1. Electrical and Mechanical properties of the heart [Part 2] Basics of ECG and its interpretation

2. At the end of the lecture, students should be able to: List the waves of the ECG and describe the relationship between electrical events of cardiac excitation and the P, QRS, and T waves, the PR and QT intervals, and the ST segment of the electrocardiogram. Describe Einthoven’s law and its basis. List and explain the ECG lead and their clinical importance. Learning Outcomes…

3. Interpret a normal electrocardiogram. Explain the term His-bundle electrogram. Discuss the normal deflections recorded in HBE and explain the physiologic mechanism of each. List its clinical utility Learning Outcomes

4. 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.

5. What types of pathology can we identify and study from ECGs? 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)

6. Normal ECG Wave It shows at least five different waves, 3 above the isoelectric line (positive waves) – –These positive waves are P, R, and T. 2 are below the line (negative). – –The negative waves are called Q, and S.

7. Waveforms and Intervals Normal ECG Wave

8. Intervals and segments PR Interval: From the start of the P wave to the start of the QRS complex PR Segment: From the end of the P wave to the start of the QRS complex J Point: The junction between the QRS complex and the ST segment QT Interval: From the start of the QRS complex to the end of the T wave QRS Interval: From the start to the end of the QRS complex ST Segment: From the end of the QRS complex (J point) to the start of the T wave

9. Intervals and segments

10. Genesis and Duration ECG WavesCauses Duration in seconds P WaveAtrial depolarization0.8-0.12 QRS ComplexVentricular depolarization0.8-0.10 T waveVentricular repolarization0.12-0.16 PR interval Atrial depolarization, and conduction through AV node (origin of p wave to beginning of Q or R wave) 0.16-0.20 OT intervalVentricular depolarization & repolarization (origin of Q wave to end of T wave) 0.40-0.43

11. Relationship between cardiac excitation and the ECG waves

12. Heart Excitation Related to ECG P wave: atrial depolarization START Atria contract. PQ or PR segment: conduction through AV node and A-V bundle P P Q Q wave R wave P Q R S wave QSQS R P ELECTRICAL EVENTS OF THE CARDIAC CYCLE Repolarization ST segment Ventricles contract. P Q R S The end T wave: ventricular Repolarization P QSQS R T P QSQS R T P

13. Analysis of normal ECG The P-Wave -It represents atrial depolarization. It starts 0.02 Sec before atrial contraction. The QRS complex It represents the process of ventricular depolarization. It includes 3 waves Q-wave: It starts 0.02 sec before mechanical response of ventricles (systole). It is caused by depolarization of ventricular septum.

14. R- wave: It is the most constant wave having the tallest amplitude. It represents depolarization of main ventricular muscles. S- wave: It represents depolarization of base of ventricles. T- wave: It represents process of ventricular repolarization. N.B: The manifestations of atrial repolarization are not normally seen because they are obscured by the more powerful QRS complex.

15. Electrocardiogram (ECG) Record of overall spread of electrical activity through heart Represents   Recording part of electrical activity induced in body fluids by cardiac impulse that reaches body surface   Not direct recording of actual electrical activity of heart   Recording of overall spread of activity throughout heart during depolarization and repolarization   Not a recording of a single action potential in a single cell at a single point in time   Comparisons in voltage detected by electrodes at two different points on body surface, not the actual potential   Does not record potential at all when ventricular muscle is either completely depolarized or completely repolarized

16. Willem Einthoven, winner of the Nobel Prize in Physiology or Medicine in 1924 for his invention of the string galvanometer, which was the first reliable electrocardiograph.

17. Einthoven’s Law Any two of the three bipolar limb leads determine the third one mathematically by simply algebraic summation.

18. ECG lead and their clinical importance

19. ECG Leads 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)

20. The 12-Leads The 12-leads include: –3 Limb leads (I, II, III) –3 Augmented leads (aVR, aVL, aVF) –6 Precordial leads (V 1 - V 6 ) The axis of a particular lead represents the viewpoint from which it looks at the heart.

21. Precordial Leads

23. Views of the Heart Some leads get a good view of the: Anterior portion of the heart Lateral portion of the heart Inferior portion of the heart

24. Anterior View of the Heart Remember the anterior portion of the heart is best viewed using leads V 1 - V 4. Limb LeadsAugmented LeadsPrecordial Leads

25. Lateral View of the Heart So what leads do you think the lateral portion of the heart is best viewed? Limb LeadsAugmented LeadsPrecordial Leads Leads I, aVL, and V 5 - V 6

26. Inferior View of the Heart Now how about the inferior portion of the heart? Limb LeadsAugmented LeadsPrecordial Leads Leads II, III and aVF

27. Summary of Leads Limb Leads Precordial Leads Bipolar I, II, III (standard limb leads) - Unipolar aVR, aVL, aVF (augmented limb leads) V 1 -V 6

28. Arrangement of Leads on the ECG

29. Interpretation of electrocardiogram.

30. Rhythm Analysis Step 1:Calculate rate. Step 2:Determine regularity. Step 3:Assess the P waves. Step 4:Determine PR interval. Step 5:Determine QRS duration. Step 6; Determine electrical axis

31. Determining the Heart Rate 10 Second Rule; 10 Second Rule; Count the # of R waves in a 6 second rhythm strip, then multiply by 10. Rule of 300; Rule of 300; Find a R wave that lands on a bold line. – –Count the # of large boxes to the next R wave

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

33. Step 1: Calculate Rate – –Find a R wave that lands on a bold line. – –Count the # of large boxes to the next R wave. If the second R wave is 1 large box away the rate is 300, 2 boxes - 150, 3 boxes - 100, 4 boxes - 75, etc. (cont) R wave

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

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

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

37. The Rule of 300 It may be easiest to memorize the following table: # of big boxes Rate 1300 2150 3100 475 560 650

38. 10 Second Rule As most ECGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the ECG and multiply by 6 to get the number of beats per 60 seconds. This method works well for irregular rhythms.

39. Step 1: Calculate Rate – –Count the # of R waves in a 6 second rhythm strip, then multiply by 10. – –Reminder: all rhythm strips in the Modules are 6 seconds in length. Interpretation? 9 x 10 = 90 bpm 3 sec

40. Step 2: Determine regularity Look at the R-R distances (using a caliper or markings on a pen or paper). Regular (are they equidistant apart)? Occasionally irregular? Regularly irregular? Irregularly irregular? Interpretation? Regular RR

41. Step 3: Assess the P waves Are there P waves? Do the P waves all look alike? Do the P waves occur at a regular rate? Is there one P wave before each QRS? Interpretation? Normal P waves with 1 P wave for every QRS

42. Step 4: Determine PR interval Normal: 0.12 - 0.20 seconds. (3 - 5 boxes) Interpretation? 0.12 seconds

43. Step 5: QRS duration Normal: 0.04 - 0.12 seconds. (1 - 3 boxes) Interpretation? 0.08 seconds

44. Rhythm Summary Rate90-95 bpm Regularityregular P wavesnormal PR interval0.12 s QRS duration0.08 s Interpretation? Normal Sinus Rhythm

45. Normal Sinus Rhythm (NSR) Etiology: the electrical impulse is formed in the SA node and conducted normally. arrhythmias. This is the normal rhythm of the heart; other rhythms that do not conduct via the typical pathway are called arrhythmias.

46. Hexaxial reference system based on the first six leads (I, II, III, aVR, aVL, and aVF) of the 12-lead ECG Application; used to determine the heart's electrical axis in the frontal plane

47. The Axis 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)

48. Determining the Axis Predominantly Positive Predominantly Negative Equiphasic

49. Determining the Axis; The Quadrant Approach lead I cuts the hexaxial reference system in half horizontally and lead aVF cuts the hexaxial reference system on half vertically. You can think of this as an x and y axis that divides the hexaxial reference system into quadrants. Hence, you can use leads I and aVF to place the heart’s electrical axis into one of the four quadrants. This is sometimes called the Quadrant Method for axis determination.

50. The Quadrant Approach 1. Examine the QRS complex in leads I and aVF to determine if they are predominantly positive or predominantly negative. The combination should place the axis into one of the 4 quadrants below.

51. The Quadrant Approach 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.

52. Quadrant Approach: Example 1 Negative in I, positive in aVF  RAD

53. Quadrant Approach: Example 2 Positive in I, negative in aVF  Predominantly positive in II  Normal Axis (non-pathologic LAD)

54. Determining the Axis; The Equiphasic Approach 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.

55. Equiphasic Approach: Example 1 Equiphasic in aVF  Predominantly positive in I  QRS axis ≈ 0°

56. Equiphasic Approach: Example 2 Equiphasic in II  Predominantly negative in aVL  QRS axis ≈ +150°

57. His bundle electrography is a test that measures electrical activity in a part of the heart that carries the signals that control the time between heartbeats (contractions).

58. His bundle electrogram (HBE) an intracardiac electrogram of potentials in the lower right atrium, atrioventricular node, and His-Purkinje system, obtained by positioning intracardiac electrodes near the tricuspid valve.

59. HBE; three intervals 1. 1. PA interval, the time from the first appearance of atrial depolarization to the A wave in the HBE, which represents conduction time from the SA node to the AV node; 2. AH interval 2. AH interval, from the A wave to the start of the H spike, which represents the AV nodal conduction time; and 3. HV interval represents conduction in the bundle of His and the bundle branches. 3. HV interval, the time from the start of the H spike to the start of the QRS deflection in the ECG, which represents conduction in the bundle of His and the bundle branches.

60. Normal Value and clinical significance Normal values for these intervals in adults are PA, 27 ms; AH, 92 ms; and HV, 43 ms. Illustrate the relative slowness of conduction in the AV node.