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ECG
Fundamentals of Electrocardiography
The conduction system is the mechanism by
which the heart contracts. Contraction is
controlled by specialized cells within the heart
that generate and distribute electrical
impulses
Fundamentals of Electrocardiography
Path of Impulse
SA Node
↓
AV Node
↓
AV Bundle (Bundle of His)
↓
Right and Left Bundle
Branches
↓
Purkinje Fibers
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Fundamentals of Electrocardiography
Path of Impulse
The pulse begins in the sinoatrial (SA) node.
The cells within the SA node spontaneously
depolarize to trigger contraction. The
contraction begins in the upper atria and
spreads toward the atrioventricular (AV)
valves. This helps move blood from the atria
to the ventricles.
Fundamentals of Electrocardiography
Path of Impulse
The pulse travels to the AV node which is located
within the interatrial septum. It than proceeds
to the AV bundle (Bundle of His). The bundle
of His then divides into the left and right bundle
branches. Each branch travels down the
septum. At the apex, the branches called
purkinje fibers transverse the ventricles back
toward the base of the heart.
Fundamentals of Electrocardiography
 Depolarization causes contraction of heart
 Repolarization causes relaxation of heart
 Contraction state of heart is systole
 Relaxation state of heart is diastole
Fundamentals of Electrocardiography
Purpose of ECG
 Identify cardiac rate
 Identify any abnormalities in rhythm
 Identify presence of abnormal transmission
impulses through conduction system of heart
Fundamentals of Electrocardiography
Indications for ECG
 Chest pain
 Rhythm disturbances
 Routine physical
 Pre-Op evaluation
Fundamentals of Electrocardiography
Basic ECG Tracing
The basic ECG tracing
consists of:
 P wave
 Q wave
 R wave
 S wave
 T wave
Fundamentals of Electrocardiography
Basic ECG Tracing
P wave: (normal <.2 sec or 5 boxes)
 First deflection from baseline
 Is a positive deflection (upward)
 Corresponds to atrial depolarization
Q wave:
 Small negative (downward) deflection
preceding the R wave
Fundamentals of Electrocardiography
Basic ECG Tracing
R wave:
 Large positive deflection preceding the S
wave
S wave:
 Small negative deflection
Fundamentals of Electrocardiography
Basic ECG Tracing
QRS Complex:
 All 3 Q, R, and S waves together(normally < 3
boxes)
 Corresponds with ventricular depolarization,
resulting in ventricle contraction
 If Q wave is absent, then QRS complex is
measured from the beginning of the first
positive deflection after the PR interval
Fundamentals of Electrocardiography
Basic ECG Tracing
T wave:
 Follows QRS complex
 Positive deflection
 Corresponds to ventricular repolarization and
relaxation
Fundamentals of Electrocardiography
Basic ECG Tracing
ST Segment
 Interval time from end of ventricular
depolarization to beginning of ventricular
repolarization
 Should normally be at baseline
 Heart’s resting period between ventricular
depolarization and repolarization
Fundamentals of Electrocardiography
Basic ECG Tracing
PR Interval
 Measured from beginning of P wave to
beginning of QRS Complex (normal <.2 sec
or 5 boxes)
 Is the time between atrial depolarization and
beginning of ventricular depolarization
 The impulse travels from the SA node to the
ventricle
Fundamentals of Electrocardiography
Basic ECG Tracing
QT Interval
 Period between onset of QRS complex and
end of T wave
 It represents the entire time of ventricular
depolarization and repolarization
Fundamentals of Electrocardiography
Standard 12-Lead ECG
 Consists of 6 chest leads and 6 limb leads
 Only ten electrodes are utilized to obtain a
12-Lead ECG
Fundamentals of Electrocardiography
Lead Placement
Limb Leads
 Right arm
 Right leg
 Left arm
 Left Leg
Chest Leads
 V1: just to the right of
the sternum
 V2: just to the left of
the sternum
 V3: placed next to &
below V2
 V4: placed next to &
below V3
 V5: laterally and over
to the left side of the
heart
 V6: laterally next to
V5
Fundamentals of Electrocardiography
Basic ECG Tracing-Limb Leads
 Lead I: Right arm and Left arm
 Lead II: Right arm and Left leg
 Lead III: Left arm and Left leg
 AVR: midway between left arm and left leg to
right arm
 AVL: midway between right arm and left leg
to left arm
 AVF: midpoint between right and left arms to
left leg
Fundamentals of Electrocardiography
Basic ECG Tracing
 Right leg lead is the ground lead
 V1, V2 and AVR are the right heart leads
 V3 and V4 are the septal leads (transition
between right and left sides of heart)
 V5, V6, I and AVL are the lateral leads (left
side of the heart)
 II, III, and AVF are the inferior heart leads
Fundamentals of Electrocardiography
Lead Placement
 Lead placement
of the six chest
leads
Fundamentals of Electrocardiography
Heart Rates & Rhythm
 Sinus rhythm: 60-100 bpm and rhythm
originates in SA node (Normal rhythm is
when there is equal distance between the
R-R intervals)
 Sinus tachycardia: SA node paces the
heart faster than 100 bpm
 Sinus bradycardia: SA node paces the
heart slower than 60 bpm
 Flutter: 250-350 bpm
 Fibrillation: >350 bpm
Fundamentals of Electrocardiography
Rate Determination
 On ECG paper, count the number of R waves
in any 6 sec. interval. Multiply that number by
10 to calculate HR
 300, 150, 100, 75, 60, 50 method
Fundamentals of Electrocardiography
Causes of Arrhythmias
 Fast or slow HR
 Skipped beats
 Heart disease
 Smoking
 Caffeine
 Alcohol
 Medications
 Stress
Fundamentals of Electrocardiography
Atrial Flutter
 Saw-tooth appearance
Fundamentals of Electrocardiography
Atrial Fibrillation
 Caused by continuous, uncontrolled firing of multiple foci
in atria, resulting in an ineffective quivering of the cardiac
muscle
 Characterized by irregular ventricular rhythm and absence
of P wave
 Difficult to get adequate pacing trigger for gated studies
Fundamentals of Electrocardiography
PVCs
 Premature ventricular contractions
 Originate from an ectopic focus in the ventricle
 It produces a wide QRS complex
 Can be unifocal or multifocal (couplet, triplet or a
run) and can occur as bigeminy or trigeminy
Fundamentals of Electrocardiography
Ventricular Bigeminy
 A repeating pattern followed by a normal beat
Fundamentals of Electrocardiography
Ventricular Trigeminy
 A pattern of PVCs followed by two normal
beats
Fundamentals of Electrocardiography
Ventricular Tachycardia
 The appearance of 3 or more rapid
consecutive PVCs.
 If not controlled, can lead to V-flutter or V-fib
Fundamentals of Electrocardiography
Ventricular Flutter
 ECG tracing
becomes wavy and
irregular with no
discernible QRS
complex or P wave
Fundamentals of Electrocardiography
Heart Block
 An electrical conduction disorder from the SA,
AV nodes or Purkinje fibers
 Heart blocks are classified by the extent of
the conduction abnormality
Fundamentals of Electrocardiography
Heart Block
First-Degree
 Electrical impulse is conducted more slowly
than normal
Second-Degree
 The electrical impulse may or may not be
conducted
Third-Degree
 The electrical impulse is totally blocked
Fundamentals of Electrocardiography
Heart Blocks
 Sinus block
 Atrioventricular block
 Bundle branch block (BBB)
Fundamentals of Electrocardiography
Heart Block-AV Block
 The AV block delays the stimulation of the
ventricles
Fundamentals of Electrocardiography
Heart Block-AV Blocks
First-Degree
 PR interval is prolonged beyond 0.2 second
because of a delay in conduction through the AV
node
Second-Degree
 PR interval becomes gradually longer and QRS
complex fails to occur
Third-Degree
 None of the atrial impulses are conducted to the
ventricles and the ventricles pace independently
Fundamentals of Electrocardiography
Heart Block-Bundle Branch Blocks
 BBB are the most common block. It
originates in the left or right bundle branches.
BBBs are caused by a block of depolarization
in the right or left bundle branches. The peak
of the QRS complex is notched.
Bundle Branch Blocks
 Right bundle branch blocks appear in leads
V1 and V2
 Left bundle branch blocks appear in leads V5
and V6
Fundamentals of Electrocardiography
Ischemia/Infarction
 ST segment depression may be seen in
subendocardial infarction, in patients on
Digitalis and in transient exercise-induced
ischemia.
 T wave inversion represents ischemia. Its
appearance may be anything from
moderately flattened to significantly inverted.
Fundamentals of Electrocardiography
 ST Depression
 T wave inversion
Fundamentals of Electrocardiography
Ischemia/Infarction
 ST segment elevation represents acute
ischemia resulting in injury. Degree of
elevation denotes severity of ischemic injury.
 Presence of Q wave denotes myocardial
infarction. Q wave is not usually visualized.
The anatomical location of the infarction is
determined by the presence of a Q wave on a
given lead.
Fundamentals of Electrocardiography
 ST segment elevation
 Presence of Q wave
Fundamentals of Electrocardiography
Abnormalities on ECG Determines Infarct Location
 Anteroseptal
 Anterior
 Anterolateral
 Lateral
 Inferior
V2 & V3
V3 & V4, not on V5 or V6
V4 & V5
I, AVL, V5 & V6
II, III, AVF
Fundamentals of Electrocardiography
Artificial Pacemakers
 An electric device used to stimulate the heart
to beat when the electrical conduction system
is unable to function properly. Can be used
for atrial, ventricular or dual-chamber pacing.
It causes a narrow line (the pacemaker spike)
in the ECG tracing.
Fundamentals of Electrocardiography
Artificial Pacemakers
References
 Crawford, MS, CNMT, Elpida S., and Syed
Sajid Husain, MD, MAS. Nuclear Cardiac
Imaging, Terminology and Technical Aspects.
Reston: Society of Nuclear Medicine, 2003.
 Taylor, MD, Andrew, David Schuster, MD,
and Naomi Alazraki, MD. A Clinician’s Guide
to Nuclear Medicine. Reston: Society of
Nuclear Medicine, 2000.
 Introduction to Nuclear Cardiology (Third
Edition). Du Pont Pharma ( a professional
education service), 1993.
References
 University of Utah School of Medicine
website, www.medstat.med.utah.edu/kw/ecg