EKG Basics 1

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EKG Basics 1

• That Squigglely Line - What Does It Really Mean ?

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The EKG Electrodes

• The tracings on the EKG paper are a reflection of
  electrical activity of the heart.

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

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• 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.

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

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• Rules Guiding The Tracings On The EKG Paper
• Making Sense Out Of Seeming Senselessness - Part I

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General Principle 1 For Depolarization
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The Isoelectric Line
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• 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
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• 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
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• 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.

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Repolarization

• Rules Guiding The Tracings On The EKG Paper
  Making Sense Out Of Seeming Senselessness - Part
  II

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General Principle 1 For Repolarization
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• 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.

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General Principle 2 For Repolarization
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• 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.

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General Principle 3 For Repolarization
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• 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.

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Time And Speed Intervals Of The EKG Tracing
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• The EKG paper is a ruled paper that is usually
  heat sensitive.

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

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EKG PaperMarriotts Practical Electrocardiography
, 9th ed., 1994
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• 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.

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• 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.

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• 1 mm box .04 seconds
• 5 mm box .20 seconds

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Determination of Heart Rate

• Because of the predictable EKG paper speed, we
  can count the number of boxes traversed over a
  period of time between heart beats to determine
  HR.

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Methods For Determining Heart Rate
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Method 1 The Cardiac Ruler

• Place the beginning point of a cardiac ruler over
  an R wave.
• Look at the number on which the next R wave falls
  and that becomes the heart rate for that patient.

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• 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

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Method 2 - A Six Second Tracing

• Obtain a six second tracing (30 five mm boxes)
  and count the number of R waves and multiply by
  10 to obtain the HR/min.

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Method 3

• Count the number of large boxes between 2 R waves
  and divide this number into 300
• Example
• 300/2.5 large boxes 120 bpm

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Method 4

• Count the number of small boxes between two R
  waves and divide this number into 1500
• Example
• 1500/12.5 small boxes120 bpm

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Amplitude or Voltage

• Amplitude of the deflected wave is measured in
  millivolts (mV).
• The voltage of a wave deflected through one large
  5 mm box deflection is 0.5 mV.

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The EKG Leads
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The Six Limb Leads

• Three Standard Leads
• Lead I
• Lead II
• Lead III

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Standard Limb Leads I, II, IIIhttp//endeavor.med
.nyu.edu/courses/physiology/courseware/ekg_pt1/EKG
stdleads.html
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The Six Limb Leads

• Three Augmented Leads
• aVF
• aVR
• aVL

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Augmented Leads aVR, aVL, aVFhttp//endeavor.med.
nyu.edu/courses/physiology/courseware/ekg_pt1/EKGa
ugleads.html
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The Precordial Chest Leads

• There are six precordial chest leads
• V1, V2, V3, V4, V5, V6

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Precordial Chest Leads V1 - V6http//endeavor.med
.nyu.edu/courses/physiology/courseware/ekg_pt1/EKG
precordial.html
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The Six Limb Leads

• The six limb leads look at the heart in the
  frontal plane.

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• 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.

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

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

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

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Hexaxial View In The Frontal PlaneMarriotts
Practical Electrocardiography, 9th ed., pg 23
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The Standard Leads

• Lead I created by making the left arm
  positive () and the right arm negative (-).
• Its angle of orientation is 0?

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Lead IThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 39, 1999
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• Lead I looks across the heart from right to left
  along the 0? axis in the frontal plane.

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• Lead II created by making the left leg
  positive () and the right arm negative (-).
• Its angle of orientation is 60?

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Lead IIThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 39, 1999
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• Lead II looks across the heart from the right
  shoulder down to the left hip along the 60?
  axis in the frontal plane.

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• Lead III created by making the left leg
  positive () and the left arm negative (-).
• Its angle of orientation is a 120?

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Lead IIIThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 39, 1999
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• 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.

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The Augmented Leads

• aVL created by making the left arm positive
  () and the al other extremities negative (-).
• Its angle of orientation is -30?

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• 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.

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Lead aVLThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 40, 1999
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• aVR created by making the right arm positive
  () and all other extremities negative (-).
• Its angle of orientation is -150?

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Lead aVRThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 40, 1999
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• aVR looks across the heart from the left hip up
  through the right shoulder along the -150?
  axis of the frontal plane.

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• aVF created by making the legs positive ()
  and all other extremities negative.
• Its angle of orientation is 90?

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Lead aVFThaylers The Only EKG Book Youll Ever
Need, 3rd ed., pg 40, 1999
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• aVF looks through the heart from the chin down to
  the feet along the 90? axis in the frontal plane.

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Leads Look At Specific Sections Of The Heart
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The Inferior Leads

• The leads looking at the inferior portion of the
  heart are II, III, aVF

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The Left Lateral Wall

• Leads looking at the left lateral portion of the
  heart (left ventricle) are I, aVL

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• Lead aVR looks toward the right side of the heart
  and is considered to be a lead in no mans land.

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

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