1. 12-Lead ECGs and Electrical Axis17
12-Lead ECGs and Electrical Axis
Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

2. Heart’s Electrical Activity
Depolarization and repolarization of the atria and ventricles are electrical events
The ECG detects this electrical activity and displays it on the oscilloscope or prints it

3. Planes of the Heart
With the 12-lead ECG, electrodes are placed at specific spots on the patient’s extremities and/or torso and chest wall to view the heart’s electrical activity from two distinct planes:
frontal
horizontal
These planes provide a cross-sectional view of the heart

4. Frontal Plane
Leads I, II, III and aVR, aVL, and aVF view the heart along this plane
Referred to as limb leads
Four electrodes are positioned either on the extremities or on the torso
Can be placed far down on limbs or close to hips and shoulders, but they must be even (right vs. left)

5. Limb Leads - Standard
Leads I, II, and III form what is known as Einthoven’s triangle, which is an electrically equilateral triangle based on these three limb leads’ positions relative to one another
leads intersect at angles of 60 degrees
Instructional Points:
The axis of lead I extends from shoulder to shoulder with the right-arm electrode being the negative electrode and the positive electrode being the left-arm electrode. The axis of lead II extends from the negative right-arm electrode to the positive left-leg electrode. The axis of lead III extends from the negative left-arm electrode to the positive left-leg electrode. I

6. Limb Leads - Lead I
Positive electrode - left arm (or left side of chest below the clavicle in the midclavicular line)
Negative electrode - right arm (or right side of chest below the clavicle in the midclavicular line)
Ground electrodes
left leg (or left side of chest in midclavicular line just beneath last rib)
Right leg (or right side of chest in midclavicular line just beneath last rib
Waveforms are positive

7. Limb Leads - Lead II
Positive electrode - left leg (or on left side of chest in midclavicular line just beneath last rib)
Negative electrode - right arm (or right side of chest below the right clavicle in the midclavicular line)
Ground electrode
Left arm (or left side of chest in midclavicular line just below the left clavicle)
Right leg (or right side of chest in midclavicular line just beneath last rib)
Waveforms are positive

8. Limb Leads - Lead III
Positive electrode - left leg (or left side of the chest in midclavicular line just beneath last rib)
Negative electrode - left arm (or left side of chest below the clavicle in the midclavicular line)
Ground electrode
Right arm (or right side of chest in midclavicular line just below the clavicle)
Right leg (or right side of chest in midclavicular line just beneath last rib)
Waveforms are positive or biphasic

9. Limb Leads - Augmented Leads
Includes aVR, aVL and aVF
Are unipolar
Enhanced by ECG machine because waveforms produced by these leads are normally small
Instructional point:
The ECG machine augments these leads by 50% to make them easier to see I

10. Limb Leads - Lead aVR
Positive electrode placed on the right arm (or right side of chest below the clavicle in the midclavicular line)
Waveforms have negative deflection

11. Limb Leads - Lead aVL
Positive electrode placed on left arm (or left side of chest below the clavicle in the midclavicular line)
Waveforms have positive deflection

12. Limb Leads - Lead aVF
Positive electrode located on left leg (or left side of chest below the last rib in the midclavicular line)
Waveforms have a positive deflection

13. Precordial Leads Includes leads V1, V2, V3, V4, V5 and V6
Positioned in order across the chest
Unipolar
Opposing pole is center of heart as calculated by ECG
Instructional point:
Provide information about horizontal plane of heart. I

14. Lead V1
Electrode positioned in fourth intercostal space just to the right of the sternum
Faces and is close to the right ventricle
Also has a view of ventricular septum

15. Lead V1
Steps for positioning the V1 electrode

16. Lead V2
Positioned in 4th intercostal space just to the left of the sternum
Horizontally, it is at the same level as lead V1 but on the opposite side the sternum
Just like lead V1, V2 faces and is close to the right ventricle
Although it has a view of the right ventricle and anterior wall of the heart, it is more recognized for its view of the ventricular septum

17. Lead V2
Steps for positioning the V2 electrode

18. Lead V3 Located midway between leads V2 and V4
Views anterior wall of the left ventricle
Depolarization of the left ventricle moves perpendicular to the positive electrode, resulting in a biphasic waveform
Instructional Point:
In this lead the QRS complex has an R wave and S wave that are relatively the same amplitude I

19. Lead V4
Is placed at the 5th intercostal space n the midclavicular line
Views the anterior wall of left ventricle and is close to the heart’s apex
Depolarization of the left ventricle moves perpendicular to the positive electrode resulting in a biphasic waveform
Instructional Points:
The QRS complexes in this lead have R waves larger than the S waves I

20. Lead V4
Steps for positioning the V4 electrode

21. Lead V5 Placed in 5th intercostal space at the anterior axillary line
Horizontally, it is even with V4 but in the anterior axillary line
Views lateral wall of the left ventricle
Depolarization of left ventricle moves toward the positive electrode, producing a tall R wave
Instructional Point:
Because of its location closest to the left ventricle, it will have the tallest R waves of the precordial leads. I

22. Lead V5
Steps for positioning the V5 electrode

23. Lead V6
Located horizontally level with V4 and V5 at the midaxillary line
Views lateral wall of left ventricle
Depolarization of left ventricle moves toward the positive electrode producing a tall R wave
Instructional Points:
Because it is not as close to the left ventricle, the R wave in lead V6 is smaller than that found in Lead V5. I

24. Lead V6
Steps for positioning the V6 electrode

25. 12 Lead ECG Waveforms Each view provides different information
When assessing the 12 lead ECG look for characteristic normalcy and changes in all leads

26. Contiguous Leads
Two leads that look at neighboring anatomical areas of the heart are said to be contiguous

27. Right Ventricular Leads
View the right ventricle
Are in addition to the 12 lead ECG and require relocating the precordial ECG electrodes

28. Posterior Leads View the posterior surface of the heart
Are in addition to the 12 lead ECG and require relocating the precordial electrodes

29. Conditions Identified by 12-Lead ECG

30. Conditions Identified by 12-Lead ECG

31. Conditions Identified by 12-Lead ECG

32. Electrical Axis
The 12-lead ECG can measure the axis of the electrical flow of energy during the cardiac cycle

33. Instantaneous Vectors
Cardiac cell depolarization and repolarization produces many small electrical currents
Sum of these currents called instantaneous vectors
Average of instantaneous vectors called the mean vector
Instructional point:
The many small electrical currents vary in intensity and direction. I

34. Mean Electrical Axis
Direction of the mean vector called the mean electrical axis
Axis is defined in the frontal plane only

35. ECG Deflection
Wave of depolarization and its affect on the ECG

36. QRS Axis
Most important and frequently determined axis

37. Ventricular Depolarization and Mean QRS Axis
Interventricular septum depolarization represents the first cardiac vector associated with ventricular depolarization
A sequence of vectors is produced as the Purkinje fibers carry the impulse from the endocardial lining of the RV and LV through the ventricular wall toward the epicardium

38. Ventricular Depolarization and Mean QRS Axis
Completion of right ventricular activation occurs first
The thinner wall of the RV transmits impulse quicker than the thicker wall of LV

39. Mean QRS Axis
The small depolarization vectors of the thicker LV are larger
Therefore, the mean QRS axis points more to the left
Instructional point:
The mean QRS axis is the sum of all the small vectors of ventricular depolarization. I

40. Position of Mean QRS Axis
Limb leads provide information about the frontal plane and are used to determine the position of the mean QRS axis
Described in degrees within an imaginary circle drawn over the patient’s chest
Instructional point:
The limb leads include leads I, II, III, aVR, aVL and aVF. I

41. Position of Mean QRS Axis
AV node is center of circle
Intersection of all lines divides circle into equal, 30-degree segments
Lead I starts at +0 degrees and is located at the three o’clock position
Lead aVF starts at +90 degrees and is located at the six o’clock position

42. Position of Mean QRS Axis
Mean QRS axis normally points downward and to patient’s left (between 0 and +90 degrees)

43. Determining Electrical Axis
Use leads I and aVF
These two leads can best detect variations in the heart’s electrical axis
Instructional Point:
Rarely does calculating the exact direction of the axis have clinical importance. It does help us detect such things as ventricular hypertrophy and detect the presence of hemiblocks that are a subset of bundle branch blocks. I

44. Determining Electrical Axis
If the mean QRS vector directed anywhere between -90º and +90º, positive QRS complex in lead I
Instructional Point:
Various condition can cause the axis to shift and this will change the configuration of the QRS complexes to be either more negative or positive than normal. I

45. Determining Electrical Axis
If mean QRS vector directed between 0º and +180º, positive QRS complex in lead aVF
Instructional Point:
Lead aVF represents essentially the bottom half of our circle.
One way to remember this information is assign your right hand as aVF and your left hand as Lead I and point your thumb in the direction of the QRS complex. Normally leads I and aVF are both positive and subsequently both thumbs are pointed up. I

46. Right Axis Deviation
An axis between +90 and ±180 degrees indicates right axis deviation

47. Left Axis Deviation
An axis between 0 and −90 degrees indicates left axis deviation

48. Determining Electrical Axis
The deflection of the QRS complexes in leads I and aVF help identify electrical axis
Instructional points:
Again, leads I and aVF can be used to quickly determine whether the mean QRS axis on any ECG is normal. If the QRS complex is positive in leads I and aVF, the QRS axis must be normal.
Right axis deviation is present When lead aVF is positive and lead I is negative. This can be represented by turning the right thumb down and the left thumb up.
Left axis deviation is present when lead aVF is negative and lead I is positive. This can be represented by turning the right thumb up and the left thumb down.
Extreme axis deviation is present when both lead aVF and Lead I are negative. This can be represented by turning both thumbs down. I

49. Determining Electrical Axis
Location of axis influenced by:
Heart’s position in the chest
Heart size
Patient’s body size
Conduction pathways
Force of electrical impulses being generated

50. Practice Makes Perfect
Determine if the mean QRS is normal or if there is axis deviation
Answer: Left axis deviation. I

51. Practice Makes Perfect
Determine if the mean QRS is normal or if there is axis deviation
Answer: Normal axis I

52. Practice Makes Perfect
Determine if the mean QRS is normal or if there is axis deviation
Answer: Left axis deviation. I

53. Practice Makes Perfect
Determine if the mean QRS is normal or if there is axis deviation
Answer: Extreme left axis deviation I

54. Summary
Because the electrodes positioned on the patient’s skin detect the heart’s electrical activity, placing them in a different location changes the lead or view.
The shape of the waveform is described from the perspective of the positive electrode of the selected lead.
Bipolar leads include leads I, II, and III.
Unipolar leads include leads aVR, aVL , and aVF and the precordial leads V1, V2, V3, V4, V5, and V6.

55. Summary
Limb leads are produced by placing electrodes on the right arm (RA), left arm (LA), left leg (LL), and right leg (RL).
The limb leads include leads I, II, and III; augmented voltage right (aVR); augmented voltage left (aVL); and augmented voltage foot (aVF).
They provide a view of the electrical activity along the heart’s frontal plane.
The precordial leads include leads V1, V2, V3, V4, V5, and V6.
They provide information about the electrical activity along the horizontal plane of the heart.

56. Summary
The mean or average of all the instantaneous vectors which the ECG detects is called the mean vector.
The direction of the mean vector is called the mean electrical axis.
When the electrical current traveling through the heart is moving toward a positive ECG electrode on a person’s chest or extremity the ECG machine records it as a positive or upright waveform.

57. Summary
The mean of all vectors that result from ventricular depolarization is called the QRS axis.
Completion of right ventricle activation occurs first as the thinner wall of the right ventricle transmits the impulse in a fraction of the time it takes the impulse to travel through the thick lateral wall of the left ventricle.
Sum of all the small vectors of ventricular depolarization is called the mean QRS vector.

58. Summary
Because the small depolarization vectors of the thicker left ventricle are larger, the mean QRS axis points more to the left.
The limb leads are used to determine the position (axis) of the mean QRS vector which is described in degrees within an imaginary circle drawn over the patient’s chest.
Lead I starts at +0 degrees and is located at the three o’clock position.
Lead aVF starts at +90 degrees and is located at the six o’clock position.

59. Summary
The mean QRS axis normally points downward and to the patient’s left, between 0 and +90 degrees.
An axis between +90 and +180 degrees indicates right axis deviation, and one between 0 and -90 degrees indicates left axis deviation.
An axis deviation between -180 and -90 degrees indicates extreme axis deviation and is called an indeterminate axis.

60. Summary
Leads I and aVF can be used to quickly determine whether the mean QRS axis on any ECG is normal.
If the QRS complex is positive in leads I and aVF, the QRS axis must be normal.

61. Summary
If the QRS complex is upright in lead I and negative in lead aVF then left axis deviation exists.
If the QRS complex is negative in lead I and positive in lead aVF then right axis deviation exists.
If the QRS complex is negative in both leads extreme right axis deviation exists.

62. Summary
Electrodes placed on patient’s skin detect heart’s electrical activity.
Graphic record or tracing is called an electrocardiogram.
Abnormalities in cardiac rate and/or rhythm are called dysrhythmias.

63. Summary
ECG paper consists of horizontal and vertical lines that form a grid.
Horizontal measurements used to determine heart rate and duration of various waveforms, segments and intervals.
Vertically on ECG paper, distance between lines, or boxes, represents amplitude in millimeters (mm) or electrical voltage in millivolts (mV).

64. Summary
Conduction of electrical impulse through the heart can be seen on ECG as P wave, PR interval, QRS complex, ST segment and T wave.
Artifact is markings on ECG tracing that have no relationship to electrical activity of the heart.

65. Summary Each lead provides a different view of heart.
Impulses traveling toward a positive electrode are recorded on ECG as upward deflections.
Impulses traveling away from a positive electrode or toward a negative electrode are recorded as downward deflections.

66. Summary
Limb leads are produced by placing electrodes on right arm (RA), left arm (LA) and left leg (LL).
Include leads I, II, III, augmented vector right (aVR), augmented vector left (aVL) and augmented vector foot (aVF).
Precordial leads include leads V1, V2, V3, V4, V5, and V6.