EKG Basics # 1
That Squigglely Line What Does It Really
Mean ?
The EKG Electrodes
 The
tracings on the EKG
paper are a reflection of
electrical activity of the heart.
 A series
of body surface
electrodes are placed at
specific points on the arms,
legs and thorax that sense and
record the heart’s electrical
activity.
 The
electrodes are assigned a
specific polarity - i.e. - either
negative or positive.
 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
Making Sense Out Of Seeming
Senselessness - Part I
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
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.
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
Rules Guiding The Tracings
On The EKG Paper : Making
Sense Out Of Seeming
Senselessness - Part II
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.
 The largest division on the
paper is a five millimeter box.
EKG Paper
Marriott’s 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
 5 mm box = .20 seconds
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.
Methods For
Determining Heart
Rate
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.
 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
 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.
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
Method # 4
 Count
the number of small boxes
between two R waves and divide
this number into 1500
 Example :
1500/12.5 small boxes=120 bpm
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.
The EKG Leads
The Six Limb Leads
 Three
Standard Leads :
Lead I
Lead II
Lead III
Standard Limb Leads I, II, III
http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_pt1/EKGstdleads.html
The Six Limb Leads
 Three Augmented
aVF
aVR
aVL
Leads :
Augmented Leads aVR, aVL, aVF
http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_pt1/EK
Gaugleads.html
The Precordial
Chest Leads
 There
are six precordial chest
leads:
V1, V2, V3, V4, V5, V6
Precordial Chest Leads V1 - V6
http://endeavor.med.nyu.edu/courses/physiology/courseware/ekg_p
t1/EKGprecordial.html
The Six Limb Leads
 The
six limb leads look at the
heart in the frontal plane.
 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
Marriott’s Practical Electrocardiography, 9th ed., pg 23
The Standard Leads
 Lead
I : created by making
the left arm positive (+) and
the right arm negative (-).
 Its angle of orientation is + 0
Lead I
Thayler’s The Only EKG Book You’ll 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 (-).
 Its angle of orientation is +60
Lead II
Thayler’s The Only EKG Book You’ll 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 (-).
 Its angle of orientation is a
+120
Lead III
Thayler’s The Only EKG Book You’ll 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
: created by making the
left arm positive (+) and the al
other extremities negative (-).
 Its angle of orientation is -30
 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
Thayler’s The Only EKG Book You’ll Ever Need, 3rd ed., pg 40, 1999
 aVR
: created by making the
right arm positive (+) and all
other extremities negative (-).
 Its angle of orientation is
-150
Lead aVR
Thayler’s The Only EKG Book You’ll 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.
 Its angle of orientation is +90
Lead aVF
Thayler’s The Only EKG Book You’ll 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
 The
leads looking at the
inferior portion of the heart
are : II, III, aVF
The Left Lateral Wall
 Leads
looking at the left
lateral portion of the heart
(left ventricle) are : I, aVL
 Lead
aVR looks toward the
right side of the heart and is
considered to be a lead in “no
man’s land”.
 Since
the inferior leads, (Leads II,
III, and aVF) will detect the presence
of vessel obstructions and MI’s 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 MI’s in the lateral
wall, what vessels might be
implicated for being occluded ??