Presentation on theme: "12-Lead ECGs and Electrical Axis"— Presentation transcript:
1 12-Lead ECGs and Electrical Axis 1712-Lead ECGs and Electrical AxisFast & Easy ECGs, 2nd E – A Self-Paced Learning Program
2 Heart’s Electrical Activity Depolarization and repolarization of the atria and ventricles are electrical eventsThe ECG detects this electrical activity and displays it on the oscilloscope or prints it
3 Planes of the HeartWith 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:frontalhorizontalThese planes provide a cross-sectional view of the heart
4 Frontal PlaneLeads I, II, III and aVR, aVL, and aVF view the heart along this planeReferred to as limb leadsFour electrodes are positioned either on the extremities or on the torsoCan be placed far down on limbs or close to hips and shoulders, but they must be even (right vs. left)
5 Limb Leads - StandardLeads 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 anotherleads intersect at angles of 60 degreesInstructional 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 IPositive 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 electrodesleft 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 ribWaveforms are positive
7 Limb Leads - Lead IIPositive 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 electrodeLeft 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 IIIPositive 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 electrodeRight 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 aVFAre unipolarEnhanced by ECG machine because waveforms produced by these leads are normally smallInstructional point:The ECG machine augments these leads by 50% to make them easier to seeI
10 Limb Leads - Lead aVRPositive 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 aVLPositive 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 aVFPositive 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 chestUnipolarOpposing pole is center of heart as calculated by ECGInstructional point:Provide information about horizontal plane of heart.I
14 Lead V1Electrode positioned in fourth intercostal space just to the right of the sternumFaces and is close to the right ventricleAlso has a view of ventricular septum
16 Lead V2Positioned in 4th intercostal space just to the left of the sternumHorizontally, it is at the same level as lead V1 but on the opposite side the sternumJust like lead V1, V2 faces and is close to the right ventricleAlthough 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
18 Lead V3 Located midway between leads V2 and V4 Views anterior wall of the left ventricleDepolarization of the left ventricle moves perpendicular to the positive electrode, resulting in a biphasic waveformInstructional Point:In this lead the QRS complex has an R wave and S wave that are relatively the same amplitudeI
19 Lead V4Is placed at the 5th intercostal space n the midclavicular lineViews the anterior wall of left ventricle and is close to the heart’s apexDepolarization of the left ventricle moves perpendicular to the positive electrode resulting in a biphasic waveformInstructional Points:The QRS complexes in this lead have R waves larger than the S wavesI
21 Lead V5 Placed in 5th intercostal space at the anterior axillary line Horizontally, it is even with V4 but in the anterior axillary lineViews lateral wall of the left ventricleDepolarization of left ventricle moves toward the positive electrode, producing a tall R waveInstructional Point:Because of its location closest to the left ventricle, it will have the tallest R waves of the precordial leads.I
23 Lead V6Located horizontally level with V4 and V5 at the midaxillary lineViews lateral wall of left ventricleDepolarization of left ventricle moves toward the positive electrode producing a tall R waveInstructional 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
32 Electrical AxisThe 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 currentsSum of these currents called instantaneous vectorsAverage of instantaneous vectors called the mean vectorInstructional point:The many small electrical currents vary in intensity and direction.I
34 Mean Electrical AxisDirection of the mean vector called the mean electrical axisAxis is defined in the frontal plane only
35 ECG DeflectionWave of depolarization and its affect on the ECG
36 QRS AxisMost important and frequently determined axis
37 Ventricular Depolarization and Mean QRS Axis Interventricular septum depolarization represents the first cardiac vector associated with ventricular depolarizationA 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 firstThe thinner wall of the RV transmits impulse quicker than the thicker wall of LV
39 Mean QRS AxisThe small depolarization vectors of the thicker LV are largerTherefore, the mean QRS axis points more to the leftInstructional 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 axisDescribed in degrees within an imaginary circle drawn over the patient’s chestInstructional 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 circleIntersection of all lines divides circle into equal, 30-degree segmentsLead I starts at +0 degrees and is located at the three o’clock positionLead 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 aVFThese two leads can best detect variations in the heart’s electrical axisInstructional 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 IInstructional 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 aVFInstructional 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 DeviationAn axis between +90 and ±180 degrees indicates right axis deviation
47 Left Axis DeviationAn 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 axisInstructional 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 chestHeart sizePatient’s body sizeConduction pathwaysForce of electrical impulses being generated
50 Practice Makes Perfect Determine if the mean QRS is normal or if there is axis deviationAnswer: Left axis deviation.I
51 Practice Makes Perfect Determine if the mean QRS is normal or if there is axis deviationAnswer: Normal axisI
52 Practice Makes Perfect Determine if the mean QRS is normal or if there is axis deviationAnswer: Left axis deviation.I
53 Practice Makes Perfect Determine if the mean QRS is normal or if there is axis deviationAnswer: Extreme left axis deviationI
54 SummaryBecause 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 SummaryLimb 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 SummaryThe 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 SummaryThe 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 SummaryBecause 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 SummaryThe 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 SummaryLeads 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 SummaryIf 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 SummaryElectrodes 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 SummaryECG 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 SummaryConduction 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 SummaryLimb 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.
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