Download The 12 Lead ECG Tutorial

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The 12 Lead ECG Tutorial
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The 12 Lead ECG Tutorial
Learn about lead placement and how they work to diagnose cardiac conditions
and other injuries.
Learn about lead placement and how they work to diagnose cardiac conditions and other
injuries.
By Sue Durkin, MSN, CCRN, CCNS
The 12 lead electrocardiogram or ECG is a valuable diagnostic tool for assisting in the
diagnosis of cardiac disorders or acute injury events. Knowledge of lead
configuration, proper skin preparation and placement are components to obtaining an
accurate picture of cardiac activity.
An understanding of cardiac anatomy, the electrophysiology of
the conduction system and basic rhythm interpretation knowledge will be helpful in
understanding the 12 lead ECG tracing. This tutorial will focus on proper lead placement
and various waveform representations.
History
While the first electrical current associated with a heart beat was identified in 1842 by an
Italian physicist, it was a British scientist, Augustus Waller, who published the first human
ECG in 1887. And, it was not until 1893 that Dutch scientist, Willem Einthoven,
introduced the term "electrocardiogram" and subsequently refined the concept of cardiac
electrical conduction, naming the deflections P, Q, R, S and T waves. Those same basic
waves are used today to identify and interpret the ECG rhythm strip.1
Einthoven's concept was a triangle, an imaginary area on the body, formed by the
intersection of the standard bipolar limb leads with the heart at the center. The bipolar
leads have a negative and positive pole that captures the direction of the electrical
activity of the heart in the various lead configurations. Looking at the heart from the
various electrical vectors that are produced helps determine which way the impulse is
traveling.
Standard Limb Leads
The hexaxial system of looking at the heart basically cuts the center of the heart in half
into two planes, a front and back, combining the view from leads I, II and III and creating
three additional augmented vector pictures in AVR, AVL and AVF. These three
additional vectors complete the standard six limb leads with the notion that the positive
electrode lies at the end of the lead name. If a camera is placed at the end of the positive
electrode, looking toward the negative pole, the electrical impulse of the heart's activity
passing under would create the ECG tracing. As the heart normally depolarizes from
right to left, a positive impulse moving away from the camera would create a negative
deflection of the QRS wave on the ECG tracing. While a positive impulse moving toward
the camera would be recorded as a positive or upward wave on the tracing. 2
The Precordial System
The six unipolar precordial leads are those that lie directly on the chest wall and in a
theoretical plane perpendicular to the limb leads. Thus creating a transverse plane
through the center of the heart cutting it into a top and bottom half. Combining all three
planes together (the front, back, top and bottom) will now give a 3D picture of the heart,
and the electrical activity recorded will be a reflection of the subsequent mechanical or
muscular contraction with each cardiac beat or cycle. The six precordial leads are those
named V1, V2, V3, V4, V5 and V6.
All leads give a view of the heart, which when combined together, transmit a wealth of
diagnostic information used to identify physiologic or pathologic processes occurring with
cardiac function. For identification of arrhythmia to hypertrophies and infarcts, the 12
lead ECG is an indispensable tool for use in daily healthcare practice.
Lead Placement
Correct electrode and lead placement is important to receive an accurate rhythm strip
tracing. Laying the patient supine or in a comfortable reclined position will allow easier
access to the upper body. Most importantly, proper skin care is the first step to assure
reliable results. Skin should be clean, dry and free of hair, oils or substances that may
interfere with placement or conduction to the electrode. Hair should be clipped and lead
placement should avoid implanted devices such as pacemakers or catheters that could
impede signal transmission.
Avoiding boney prominences, open wounds and very muscular or hairy areas will
provide a more accurate picture. While exact placement for limb leads can vary,
correct precordial lead position is essential.
What the Leads Tell Us
The sum of the total 12 lead system gives a wealth of information to the practitioner
during controlled or emergent situations, essentially directing treatment and interventions
for care.
An important concept to remember is that the electrical activity of the heart precedes the
mechanical function of the cardiac cycle. And, each wave is associated with a particular
function of the cardiac cycle. Changes in the specific waves can indicate abnormalities in
heart chamber size, electrolyte values, acute myocardial infarction or STEMI
patterns and electrical conduction defects as a result of ischemia, clot
or hypertrophic changes related to congestive failure.
Reviewing trends in wave form pattern changes can be important in determining
treatment for acute coronary syndromes where ischemia often precedes injury. Follow
the guidelines from the American Heart Association for prevention of cardiac syndromes
and when determining treatment for acute injury events. Use the12 lead ECG as a tool in
the armamentarium of options to prevent and treat cardiac emergencies.
Definition Page
12 lead electrocardiogram: This is a recording that provides a trans thoracic view of the
electrical activity of the heart on a strip of paper in a set time frame used to assist with
diagnosis of cardiac events.
(Permission to use the following ECG rhythm strips granted by NewCardio Inc.)
Lead configuration: The placement of electrodes on the body in a set order to relay a
picture of the electrical activity of the heart. Through analysis of the 12 specific leads,
physiologic and pathologic patterns of activity can be determined.
Cardiac anatomy: The specialized structures within the heart muscle that encompass
the four chambers, two atria, two ventricles, four heart valves, sinoatrial node, AV node,
bundle branch and purkinje system of conduction. This also includes the great vessels of
the heart, the aorta, superior and inferior vena cava, and pulmonary veins and coronary
circulation system.
Electrophysiology: This relates to the electrical activity of the heart and analysis of
function.
Conduction: The intrinsic electrical stimulation of specialized cells of the cardiac system
that allows impulses to propagate throughout the various nodes, fibers and chambers
within the heart.
P, Q, R, S, T Waves:
The P wave denotes the specific electrical action of the synchronous depolarization of
both atria. The QRS complex represents a simultaneous depolarization of the right and
left ventricles. While the ST segment and T wave symbolize the re polarization of the
ventricles.3http://library.med.utah.edu/kw/ecg/ecg_outline/Lesson1/index.html#orientation
Permission to use image granted byHealth Education And Rescue Training,
at [email protected].
Vectors: This term relates to a force or velocity and is often shown by an arrow
indicating direction.
Hexaxial: A system referring to the imaginary planes of the heart that divide it into front,
back and top and bottom through the intersection of the limb leads.
Precordial leads
The precordial leads lie on a horizontal plane on the chest with a unipolar view of the
heart where the anatomically placed position is the positive pole. As the impulse moves
toward or away from the positive pole, the waveform will be noted as positive or
negative. In the case of V1, the QRS wave form is negative.
Limb leads: The standard limb leads are placed so the tracing on the recording device,
the ECG machine, reflects the activity of the heart with a view of the impulse moving
either toward or away from the positive electrode in various locations, giving the resulting
picture of an upward or downward QRS waveform. In other words, the ECG is a
reflection of the many vectors or courses of an electrical impulse as it passes beneath
the electrode. The electrodes placed on the upper and lower extremities give origin to
leads I, II, III, AVR, AVL and AVF.
Angle of Louis: The sternal or the anterior angle that is formed by the junction of the
manubrium and sternum.
Acute myocardial infarct: The rhythms on the ECG strip that denote acute myocardial
muscle injury.
STEMI patterns: ST elevation myocardial infarction waves denoted on the ECG rhythm
strip.
Ischemia: Inadequate blood supply to the heart muscle.
Hypertrophy: The enlargement or dilation of a chamber of the heart in an effort to
overcome increasing pressure.
Acute coronary syndromes: A generalized term for conditions such as chest pain
where there is a disruption or blockage of the blood supply to the heart muscle. 4
TAKVIN TEB R&D
1/13/2014
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