Download The Function of an ECG in Diagnosing Heart Conditions

The Function of an
ECG in Diagnosing
Heart Conditions
A useful guide to the function of the heart’s electrical system for
patients receiving an ECG
Written by Erhan Selvi
July 28, 2014
Audience and Scope
The purpose of this document is to describe the electrical activity of the heart and how it relates to an
ECG (electrocardiogram) for patients if they receive one during a visit to the hospital. The focus will be
placed on anatomical and physiological functions of the heart. Further focus will be directed towards
abnormalities in the heart and how they can be diagnosed by using an ECG.
Everyone is familiar with the typical heart beat pulse that is monitoring a patient in the hospital that is
often dramatized in TV shows or movies. That waveform that is displayed on the machine’s screen is a
typical readout from an ECG machine. This document will inform patients who are going to receive an
ECG test about how the ECG produces the waveform and what different waveforms in the readout
mean. Receiving an ECG is very simple procedure that is both brief and painless, but is very important to
ensure the wellbeing of an individual.
Background Anatomy Information
An ECG is a test that is used to determine the electrical activity of the heart. The information produced
from an ECG can be further interpreted to determine the heart rate, heart rhythm, and contractility of
the heart. If a patient experience symptoms that include, chest pain, uneven heartbeat, breathing
issues, unusual fatigue, and abnormal heart sounds, a physician may recommend an ECG to diagnose
the cause of these symptoms. An ECG examination may also be part of a routine checkup to ensure the
heart’s conduction system is working properly.
Autorhythmic cells and Action Potentials
The heart beats with a steady rhythm, and that is the result of specialized cells in the heart that
are known as autorhythmic cells. Autorhythmic cells have the ability to contract the entire heart
due to its electrical features. These cells go through a process known as an action potential. The
action potential is a quick change in the electrical voltage of a heart cell. For instance, when a
heart cell is at rest it has a voltage of -70 millivolts. During an action potential, the voltage of the
cell will quickly become very positive (depolarization), and then immediately drop back down to
a negative value (repolarization). This action potential causes the heart cells to contract. Most
cells in the human body need an external agent to cause an action
potential; autorhythmic cells can go through an action potential on
their own.
Autorhythmic cell – a cell that can undergo
action potentials, and thus contractions, on
its own
Action potential – a rapid depolarization
and repolarization that results in the cell
contracting
SA node – specialized group of autorhythmic
cells that undergo action potentials at the
fastest rate; sets heart rate in healthy hearts;
causes atrial contraction
AV node – specialized group of autorhythmic
cells that that receives action potential
signals from the SA node; spreads signal to
ventricular fibers
Specialized Autorhythmic Cells
Multiple regions of the heart have autorhythmic cells so that
different parts of the heart may contract at different times. The first
and most important group of autorhythmic cells is located at the top
of the right atrium and is called the SA node as seen in figure 1.
These cells undergo action potentials at the fastest rate. Therefore
the SA node is often called the “pacemaker” of the heart. Another
important node of autorhythmic cells is the AV node, located at the
bottom of the right atrium. When the SA node undergoes an action
potential, its action potential signal is sent to the AV node which
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signals cells in that region to contract. After the AV node, there are fibers that are spread out in
the ventricles that will also relay the action potential signals that were started at the SA node.
Therefore, after the AV node receives the signal from the SA node, the signal moves to the fibers
which signal the ventricles to contract last.
Figure 1 – Anatomical Structure of the Heart
Source: http://www.heart-valve-surgery.com/Images/cardiac-conduction-system.jpg
Gap Junctions
Gap junctions play an important role in the contraction of the entire heart. The heart can
contract without the help of the SA node. While there are the
specialized groups of autorhythmic cells such as the SA and AV
nodes, autorhythmic cells are located all throughout the
Ventricular fibers – receive action potential
heart. All cells in the heart also contain a structure called gap
signals from AV node and send the signal to
junctions that link neighboring cells together. This structure
ventricular cells; causes ventricular
contraction
acts like an open doorway for voltages to move from one cell
to the other. This is important because the spread of voltages
Gap junction – linkage between neighboring
all cardiac cells that allows for the spread of
from one cell will cause depolarization of the cells in that
voltage and thus contraction
region, and this ultimately results in the spread of action
potentials and contractions in the heart cells (heart cells
contract after undergoing an action potential). With the ability
for a heart to spread an action potential through the gap
junctions in its cells with the help of the SA node setting the
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pace of action potentials, the heart can successfully beat in a coordinated rhythm that delivers
blood efficiently to the body.
ECG Readings
Electrical signals can be detected using an ECG with sensors placed on different parts of the body. The
sum of all the action potentials that occur in the heart is what an ECG measures. Sensors (or leads) can
be placed on several parts of the body such that when action potentials occur in its region, they will
detect the change in voltages. These changes in voltage from the ECG waveform are seen in figure 2.
Figure 2 – Typical ECG Waveform
Source: lessons4medicos.blogspot.com
The level point at the beginning of the wave is known as the isoelectric point and this represents a
resting voltage value where there are no major action potentials occurring. The first major event to
occur is the P wave. The P wave is the depolarization of the atria of
the heart. Following the P wave is the QRS complex, and this
represents the ventricular depolarization. Lastly is the T wave which
P wave – atrial depolarization and
is the ventricular repolarization. The atrial repolarization is not seen
contraction
in the ECG waveform because the overall magnitude of it is not
QRS complex – ventricular depolarization
strong enough for it to create a distinguishable wave. For the sake
and contraction
of simplicity, when a region of the heart depolarizes, that means the
T wave - ventricular repolarization and
cells in that region will contract.
relaxation
Arrhythmia – heartbeat that is out of
rhythm
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Diagnosis of Heart Abnormalities
An ECG test is useful to detect when the heart does not beat in rhythm (arrhythmia). The sequence of
contraction in the heart starts with both atria at the same time contracting into the ventricles, followed
by both ventricles contracting blood into the body. This order is dependent upon the heart’s electrical
system and the order that it follows. The order of contraction is SA node, atrial cells, AV node, then
ventricular cells. Any abnormalities that occur in this sequence result in arrhythmia. Figure 3 shows the
ECGs of several different arrhythmias that reveal abnormalities in the heart and how it contracts.
Second-degree block is seen as the first graph in figure 3. This ECG differs from the normal waveform in
that not every P wave is followed by a QRS complex. Since the P wave is associated with atrial
contraction and the QRS complex is associated with ventricular contraction, so there must be a problem
with the conduction of action potentials from the atria to the ventricles, so the malfunctioning
component of the heart is the AV node.
Figure 3 – ECG waveform of second-degree heart block
Source: cnx.org/content/m46664/latest/
In the atrial fibrillation ECG graph in figure 4, the only noticeable wave is the QRS complex, and the rest
of the graph is just peaks and valleys of weak signals. These small deflections in the wave are
representative of the autorhythmic cells of the atria contracting on their own without any order. Thus, it
can be concluded that atrial fibrillation is the result of the SA node not properly sending its signal and
setting the pace for the rest of the heart to beat.
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Figure 4 – ECG waveform of atrial fibrillation
Source: cnx.org/content/m46664/latest/
Ventricular fibrillation, seen in figure 5, is similar to atrial fibrillation, except that there are none of the
distinguishable waves of a normal ECG present. The peaks and valleys are seen again, however they are
slightly larger. This is due to there being more ventricular cells than atrial cells. Thus in ventricular
fibrillation, all of the autorhythmic cells of the ventricles are beating on their own without any
coordination and this results in an extremely ineffective pumping of the blood. The cause of ventricular
fibrillation is not exactly known, but it is clear that the pathway of the electrical signal starting in the SA
node is not reaching the AV node and the subsequent fibers that allow the heart to contract in order
and rhythm.
Figure 5 – ECG waveform of ventricular fibrillation
Source: cnx.org/content/m46664/latest/
Third-degree heart block is represented in figure 6 and is the result of miscommunication between the
atria and ventricles contracting. The most notable feature is the slower rate of QRS complexes
appearing. The reason for this is that the since the SA node and AV node are not communicating
correctly, the AV node takes over the pace-making responsibilities because it contracts and the next
fastest rate. However, this rate is significantly slower than the rate at which the SA node sets for the
heart. This loss of signal from the SA node that doesn’t reach the ventricles results in the long interval
between ventricular contractions.
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Figure 6 – ECG waveform of third-degree heart block
Source: cnx.org/content/m46664/latest/
Conclusion
The heart’s electrical system is the spark behind the action of the heart and is the reason why it can
pump blood through the human body so effectively. The electrical system of the heart is made up of the
SA node, AV node, ventricular fibers, and all the other
autorhythmic cells located throughout the heart. The SA
node sets the pace for the heartbeat because it undergoes
Second-degree heart block - AV node malfunctioning resulting
action potentials at the fastest rate, and that action
in atrial contraction that is always not immediately followed by
potential spreads throughout the entire heart resulting in
ventricular contraction
ordered contraction of the atria first, followed by the
Atrial fibrillation – SA node malfunctioning resulting in
ventricles. When the SA node fails, the AV will take up
uncoordinated contractions of atrial cells
responsibility for setting the pace of the heartbeat, but it
Ventricular fibrillation – uncoordinated contraction of
will produce action potentials at a slower rate than the SA
ventricular cells most likely due to a malfunctioning SA and AV
node
node. If both the SA node and the AV node fail, then the
remaining autorhythmic cells in the heart will contract
Third-degree block – SA node malfunctioning resulting in
slower heart rate and uncoordinated atrial contraction
independently without any specific pace or rhythm.
Understanding how the heart works and how an ECG
monitors its functions is useful to become an informed
patient. Some of these abnormalities are asymptomatic, so
it would be impossible to determine whether or not one
has it without an ECG examination. Therefore, it is
important to have regular appointments with a physician
to ensure that the heart it pumping properly and that
there are no issues in the wiring of the heart’s electrical
system.
Note: Most of the information comes from prior knowledge of
the author from college-level coursework so cited materials
were used as general reference
Works Cited
"Cardiac Muscle and Electrical Activity." OpenStax CNX. OpenStax College, 19 June 2013. Web. 27 July
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2014.
"Understanding the Heart's Electrical System and EKG Results." National Heart, Lung, and Blood
Institute. National Institute of Health, n.d. Web. 27 July 2014.
"What Is Atrial Fibrillation?" National Heart, Lung, and Blood Institute. National Institute of Health, n.d.
Web. 27 July 2014.
"What Is an Electrocardiogram?" National Heart, Lung, and Blood Institute. National Institute of Health,
n.d. Web. 27 July 2014.
Figure 1. (cropped)
Electrical System of the Heart. Digital image. HeartValveSurgery.com. HeartValveSurgery.com, 2012.
Web. 27 July 2014.
Figure 2.
Normal ECG waveform. Digital image. Medicine Decoded. N.p., 18 June 2008. Web. 27 July 2014.
Figure 3. (cropped)
Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19
June 2013. Web. 27 July 2014.
Figure 4. (cropped)
Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19
June 2013. Web. 27 July 2014.
Figure 5. (cropped)
Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19
June 2013. Web. 27 July 2014.
Figure 6. (cropped)
Common ECG Abnormalities. Digital image. Cardiac Muscle and Electrical Activity. OpenStax College, 19
June 2013. Web. 27 July 2014.
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