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Electrophysiology of the Heart
ECG Monitoring
• The ECG is a graphic representation of the
heart's electrical activity generated by
depolarization and repolarization of the atria
and ventricles
ECG Monitoring
• Valuable diagnostic tool for identifying
cardiac abnormalities including:
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Abnormal heart rates and rhythms
Abnormal conduction pathways
Hypertrophy or atrophy of portions of the heart
Approximate location of ischemic or infarcted
cardiac muscle
ECG Monitoring
• The ECG tracing is only a reflection of the
heart's electrical activity
• It does not provide information regarding
mechanical events such as force of
contraction or blood pressure
Voltage
• Voltage may be:
– Positive--seen as an upward deflection on the
ECG tracing
– Negative--seen as a downward deflection on the
ECG tracing
– Isoelectric--no electrical current detected
• Seen as a straight baseline on the ECG
Application of Monitoring
Electrodes
• Electrodes are pre-gelled, stick-on disks that
can easily be applied to the chest wall
• When applying electrodes:
– Cleanse the area with alcohol to remove dirt
and body oil
– Use the inner surfaces of the arms and legs
when attaching electrodes to extremities
Application of Monitoring
Electrodes
• When applying electrodes:
– Trim excess body hair (if necessary) before
placing the electrodes
– Attach the electrodes to the prepared site
– Attach the ECG cables to electrodes
– Turn on the ECG monitor and obtain a baseline
tracing
Monitoring Electrodes
• If the signal is poor, recheck the cable
connections and electrode contact with the
patient's skin
• Other causes of a poor signal include:
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Excessive body hair
Dried conductive gel
Poor electrode placement
Diaphoresis
ECG Monitoring
ECG Monitoring
ECG Monitoring
Calibration
• The sensitivity of the 12-lead ECG machine
is standardized
• When properly calibrated, a 1-mV electrical
signal produces a 10-mm deflection (two
large squares) on the ECG tracing
Figure 28-21
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
The Electrocardiogram
– Refractory Periods
• Absolute – no other electrical impulse can be
generated
• Relative – very dangerous time another electrical
impulse can interfere with the rhythm – V Tach can
occur or V Fib
Conduction System of the Heart
• Sinoatrial (SA) node
• Atrioventricular (AV)
node / bundle of His
• Purkinje fibers
Figure 6-41
Normal Conduction
• Sequence of normal
impulse conduction
– SA node
– Both atria
• Atrial contraction
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AV node
Bundle of His
Purkinje fibers
Both ventricles
• Ventricular contraction
Figure 6-41
Inherent Rates
• SA node – 60-100
• AV node – 40-60
• Ventricular – 20-40
P Wave
• First positive (upward) deflection on ECG
• Represents atrial depolarization
• Usually rounded and precedes the QRS complex
– Begins with first positive deflection from baseline
– Ends at point where wave returns to baseline
Figure 28-22
P Wave
• Duration normally 0.10 second or less
• Amplitude normally 0.5 to 2.5 mm
• Usually followed by a QRS complex unless
conduction disturbances are present
Figure 28-22
PR Interval
• Represents the time it takes for
an electrical impulse to be
conducted through the atria and
the AV node up to the instant of
ventricular depolarization
– Measured from the beginning of
the P wave to the beginning of the
next deflection on the baseline
(the onset of the QRS complex)
• Normal is 0.12-0.20 second
Figure 28-25
PR Interval
• A normal PR interval
indicates that the
electrical impulse has
been conducted through
the atria, AV node, and
bundle of His normally
and without delay
• PRI < 0.2 secs
Figure 28-25
QRS Complex
• Generally composed of three
individual waves: the Q, R, and
S waves
• Begins at the point where the
first wave of the complex
deviates from the baseline
• Ends where the last wave of
the complex begins to flatten
at, above, or below the baseline
Figure 28-25
QRS Complex
• Direction of the QRS complex may be:
– Predominantly positive (upright)
– Predominantly negative (inverted)
– Biphasic (partly positive, partly negative)
QRS Complex
• The normal QRS complex
is narrow and sharply
pointed
• Duration is generally 0.08
to 0.12 second
• Amplitude normally
varies from less than 5
mm to more than 15 mm
Figure 28-25
Q Wave
• The first negative
(downward) deflection of
the QRS complex on the
ECG
– May not be present in all
leads
• Represents depolarization
of the interventricular
septum
Figure 28-25
R Wave
• First positive deflection after the P wave
– Subsequent positive deflections in the QRS complex
that extend above the baseline and that are taller than
the first R wave are called R prime (R’), R double
prime (R’’), and so on
Figure 28-23
S Wave
• Negative deflection that
follows the R wave
– Subsequent negative
deflections are called S prime
(S’), S double prime (S”), and
so on
• R and S waves represent the
sum of electrical forces
resulting from
depolarization of the right
and left ventricles
Figure 28-25
QRS Complex
• Follows the P wave
• Marks the approximate beginning of mechanical
systole of the ventricles, which continues through
the onset of the T wave
• Represents ventricular depolarization
– Conduction of an electrical impulse from the AV node
through the bundle of His, Purkinje fibers, and the right
and left bundle branches
QRS Complex
Figure 28-22
ST Segment
• Represents the early
phase of repolarization
of the right and left
ventricles
• Immediately follows
the QRS complex and
ends with the onset of
the T wave
Figure 28-24 A
ST Segment
• The point at which the ST segment “takes
off” from the QRS complex is called the J
point
Figure 28-24 D
ST Segment
• The position of the ST segment is commonly
judged as normal or abnormal using the baseline
of the PR or TP interval as a reference
– ST segment elevation
– ST segment depression
Figure 28-24 A, B, & C
ST Segment
• Abnormal ST segments may be seen in:
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Infarction
Ischemia
Pericarditis
After digitalis administration
Other disease states
T Wave
• Represents repolarization of
ventricular myocardial cells
• Occurs during the last part of
ventricular systole
• May be above or below the
isoelectric line and is usually
slightly rounded and slightly
asymmetrical
Figure 28-25
T Wave
• Deep and symmetrically inverted T waves
may suggest cardiac ischemia
• A T wave elevated more than half the height
of the QRS complex (peaked T wave) may
indicate a new onset of myocardial ischemia
or hyperkalemia
Artifact
• A series of deflections on the ECG display or
tracing produced by factors other than the heart's
electrical activity
• Common causes of artifact:
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Improper grounding of the ECG machine
Patient movement
Loss of electrode contact with the patient's skin
Patient shivering or tremors
External chest compressions
Artifact - Muscle Tremors
Figure 28-26 A
Artifact - AC (60 cycle) Interference
Figure 28-26 B
Artifact – Loose Electrode
Figure 28-26 C
Reading ECG’s
ECG Measurements
1 sec
0.20 sec
0.04 sec
5 large squares= 25 mm = 1 second
1 large square = 5 mm = 5 small squares =0.20 seconds
1 small square = 1 mm = 0.04seconds
0.1 mV
Voltage
Standard
calibration for
an ecg is
10mm=1 mV, or
1mm = 0.1mV
Time
Rate Determination
The Six Second Method
1. ECG paper is marked at three second intervals (15 large boxes)
2. Count the R Waves in 6 seconds (30 large boxes/two 3-second intervals)
3. Multiply the number of complexes by 10 to determine the rate
(in this example 7 X 10 = 70)
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Interpreting ECG’s
• Questions to ask
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Are there P waves?
Do they all look alike?
PRI – short or long?
QRS – tight or wide?
Any extra P waves?
Any extra QRS complexes?
What is the rate?
WHAT IS THE UNDERLYING RHYTHM?
Atrial Rhythms
• Sinus rhythm
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P waves
PRI – normal
No extra beats
Rate 60 - 100
• Sinus Bradycardia
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P waves
PRI – normal
No extra beats
Rate is < 60
• Sinus Tachycardia
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P waves
PRI – normal
No extra beats
Rate is 100-150
• Wandering Atrial Pacemaker
– P waves are present but they do not look alike
– QRS intervals are regular
• Wolfe-Parkinson-White
– P waves are present
– Delta wave
– Reentry rhythm
• Sinus Rhythm with unifocal Premature Ventricular Contractions
(PVC)
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P waves are present
PRI is normal
Some QRS complexes come early and are wide
Premature QRS complexes look alike
Must differentiate whether the PVC’s look alike or not
• Sinus Rhythm with multifocal PVC’s
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P waves are present
PRI is normal
Some QRS complexes come early and are wide
Early QRS complexes are different
Must differentiate whether the PVC’s look alike or not
• Sinus Rhythm with couplets
– P waves are present
– PRI is normal
– 2 QRS complexes come early together and are wide
• Sinus Rhythm with a run of V Tach
– P waves are present
– PRI is normal
– 3 or more PVC’s together
• Sinus Rhythm with R on T phenomenon
– P waves are present
– PRI is normal
– Can go into V Fib
• Sinus Rhythm with bigeminy
– Underlying rhythm is a SR
– Every other QRS complex is a PVC
• Sinus Rhythm with an aberrant conduction or Bundle
Branch Block
– P waves are present
– PRI is normal
– QRS complex is wide
• Atrial fibrillation
– No discernable P waves
– QRS – irregularly irregular
• Atrial Flutter
– P waves are flutter waves or “sawtooth pattern”
– QRS can be regular or irregular
Heart Blocks
• Sinus Rhythm with 1st degree HB
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P waves are present
PRI – longer than normal
QRS complex is usually narrow
MUST GIVE AN UNDERLYING RHYTHM
• 2nd degree HB Type I or Wenckebach
– P waves are present
– Extra P waves
– PRI gets progressively longer then you get a “dropped
QRS”
• 2nd degree HB Type II or Classic 2nd degree HB
– P waves are present
– Extra P waves
– PRI stays the same but then you get a “dropped QRS”
• 3rd degree HB or Complete HB
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P waves are present
P waves map out
QRS complexes are usually wide
QRS complexes map out
P waves do not map out with the QRS complexes
PRI has no consistency
Sometimes it looks like the P waves are marching towards the QRS
Junctional Rhythms
• Junctional Rhythm
– No P waves
– QRS complex is tight
– Rate 40-60
• Junctional Bradycardia
– No P waves
– QRS complex is tight
– Rate < 40
• Accelerated Junctional Rhythm
– No P waves
– QRS complex is tight
– Rate 60-100
• Junctional Tachycardia
– No P waves
– QRS complex is tight
– Rate 100-150
Ventricular Rhythms
• Idioventricular Rhythm
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No P waves
QRS complex is wide
Rhythm is regular
Rate 20-40
• Agonal Idioventricular Rhythm
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No P waves
QRS complex is extremely wide
Will not produce a pulse
Rate < 20
• Accelerated Idioventricular Rhythm
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No P waves
QRS complex is wide
Rhythm is regular
Rate 40-100
• “Slow” Ventricular Tachycardia
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No P waves
QRS complex is wide
Rhythm is regular
Rate 100-150
• Ventricular Tachycardia
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No P waves
QRS complex is wide
Rhythm is regular
Rate > 150
• Ventricular Fibrillation
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No P waves
QRS complex is wide and bizarre
Rhythm is erratic
Rate > 150
• Fine Ventricular Fibrillation
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No P waves
QRS complex is wide, bizarre and small
Rhythm is erratic
Rate > 150
• Coarse Ventricular Fibrillation
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No P waves
QRS complex is wide, bizarre and large
Rhythm is erratic
Rate > 150
• Torsades de Pointes (Turning of the Points)
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No P waves
QRS complex is wide, bizarre and large
QRS complexes seem to face different directions
Rhythm is erratic
Rate > 150
• Asystole
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No P waves
No QRS complexes
“Flatline”
Must confirm in at least two leads
• Ventricular Pacemaker
– P waves may or may not be present
– Pacemaker spikes just prior to QRS complex
– QRS complexes are wide
• Atrial Pacemaker
– Pacemaker spikes just prior to P waves
– QRS complexes are narrow
• Atrial Ventricular Pacemaker or AV Sequential
Pacemaker
– Pacemaker spikes just prior to P wave
– Pacemaker spikes just prior to QRS complex
– QRS complexes are wide
Any Questions???