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2. • Review of the conduction system
• ECG waveforms and intervals
• ECG leads
• Determining heart rhythm / rate
• Determining QRS axis
• Normal waves / intervals

4. 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.

5. Runs at a paper speed of 25 mm/sec
• Each small block of ECG paper is 1 mm2
• At a paper speed of 25 mm/s, one small block equals
0.04 s
• Five small blocks make up 1 large block which
translates into 0.20 s (200 msec)
• Hence, there are 5 large blocks per second
• Voltage: 1 mm = 0.1 mV between each individual block
vertically

7. • Arrhythmias
• Myocardial ischemia and infarction
• Pericarditis
• Chamber hypertrophy
• Electrolyte disturbances (i.e.
hyperkalemia, hypokalemia)
• Drug toxicity (i.e. digoxin and drugs
which prolong the QT interval)

10. • 3 distinct waves are
produced during
cardiac cycle
• P wave caused by
atrial depolarization
13-63

11. • QRS complex caused
by ventricular
depolarization

12. • T wave results from
ventricular repolarization
13-63

13. Leads are electrodes which measure
the difference in electrical potential
between either:
1. Two different points on the body (bipolar
leads)
2. One point on the body and a virtual
reference point with zero electrical potential,
located in the center of the heart (unipolar
leads)

14. The standard ECG 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.

22. Limb Leads Precordial
Leads
Bipolar I, II, III -
(standard limb leads)
Unipolar aVR, aVL, aVF V1-V6
(augmented limb leads)

24. (Septum)

25. (Anterior Wall)

26. (Lateral Wall)

27. (Inferior Wall)

29. Normal Sinus Rhythm
Each P wave is followed by a QRS
o P wave rate 60 - 100 bpm with <10%
variation
o rate <60 = sinus bradycardia
o rate >100 = sinus tachycardia
o variation >10% = sinus arrhythmia

30. • Rule of 300
• 10 Second Rule

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

32. (300 / 6) = 50 bpm

33. (300 / ~ 4) = ~ 75 bpm

34. (300 / 1.5) = 200 bpm

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

36. 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.

37. The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
33 x 6 = 198 bpm

38. 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)

39. By near-consensus, the
normal QRS axis is defined
as ranging from -30 to +90 .
-30 to -90 is referred to as a
left axis deviation (LAD)
+90 to +180 is referred to as
a right axis deviation (RAD)

40. • The Quadrant Approach
• The Equiphasic Approach

41. Predominantly Predominantly Equiphasic
Positive Negative

42. 2. In the event that LAD
is present, examine
lead II to determine if
this deviation is
pathologic.
If the QRS in II is
predominantly
positive, the LAD is
non-pathologic (in
other words, the axis
is normal). If it is
predominantly
negative, it is
pathologic.

43. The Alan E. Lindsay
ECG Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Negative in I, positive in aVF  RAD

44. The Alan E. Lindsay
ECG Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Positive in I, negative in aVF  Predominantly positive in II 
Normal Axis (non-pathologic LAD)

45. 1. Determine which lead contains the most equiphasic
QRS complex. The fact that the QRS complex in this
lead is equally positive and negative indicates that
the net electrical vector (i.e. overall QRS axis) is
perpendicular to the axis of this particular lead.
2. Examine the QRS complex in whichever lead lies 90°
away from the lead identified in step 1. If the QRS
complex in this second lead is predominantly
positive, than the axis of this lead is approximately
the same as the net QRS axis. If the QRS complex is
predominantly negative, than the net QRS axis lies
180° from the axis of this lead.

47. The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
Equiphasic in aVF  Predominantly positive in I  QRS axis ≈ 0

48. The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
Equiphasic in II  Predominantly negative in aVL  QRS axis ≈ +150°

49. Normal P Waves
height < 2.5 mm in lead II (higher = ? P-pulmonale)
width < 0.11 s in lead II (wider = ? P-mitrale)
Normal PR interval
0.12 to 0.20 s (3 - 5 small squares)
Short PR interval (Wolff-Parkinson-White
syndrome / Lown-Ganong-Levine syndrome)
Long PR interval (first degree heart block /
'trifasicular' block)

50. Normal
P - pulmonale

51. P - mitrale
Long P-R

52. Normal QRS complex
< 0.12 s duration (3 small squares)
No pathological Q waves
Pathologic “Q”: - > 0.04 sec (small box)
- > 25% of “R” amplitude
Wide QRS (right or left bundle branch block, ventricular
rhythm, hyperkalemia)
o No evidence of left or right ventricular
hypertrophy

53. Normal
LVH

54. Wide Complex
Pathologic “Q” wave

55. Normal QT interval:
– Males: < 450 ms.
– Females: < 470 ms.
o Calculate the corrected QT interval (QTc) by dividing
the QT interval by the square root of the preceeding R -
R interval.
o Long QT interval is a risk factor for VT / Torsades de
Pointes.
o Long QT interval (MI, myocarditis, diffuse myocardial
disease / hypocalcaemia / hypothyrodism / intracerebral
haemorrhage / drugs (sotalol, amiodarone) / hereditary
(Romano Ward syndrome (autosomal dominant) / Jervill
Lange Nielson syndrome (autosomal recessive)

57. Normal ST segment
no elevation or depression
ST elevation: acute MI / left bundle branch
block, normal variants (e.g. athletic heart)
acute pericarditis
ST depression: myocardial ischaemia,
digoxin effect / ventricular hypertrophy /
acute posterior MI / right bundle branch
block

58. Normal ST
ST depression
ST elevation

59. Normal T wave: variable morphology & amplitude / usually same
direction as the QRS except in V1-2 leads.
In the normal ECG the T wave is always upright in leads I, II, V3-6,
and always inverted in lead aVR.
Tall T: hyperkalemia / hyperacute myocardial infarction.
Small, flattened or inverted T waves: ischaemia / LVH / drugs (e.g.
digoxin) / pericarditis / PE / RBBB / electrolyte disturbance.
Normal U wave: usually < 1/3 T wave amplitude & same direction
in the same lead / prominent at slow heart rates.
o Origin of the U wave is thought to be related to after
depolarizations which interrupt or follow repolarization

60. The U Wave

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