EKG Basics # 1

8/30/00


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EKG Basics # 1

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The EKG Electrodes

A series of body surface electrodes are placed at specific points on the arms, legs and thorax that sense and record the heartís electrical activity.

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The electrodes are assigned a specific polarity - i.e. - either negative or positive.

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For any lead, the EKG machine looks at a specific combination of electrodes in order to configure the tracing that we expect to see for Lead I, II, III, aVF, aVR, aVL or for V1-V6.

The electrode we care about the most is the Sensing Electrode which is always given a positive polarity (+). Hence, we call it the Positive Sensing Electrode.

In order to understand what the EKG tracing is saying to us, there are a few general principles to remember for depolarization and repolarization.

Rules Guiding The Tracings On The EKG Paper

General Principle # 1 For Depolarization

The Isoelectric Line

If the wave of depolarization is generally moving toward the positive sensing electrode, that electrode will record a positive deflection above the isoelectric line on the EKG paper

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General Principle # 2 For Depolarization

If the wave of depolarization is generally moving away from the positive sensing electrode, then the electrode will record a negative deflection below the isoelectric line on the EKG paper.

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General Principle # 3 For Depolarization

If the wave of depolarization moves perpendicular to the line of sight of the positive sensing electrode, the sensing electrode will record a biphasic tracing on the EKG paper.

Repolarization

General Principle # 1 For Repolarization

If a wave of repolarization is moving toward the positive sensing electrode, then it will inscribe a negative deflection below the isoelectric line on the EKG paper.

General Principle # 2 For Repolarization

If a wave of repolarization moves away from the positive sensing electrode, then it will inscribe a positive deflection above the isoelectric line on the EKG paper.

General Principle # 3 For Repolarization

If a wave of repolarization that moves perpendicular past the line of sight of the positive sensing electrode, then it will inscribe a biphasic deflection on the EKG paper.

Time And Speed Intervals Of The EKG Tracing

The EKG paper is a ruled paper that is usually heat sensitive.

The smallest division on the paper is a one millimeter box.

EKG Paper Marriottís Practical Electrocardiography, 9th ed., 1994

By international convention, the speed of the paper is 25 millimeters per second. This is the speed that makes all of the tracings look appropriate for any lead that is being viewed.

Because the speed of the paper is 25 mm/second, a small 1 mm box is traversed in .04 seconds and a large 5 mm box is traversed in .2 seconds.

1 mm box = .04 seconds

Determination of Heart Rate

Methods For Determining Heart Rate

Method # 1 : The Cardiac Ruler

Use the following numbers to indicate what the heart rate is between two successive R waves : 300, 150, 100, 75, 60, 50, 43, 37, 33, 30

Method # 2 - A Six Second Tracing

Method # 3

Method # 4

Amplitude or Voltage

The EKG Leads

The Six Limb Leads

Standard Limb Leads I, II, III http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_pt1/EKGstdleads.html

The Six Limb Leads

Augmented Leads aVR, aVL, aVF http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_pt1/EKGaugleads.html

The Precordial Chest Leads

Precordial Chest Leads V1 - V6 http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_pt1/EKGprecordial.html

The Six Limb Leads

By international convention, a circle is drawn from the chin to the symphysis pubis to describe the area in which the limb leads view the heart.

The circle is divided along the horizontal X axis and the vertical Y axis.

The top half of this circle is wholly negative with the circle enumerated as 0 to -180 degrees.

The bottom half of this circle is enumerated as wholly positive with the circle enumerated from 0 to +180 degrees.

Hexaxial View In The Frontal Plane Marriottís Practical Electrocardiography, 9th ed., pg 23

The Standard Leads

Lead I Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 39, 1999

Lead I looks across the heart from right to left along the +0? axis in the frontal plane.

Lead II : created by making the left leg positive (+) and the right arm negative (-).

Lead II Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 39, 1999

Lead II looks across the heart from the right shoulder down to the left hip along the + 60? axis in the frontal plane.

Lead III : created by making the left leg positive (+) and the left arm negative (-).

Lead III Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 39, 1999

Lead III looks at the heart from the left shoulder down through the heart to the right hip along the +120? axis in the frontal plane.

The Augmented Leads

aVL looks across the heart from the right leg up through the heart to the left shoulder along the -30? axis in the frontal plane.

Lead aVL Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 40, 1999

aVR : created by making the right arm positive (+) and all other extremities negative (-).

Lead aVR Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 40, 1999

aVR looks across the heart from the left hip up through the right shoulder along the -150? axis of the frontal plane.

aVF : created by making the legs positive (+) and all other extremities negative.

Lead aVF Thaylerís The Only EKG Book Youíll Ever Need, 3rd ed., pg 40, 1999

aVF looks through the heart from the chin down to the feet along the +90? axis in the frontal plane.

Leads Look At Specific Sections Of The Heart

The Inferior Leads

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The Left Lateral Wall

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Lead aVR looks toward the right side of the heart and is considered to be a lead in ďno manís landĒ.

Since the inferior leads, (Leads II, III, and aVF) will detect the presence of vessel obstructions and MIís in the inferior wall of the heart, what vessels might be implicated for being occluded ??

Since the lateral wall leads (aVL and Lead I) detect flow obstructions and the presence of MIís in the lateral wall, what vessels might be implicated for being occluded ??

Author: David Arnall

Email: David.Arnall@NAU.EDU , DAArnall@AOL.COM

Home Page: http://jan.ucc.nau.edu/~daa/heartlung/

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