12-Lead ECGs and Electrical Axis

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11Fast & Easy ECGs, 2EFast & Easy ECGs, 2E

Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

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

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

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)

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

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

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

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

Limb Leads - Augmented Leads

• Includes aVR, aVL and aVF• Are unipolar• Enhanced by ECG machine because

waveforms produced by these leads are normally small

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

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

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

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

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

Lead V1

• Steps for positioning the V1 electrode

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

Lead V2

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

Lead V4

• Is placed at the 5th intercostal space n the midclavicular line

• Views the anterior wall of left ventricleand is close to the heart’s apex

• Depolarization of the left ventricle moves perpendicular to the positive electrode resulting in a biphasic waveform

Lead V4

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

Lead V5

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

Lead V6

12 Lead ECG Waveforms

• Each view provides different information

• When assessing the 12 lead ECG look for characteristic normalcy and changes in all leads

Contiguous Leads

• Two leads that look at neighboring anatomical areas of the heart are said to be contiguous

Right Ventricular Leads

• View the right ventricle

• Are in addition to the 12 lead ECG and require relocating the precordial ECG electrodes

Posterior Leads

• View the posterior surface of the heart

• Are in addition to the 12 lead ECG and require relocating the precordial electrodes

Conditions Identified by 12-Lead ECG

Electrical Axis

• The 12-lead ECG can measure the axis of the electrical flow of energy during the cardiac cycle

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

Mean Electrical Axis

• Direction of the mean vector called the mean electrical axis

• Axis is defined in the frontal plane only

ECG Deflection

• Wave of depolarization and its affect on the ECG

QRS Axis

• Most important and frequently determined axis

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

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

Mean QRS Axis

• The small depolarization vectors of the thicker LV are larger

• Therefore, the mean QRS axis points more to the left

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

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

Position of Mean QRS Axis

• Mean QRS axis normally points downward and to patient’s left (between 0 and +90 degrees)

Determining Electrical Axis

• Use leads I and aVF– These two leads can best detect variations in the

heart’s electrical axis

• If the mean QRS vector directed anywhere between -90º and +90º, positive QRS complex in lead I I

• If mean QRS vector directed between 0º and +180º, positive QRS complex in lead aVF

Right Axis Deviation

• An axis between +90 and±180 degrees indicates right axis deviation

Left Axis Deviation

• An axis between 0 and −90 degrees indicates left axis deviation

• The deflection of the QRS complexes in leads I and aVF help identify 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

Practice Makes Perfect

• Determine if the mean QRS is normal or if there is axis deviation

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

12-Lead ECGs and Electrical Axis

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