V6

13. Arrangement of Leads on the EKG

14. Precordial Leads
Adapted from:

15. Precordial Leads

16. one small block equals 0.04 s
ECG Graph Paper
one small block equals 0.04 s
Five small blocks make up 1 large block which translates into 0.20 s (200 msec)

17. Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

20. ECG 3 distinct waves are produced during cardiac cycle
P wave caused by atrial depolarization
QRS complex caused by ventricular depolarization
T wave results from ventricular repolarization

21. Elements of the ECG:
P wave: Depolarization of both atria;
Shape and duration of P may indicate atrial enlargement
Relationship between P and QRS helps distinguish various cardiac arrhythmias

23. Waveforms and Intervals

26. QRS complex: Ventricular depolarization
Larger than P wave because of greater muscle mass of ventricles
Normal duration = seconds
Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.
Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

27. PR interval: from onset of P wave to onset of QRS
Normal duration = sec ( ms) (3-4 horizontal boxes)
Represents atria to ventricular conduction time (through His bundle)
Prolonged PR interval may indicate a 1st degree heart block

28. ST segment:
Connects the QRS complex and T wave
Duration of sec ( msec
T wave:
Represents repolarization
QT Interval
Measured from beginning of QRS to the end of the T wave
Normal QT is usually about 0.40 sec
QT interval varies based on heart rate

36. Determining the Heart Rate
Rule of 300
10 Second Rule

37. Rule of 300
Count the number of “big boxes” between neighboring QRS complexes, and divide 300 by this number. The result will be approximately equal to the rate
Although fast, this method only works for regular rhythms.

38. What is the heart rate?
(300 / 6) = 50 bpm

39. What is the heart rate?
(300 / ~ 4) = ~ 75 bpm

40. What is the heart rate?
(300 / 1.5) = 200 bpm

41. The Rule of 300 It may be easiest to memorize the following table:
# of big boxes
Rate 1
300 2
150 3
100 4 75 5 60 6 50

42. 10 Second Rule
As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the EKG and multiply by 6 to get the number of beats per 60 seconds.
This method works well for irregular rhythms.

43. The QRS Axis
The QRS axis represents the net overall direction of the heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)