Download - Measurement of EKG
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Measurement of EKG
Removing the Mysteries
http://course1.winona.edu/LReuter/212/2012
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View the heart as a
signal source.
The signal center
moves.
The signal amplitude
varies
From where does the EKG signal arise?
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From where does the EKG signal arise? Depolarization of cardiomyocytes and Purkinje Fibers
Repolarization of cardiomyocytes and Purkinje Fibers De/re/polarization results in a change of
transmembrane ions in a heart region
The de/re/polarization change migrates to different
regions of the heart The de/re/polarization signal is greatest when many
cells have polarization changes at the same time
Other de/re/polarizations, such as from skeletal
muscle contraction, can interfere with the heartsignal
Many different ions (Na+, K+, Ca++, Cl- contribute tothe electrical measurement
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ELECTRODESmeasure voltage
Bipolar Electrodes:
Measure the electrical difference between two points
Example: Leads: I, II, & III
Unipolar Electrodes:
Measure the electrical difference between one point
and an established zero reference point.
Example: Leads: aVR, aVL, aVF, V1, V2, V3, V4, V5, &V6
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LEADSspecify location of electrodes
Limb Leads I, II, III, aVL, aVR, aVF
Measure the electrical difference in the Frontal Plane (has
directions of Left->Right & Inferior->Superior)
a == augmented (an electrode arrangement to increase
signal and change the axis of measurement hexaxialreference system.)
V == vector (a quantity that has magnitude and direction)
Precordial Leads V1, V2, V3, V4, V5, & V6 Measure the electrical difference in the Transverse Plane
(has directions of Left->Right & Anterior->Posterior)
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Limb Leads
Right
Left
Foot
In Frontal Plane
aVR
aVL
aVF
Precordial Leads
V1, V2, V3, V4, V5, V6
In Transverse Plane
Einthovens Triangle
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The Positioning of Electrodes is Important
When an electrical signal from the heart arrives
at two electrodes at different times, a voltagewill appear between the two electrodes. (Thevelocity of an electrical signal in tissue is ~0.1C)
For LA and RA, place the electrodes on
corresponding parts of the anatomy: the twowrists, the two elbows, the two shoulders.
Try to keep an Einthoven triangle between limb
electrodes, with the heart in the triangles center. Try to keep electrodes away from major skeletal
muscles, especially when patient is ambulatory.
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LEAD I: (+)LeftArm; (-)RightArm
Measures electrical potential difference from the
mid-sagittal plane == 0 degrees axis
Measures along an axis corresponding to the base of
Einthovens Triangle
Measures mainly across the heart
+
+-
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An EKG Trace
Baseline = end of P wave
Y dimensions = mV or V
X dimensions = sec or mm/sec
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LEAD II: (+)LeftLeg; (-)RightArm
Measures electrical potential difference from a
plane that has been rotated 60 degrees from the
mid-sagittal plane == 60 degree axis
Measures along an axis corresponding to the right side
of Einthovens Triangle (on patients right side) Measures approximately along the length of the heart
0v
+
-
+
-
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LEAD III: (+)LeftLeg; (-)LeftArm
Measures electrical potential difference from a
plane that has been rotated 120 degrees from the
mid-sagittal plane == 120 degree axis
Measures along an axis corresponding to the left side
of Einthovens Triangle (on patients left side) Measures approximately across the width of the heart
0v+
-
+
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More Leads: Hexaxial Reference System
Augmented leads, with a constructed reference zero
aVR(+) RightArm (-) [LeftArm + LeftLeg] == [I +II]/2 aVL(+) LeftArm (-) [RightArm + LeftLeg] == I [II/2] aVF(+) LeftLeg (-) [RightArm + LeftArm] == II [I/2]
One electrode is a basic Limb Lead
The other electrode is an average of
the two remaining basic Limb Leads
The augmented leads have axes that
are located between those of the
basic Limb Leads
Allows for an easier estimation of
the Mean Electrical Vector
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Meaning of trace deflection Positive components:
Potential increasing in the positive axis direction
Potential decreasing in the negative axis direction
Negative components:
Potential decreasing in the positive axis direction
Potential increasing in the negative axis direction
No measurable components:
No change in electrical potential
Identical changes in electrical potential
Views From Different Axes:
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Limb Leads: Mean Electrical Vector in frontal plane: may
show greatest magnitude along axis of a particular lead,
but two axes are needed to accurately establish MEV.
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Determining the Cardiac QRS Mean Electrical Vector
1. All measured waves and segments of the EKG have associated electrical vectors.
2. We will measure the MEV of the QRS complex (waves).
3. The most accurate measurement requires a measurement of the area under the three
waves; we are more interested in the angle of the MEV and will estimate area by the
amplitude of each wave.
4. Obtain graphic recordings of at least two Limb Leads, such as Lead I and Lead II.
5. Make a 0o, 60o, 120o triaxial graph with correct axes for these Leads, as shown below.
6. Establish a base line (using the PR segment, from the end of the P wave to the beginning
of the Q wave).7. From the base line (zero) Measure the (+/- signed) amplitudes of the Q, R, and S waves .
If your two Leads have different Y scales, convert the amplitudes to millivolts.
8. Algebraically add the Q, R, and S amplitudes to achieve a parameter of the QRS complex
area. Plot the resultant amplitudes on the respective Lead axes.
9. Draw equipotential perpendiculars at the ends of each vector.
10. Draw a line from the origin to the intersection of the equipotential lines. This vector is
the MEV, which has magnitude and direction of the mean electrical axis
11. View example on the next slide.
(+) Lead I
[+0o]
(+) Lead III[+120o]
(+) Lead II[+60o]
0(-)
(-) (-)
l [ / ]
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(+) Lead III
[+120o]
(+) Lead II
[+60o]
0(-)
(-) (-)
Lead I
Equipotential
Line
Lead II
Equipotential
Line
Lead III
Equipotential
Line
Lead II: 13mV R -1.5mV Q - 0mV S = 10.5mV
Lead I: 6mV R -0.5mV Q 2.5mV S = 3mV
(+) Lead I[+0o]
MEV Example: 11 mV; invCos-[3/-11]=74o
Lead III: 8mV R -1mV Q - 0mV S = 7mV
3mV
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General Comments on Trace Patterns:Shortened QT interval Hypercalcemia (increased serum Ca++)
Prolonged QT interval Hypocalcemia (decreased serum Ca++)
Flattened or inverted T waves Coronary ischemia, hypokalemia, leftventricular hypertrophy
Hyperacute T waves Maybe acute myocardial infarction
Prominent U waves Hypokalemia (decreased serum K+)
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Wiggers Diagram