EKG Basics # 1 That Squigglely Line - What Does It Really Mean ?

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

l 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.

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

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

l 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.

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

General Principle # 1 For Depolarization

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

l 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

l 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

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

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

EKG Paper Marriott’s Practical Electrocardiography, 9th ed., 1994

l 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.

l 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.

l 1 mm box =.04 seconds l 5 mm box =.20 seconds

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

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

Method # 4 l Count the number of small boxes between two R waves and divide this number into 1500 l Example : 1500/12.5 small boxes=120 bpm

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

The EKG Leads

The Six Limb Leads l Three Standard Leads : Lead I Lead II Lead III

Standard Limb Leads I, II, III

The Six Limb Leads l Three Augmented Leads : aVF aVR aVL

Augmented Leads aVR, aVL, aVF Gaugleads.html

The Precordial Chest Leads l There are six precordial chest leads: V1, V2, V3, V4, V5, V6

Precordial Chest Leads V1 - V6 pt1/EKGprecordial.html

The Six Limb Leads l The six limb leads look at the heart in the frontal plane.

l 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.

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

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

l 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 l 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.

l 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.

l 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 l 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

l 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.

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

The Left Lateral Wall l Leads looking at the left lateral portion of the heart (left ventricle) are : I, aVL

l Lead aVR looks toward the right side of the heart and is considered to be a lead in “no man’s land”.