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Transcript of Shadechapter13.ppt [read only]
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Q
I
A
13
Fast & Easy ECGs – A Self-Paced Learning Program
Electrical Axis
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Electrical Axis • 12-lead ECG can measure the axis of the
electrical flow of energy during the cardiac cycle
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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
I
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Mean Electrical Axis • Direction of the mean
vector called the mean electrical axis
• Axis is defined in the frontal plane only
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ECG Deflection
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QRS Axis
• The most important most frequently determined axis
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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
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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
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Mean QRS Axis • The small
depolarization vectors of the thicker LV are larger
• Therefore, the mean QRS axis points more to the left
I
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Position of the 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
I
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Position of the 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
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Position of the Mean QRS Axis • Mean QRS axis normally points downward and
to patient’s left (between 0 and +90 degrees)
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Determining Electrical Axis
• Use leads I and aVF – The two leads that can best detect variations
in the heart’s electrical axis
I
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• If the mean QRS vector directed anywhere between -90º and +90º, positive QRS complex in lead I
I
Determining Electrical Axis
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• If mean QRS vector directed between 0º and +180º, positive QRS complex in lead aVF
I
Determining Electrical Axis
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I
Determining Electrical Axis
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I
Determining Electrical Axis
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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
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Practice Makes Perfect • Determine if the mean QRS is normal or if there is axis deviation
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Practice Makes Perfect • Determine if the mean QRS is normal or if there is axis deviation
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Practice Makes Perfect • Determine if the mean QRS is normal or if there is axis deviation
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Practice Makes Perfect • Determine if the mean QRS is normal or if there is axis deviation
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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.
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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.
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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.
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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.
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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.
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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.