1. Introduction to the 12-lead ECG
© G Schellack; 2012
Gustav Schellack
SASOCP Workshop
10 March 2012

2. Electrocardiogram (ECG)
The ECG is a graphic recording of the electrical activity of the heart, at a specific moment in time.
Note: ‘Electrical activity’ refers to the depolarisation and repolarisation of the atria and ventricles.
Note: The ECG cannot tell us anything about the ‘mechanical activity’ (i.e. contraction and relaxation) of the myocardium.

3. Cardiac Conduction System
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from:
(Marquette Electronics, 1996)
Anatomical orientation:
Figure 1:

4. The Cardiac Impulse RA LA RBB LBB
Spreads through both atria [virtually] simultaneously RA
Originates from the SA-node (in the RA)
Reaches the AV-node (where it is momentarily delayed)
Spreads through both atria [virtually] simultaneously LA
Enters the Bundle of His and the right and left bundle branches
RBB
Enters the Bundle of His and the right and left bundle branches
LBB
Activates both ventricles via the His-Purkinje network; first the septum from (L) to (R), then the RV, then the LV, and finally the apex
Repolarisation of the ventricles (note that the repolarisation of the atria is obscured by the depolarisation of the ventricles)

5. Figure 2: Diagram of the functional SA- and AV-nodes
The Cardiac Impulse
SA-node
Bachmann’s bundle
Ca2+
AV-node
Ca2+
Internodal pathways
Bundle of His
Right atrium (RA)
Bundle branches
Figure 2: Diagram of the functional SA- and AV-nodes
© G Schellack, 2012

6. The Standard 12-lead ECG You have each received a copy of this ECG…
This space should contain the following information:
Patient identifiers (e.g. name, hospital number, treating physician, etc.)
Date and time that the ECG was obtained
Vital signs, including BP and HR
Comment on any relevant symptoms (e.g. chest pain), cardiovascular drugs and special lead placements…
You have each received a copy of this ECG…
What is missing in this blank space?

7. The Standard 12-lead ECG The 12 leads are:
Six limb leads (I, II, III, aVR, aVL and aVF)
Six precordial (chest) leads: V1 to V6
The 12 leads are displayed at a standardised tracing speed of 25 mm per second, and with 1 cm representing 1.0 mV on the vertical axis.
Check the calibration mark.

8. The Standard 12-lead ECG Criteria:
Standardisation: 10 mm in height = 1 mV; 25 mm per second.
Double and half-standardisation may be required.
Sinus rhythm:
Each P-wave must be followed by a QRS-complex, and vice versa.
RR-intervals must be constant (or ≤ than 0.12 s of physiological variance).
Normal heart rate ( beats per minute).
Normal PR-interval (0.12 to 0.2 s in duration).
P-waves: Examine for normality in Lead I, II and V1.
QRS-complexes: Must be ‘positive’ in Lead I, II, III, aVF and aVL, and ‘negative’ in aVR.
Narrow QRS-complexes (0.08 to 0.12 s in duration); Q-waves in V5 and V6 are < 0.04 s and < 3 mm deep.
R-S-wave progression from V1 to V6; QTc-interval is normal.
Normal cardiac axis: Positive QRS-complexes in Lead I, III and aVF.
Normal ST-segments and T-waves.

9. Limb Lead Placement and Einthoven’s Triangle
Lead I + -
Lead II
Lead III
aVR (+)
aVL (+)
aVF
(+) RA LA LL

10. Limb Lead Placement and Einthoven’s Triangle
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from:

11. Limb Lead Placement and Einthoven’s Triangle
(From: Yanowitz, FG. The Alan E Lindsay ECG Learning Centre in Cyberspace [homepage on the Internet]. c2012. Available from:

12. Placement of the Six Chest Leads
Midclavicular
Anterior axillary
Midaxillary
Antero-lateral view
Transverse plane
Placement of the Six Chest Leads

13. Important clinical considerations
Correct lead-placement and good skin contact are essential.
Avoid electrical interference (machine to be earthed).
Compare serial tracings, if available.
Relate any changes to age, gender, clinical history, etc.
Consider co-morbidities and intercurrent illnesses that may have an effect on the ECG.
For thermal paper: Obtain a photocopy for future reference.
Interpret the ECG systematically to avoid errors. 13

14. The Standard 12-lead ECG [5/5]
What/where are the 12 leads?
3 Augmented (unipolar) limb leads
[aVR, aVL and a VF]
3 Standard (bipolar) limb leads
[I, II and III]
6 Precordial (chest) leads
[V1 to V6]
Calibration mark

15. Voltage (millivolts; mV)
ECG Graph Paper
Voltage (millivolts; mV)
(1  1) mm2
0.04 s
0.1 mV y x’ x
Time (seconds; s)
Question: What would the bigger square, i.e. the (5  5) mm2, represent?
Answer: 0.2 s on the x-axis; 0.5 mV on the y-axis. y’

16. Deflections on the ECG R QRS-complex (qRs) P-wave T-wave U-wave Q (q)
S (s)

17. Segments and Intervals on the ECG
QRS-duration
PR-interval
ST-segment
J-point
Isoelectric (base) line [0 mV]
QT-interval

18. The P-wave P-wave PR-interval
Conduction through the Right Atrium (RA)
Conduction through the Left Atrium (LA)
Conduction through the His-Purkinje network
PR-interval: Represents the depolarisation of the atria and the time it takes for the impulse to spread from the SA-node, through the AV-junction, the bundle of His, and to the bundle branches.

19. Drawing an Isoelectric Baseline
Why do we need to do this?
[A portion of the rhythm strip.]

20. Drawing an Isoelectric Baseline

21. Rate Determination:
First R-wave
Next R-wave 21 21

22. Example: What is the heart rate ?
Are the RR-intervals constant?
RR-intervals are approximately 3 ‘big’ squares apart.
Thus, the heart rate is: 300 ÷ 3 = 100 beats per minute.
Count the number of squares between two consecutive R-waves… 22 22

23. Calculating Heart Rate (HR)
‘Counting squares’
‘Accurate measurement’

24. Calculating Heart Rate (HR)

25. Calculating Heart Rate (HR)
Measure the shortest and the longest distance in mm, and divide 1,500 by each measurement, for example:
Shortest distance measured = 10 mm
Longest distance measured = 15 mm
Then 1,500  10 = 150 and 1,500  15 = 100
Thus, the HR = beats per minute

26. Correct answer: Shortest measurement = 16 mm
Longest measurement = 19 mm
Remember: Each small square represents 0.04 s, and 1,500 small squares represent one minute on the horizontal axis.
Therefore:
1,500  16 = (~ 94 beats per min.)
1,500  19 = (~ 79 beats per min.)
Thus, this patient’s heart rate (HR) is 79 to 94 beats per minute.

27. Calculating Variance in HR
Following from the previous example, subtract the shortest distance measured, from the longest distance measured (i.e. 150 mm – 100 mm = 50 mm).
Then, multiply the answer by 0.04 s (i.e. 50  0.04 = 2.0 seconds).
Normal physiological variance should be  0.12 seconds.

28. Correct answer: Shortest measurement (RR-interval) = 16 mm
Longest measurement (RR-interval) = 19 mm
Therefore:
19 mm – 16 mm = 3 mm
3 mm  0.04 s = 0.12 s (i.e. the upper limit of normal)
Normal physiological variance:  0.12 s.

29. The ‘PQRSTU-complex’ Summary:
P-wave and PR-interval:
Represents atrial depolarisation – Normal: ~ 0.10 s in duration, with an amplitude of 0.50 to 2.50 mm.
PR-interval is measured from the beginning of P-wave to the beginning of the QRS-complex (whether this a Q-wave or an R-wave) – Normal: 0.12 to 0.20 s in duration.
Digoxin prolongs the PR-interval, effectively causing a 1st-degree AV-block, which will worsen in the event of digitalis toxicity. 29

30. (Cont’d): QRS-complex and ST-segment: T-wave:
Represents depolarisation of the ventricles Ventricular contraction – Normal: 0.08 to 0.12 s in duration; amplitude of 5 to 15 mm.
ST-segment is the interval between the end of the QRS-complex and the beginning of the T-wave – Normal: on the isoelectric line (‘electric silence’); deviation of < 1 mm on either side.
T-wave:
Should be a positive deflection (except in aVR, V1 and V2).
Represents repolarisation of the ventricles. Normal: 0.10 to 0.25 s in duration; amplitude < 5 mm. 30

31. (Cont’d): QT-interval:
Is measured from the beginning of QRS-complex to the end of T-wave; requires rate-correction for heart rate (QTc).
Certain drugs may prolong the QTc (i.e. cardiotoxicity) and elicit torsades de pointes.
Example of QT-interval correction: Bazett’s formula. 31

32. (Cont’d): U-wave: ? (not necessarily present on all normal ECGs).
RR-interval:
Interval between two consecutive R-waves. Each R-wave should immediately precede a contraction of the ventricles.
We use the RR-interval to calculate HR (heart rate); heart monitors “beep” whenever R-waves are recognised.
Remember: Electromechanical dissociation (EMD)… 32

33. Rate Correction of the QT-interval
Bazett’s Formula:
RR-interval (s)
[Measured] QT-interval (s)
QTc (s) =

34. Correct answer: Measurement of the QT-interval = 9.5 mm
Therefore: QTm (s) = 9.5 mm  0.04 s = 0.38 s
Using Bazett’s formula:
Shortest RR-interval: 16 mm  0.04 s = 0.64 s
Longest RR-interval: 19 mm  0.04 s = 0.76 s
Thus:
QTc (s) = 0.38 s  s = s (~ 0.48 s)
QTc (s) = 0.38 s  s = s (~ 0.44 s)
A normal QT-interval (corrected for HR) should measure between: 0.35 and 0.45 s.

35. Cardiac Axis Determination
Complexes moving towards one another
‘Positive’ complexes
Complexes deflecting away from one another
Normal axis
Left axis deviation
Right axis deviation 35

36. Atrial P-wave Variants
Cardiac Axis Determination
Atrial P-wave Variants
Normal
Right Axis Deviation
Left Axis Deviation II V1 36

37. Questions?

38. Back-up Slides…

39. Ventricular Hypertrophy
Ventricular muscle hypertrophy
Increased QRS-voltages in V1 and V6, Lead I and aVL
May need to change the tracing to half of the normal standardisation
T-wave changes in the opposite direction to the QRS-complexes
Associated axis-deviation
Associated atrial hypertrophy 39

40. Blood Supply to the Heart
RCA
LCX
LAD
RCA
LCA 40

41. Blood Supply to the Heart
The heart has four surfaces:
Anterior surface: LAD, Left Circumflex (LCx)
Left-lateral surface: LCx, and partly via LAD
Inferior surface: RCA, terminal portion of the LAD
Posterior surface: RCA, branches of the LCx.
(R) and (L) coronary arteries arise from the root of the aorta (just above the aortic valve opening). 41

42. ST-segment Abnormalities
Source: Bing Images, at

43. Lead aVR
Flip the ECG vertically

44. Lead aVL ? ?