measurement of ekg

7/29/2019 Measurement of EKG

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Measurement of EKG

Removing the Mysteries

http://course1.winona.edu/LReuter/212/2012


2/18

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

measurement of ekg

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