1. Practical Electrocardiography - Introduction -
Scott E. Ewing, D.O.
Cardiology Fellow
Lecture #1

3. Introduction Electrophysiology Depolarization Anatomy ECG Paper
Lead Placement
Normal ECG
Waves / Intervals / Segments

4. Cardiac Action Potential

5. Cardiac Action Potential
Phase 0: Rapid depolarization
Sodium moves rapidly into the cell
Calcium moves slowly into the cell
Phase 1: Early repolarization
Sodium channels close
Phase 2: Plateau
Calcium continues to flow in
Potassium continues to flow out
Phase 3: Rapid repolarization
Calcium channels close
Potassium flows out rapidly
Active transport Na/k pump
Phase 4: Resting
Cell membrane is impermeable to sodium
Potassium moves out

6. Sequence of Excitation
Sinoatrial (SA) node generates impulses about 75 times/minute
Atrioventricular (AV) node delays impulse approximately 0.1 second
Impulse passes from atria to ventricles via atrioventricular bundle (bundle of His)

7. Depolarization

10. Heart Physiology: Sequence of Excitation
Figure 17.14a

11. ECG Paper
Time intervals indicated for the thick and thin vertical grid lines
Amplitudes indicated for the thick and thin horizontal grid lines
Each small square 0.04 s × 0.1 mV and each large square 0.20 s × 0.5 mV

12. ECG Limb Leads

13. ECG Limb Leads

14. Willem Einthoven (1860 – 1927) Dutch doctor and physiologist
Invented the first practical electrocardiogram in 1903
Nobel Prize Medicine in 1924
Died in Leiden in the Netherlands and is buried in the graveyard of the Reformed Church at 6 Haarlemmerstraatweg in Oegstgeest

15. ECG Augmented Leads

16. ECG Frontal Plane

17. ECG Frontal Plane

18. ECG Precordial Leads

19. ECG Precordial Leads

21. Normal ECG

22. Normal Findings Tall R waves Prominent U waves
ST segment elevation (high­take off, benign early repolarization)
Exaggerated sinus arrhythmia
Sinus bradycardia
Wandering atrial pacemaker
Wenckebach phenomenon
Junctional rhythm
1st degree heart block

23. Waveform Review

24. P Wave

25. P Wave

26. P Wave Atrial activation begins in the SA node Normal amplitude
Spreads in radial fashion to depolarize the right atrium, interatrial septum, then the left atrium
Last area of the left atrium to be activated is the tip of the left atrial appendage
Normal amplitude
Seldom exceeds 0.25 mV normally in limb leads
In precordial leads, positive component is normally less than 0.15 mV

27. P Wave Characteristics
Positive in leads I and II
Best seen in leads II and V1
Commonly biphasic in lead V1
< 3 small squares in duration
< 2.5 small squares in amplitude

28. P Wave – Lead II

29. P Wave – Lead V1

30. PR Interval / Segment

31. PR Interval / Segment

32. PR Interval Beginning P wave to beginning QRS complex
Interval between onset atrial depolarization and onset ventricular depolarization
Time required for the activation impulse to advance from atria through the AV node, bundle of His, bundle branches, and the Purkinje fibers until ventricular myocardium begins to depolarize
Normal PR Interval
seconds (adults)
Should be taken from lead with the largest and widest P wave and longest QRS duration

33. PR Segment
Horizontal line between the end of the P wave and the beginning of the QRS complex
Duration depends on the duration of the P wave as well as the impulse conduction through the AV junction
Usually isoelectric, however it is often displaced in a direction opposite to the polarity of the P wave
Depressed in most of the conventional leads except aVR
Displacement is mainly due to atrial repolarization

34. PR Interval – Lead II

35. QRS Complex

36. QRS Complex

37. QRS Complex Q wave – Any initial negative deflection
R wave – Any positive deflection
S wave – Any negative deflection after an R wave

38. QRS Complex

39. QRS Complex
Resultant electrical forces generated from ventricular depolarization
Begins at middle third of left interventricular septal surface
Spreads in a rightward direction
RV begins to depolarize shortly after initiation of LV activation
Basal portion of septum and posterobasal portion of LV free wall are last areas of depolarization
LV contributes most of the QRS forces due to larger muscle mass

40. Ventricular Depolarization

41. QRS Complex QRS duration represents duration of ventricular activation
Should be measured from lead with widest QRS complex
Traditionally measured from the limb leads, but V1 or V2 may have the widest complex
Normal QRS varies between second

42. QRS Complex Lead I: usually records a dominant R wave
Lead II: invariably has prominent R wave since mean vector is always toward II if QRS axis is normal
Lead aVR: always records negative deflection
Lead III: variable

43. QRS Complex – Lead V3

44. ST Segment

45. ST Segment
Segment between end of QRS complex (J point) and beginning of T wave
Represents state of unchanged polarization between end of depolarization and beginning of repolarization
Stage when terminal depolarization and starting repolarization are superimposed and cancel each other

46. ST Segment
Most important information regarding ST segment is presence or absence and degree of displacement from isoelectric line
TP segment is used as reference baseline
Limb leads – elevation or depression < 1mm
Precordial leads- elevation sometimes seen and normal in V2-V3 (< 2mm), rarely > 1mm in V5-V6
Any ST depression in precordial leads is abnormal (normal vector in horizontal plane is anterior and leftward)

47. ST Segment – Lead aVF

48. T Wave

49. T Wave Represents potential for ventricular repolarization
Proceeds in general direction of ventricular excitation
Polarity of resultant T wave is similar to that of the QRS vector
Upright in I, II, V5-V6
Inverted in aVR
When inverted in 2 or more of right precordial leads, referred to as persistent juvenile pattern

50. T Wave Limb leads Precordial leads: Tallest in lead II
Normally < 6mm in all limb leads
Should never be < 0.5mm
Precordial leads:
Tallest in V2-V3 (average 6mm)
Smaller in left precordial leads

51. T Wave – Lead aVF

52. QT Interval

53. QT Interval Represents duration of ventricular electrical systole
Measured from beginning of QRS complex to end of the T wave
Lead with a large T wave and distinct termination is used
Leads V2-V3 are usually best for this specific measurement

54. QT Interval Varies with heart rate Increases slightly with age
Lengthens as heart rate decreases
Shortens as heart rate increases
Increases slightly with age
Diurnal variation of QT has been documented
Longer during sleep than during waking hours

55. QT Interval Normal = 0.35 – 0.45 second for HR=70 Bazett's correction
For every 10-beat increase or decrease of the rate, 0.02 second is deducted or added to the QT
Bazett's correction
QTc = measured QT divided by square root of RR interval
Obvious abnormality if QT > ½ RR interval

56. QT Interval – Lead aVF

57. U Wave

58. U wave Small, low-frequency deflection that appears after the T wave
Genesis is controversial
Afterpotentials of ventricular myocardium
Repolarization of the Perkinje fibers
Amplitude is proportional to T wave
Usually 5-25% of T wave voltage
Largest in leads V2-V3
Prominent during slower heart rates
Initial portion is normally steeper than terminal portion

59. 8-Step Method ECG Interpretation
Rate
Rhythm
Axis
P wave
PR interval
QRS complex
QT interval
ST segment and T wave

61. Questions?

62. His-Purkinje Conduction System