Transcript Egan Ch 17 Interpreting the Electrocardiogram

Chapter 17
Interpreting the Electrocardiogram
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Learning Objectives
• Describe the value and limitations of the
electrocardiogram (ECG).
• Describe the electrophysiology of cardiac cells.
• Describe how the cardiac impulse is conducted
through the different structures of the heart.
• Recognize various abnormal ECG recordings.
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The Electrocardiogram
• An ECG is a popular tool because it’s
inexpensive, noninvasive, & easy to obtain
• Primarily used to assess patients suspected of
having an acute myocardial condition
• Also used as health screening tool in patients
over age 40 years
• Cannot predict future heart attacks or detect
structural defects (e.g., valve stenosis)
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Electrophysiology
• Impulse-conducting system provides electrical
stimulus to activate & pace myocardium
• Cardiac cells polarized with positive charge on
the outside & negative charge on the inside
• When stimulated, cardiac cells depolarize as
sodium rushes inside cells
• Depolarization causes muscle cells to contract
momentarily
• Repolarization reestablishes electrical
imbalance across cell membrane
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An ECG may help to evaluate a patient for all of
these conditions, except:
A. Congestive heart failure
B. Valvular defects
C. Heart rate anomalies
D. Myocardial Infarction
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Electrophysiology (cont.)
• Impulse-conducting system has 3 different types
of cardiac cells capable of electrical excitation:
1. Pacemaker cells (e.g., sinoatrial node)
2. Specialized rapidly conducting tissue (e.g., Purkinje
fibers)
3. Atrial & ventricular muscle cells
• All these cells have intrinsic ability to
spontaneously depolarize (automaticity)
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Electrophysiology (cont.)
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The Impulse-Conducting System
• This system is responsible for:
 Initiating heartbeat
 Controlling heart rate
 Coordinating contraction of heart chambers
• A defect in system may lead to:
 Inadequate cardiac output
 Decreased tissue perfusion
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The Impulse-Conducting System
(cont.)
• Normally, SA node has greatest degree of
automaticity & paces heart
• AV node serves as back-up pacemaker when
SA node fails
• After leaving AV node, impulse travels through
bundle of HIS, bundle branches, & Purkinje
fibers.
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The Impulse-Conducting System
(cont.)
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What is the purpose of the normally occurring,
temporary delay of the electrical impulse at the AV
node serve?
A. Allows the ventricles time to fill with blood
B. Increases ventricular stimulation
C. Provides rest for the ventricles
D. Allows the atria time to fill with blood
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ECG Procedural Summary
• Physician order is required for 12-lead ECG
• Requires use of:
 Portable ECG unit
 Lead wires
 Electrodes
• Placement of 12 leads (subdivided into 2
groups)
1. 6 extremity (limb) leads
2. 6 chest (precordial) leads
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ECG Procedural Summary
• Limb leads are bipolar & are able to monitor
heart’s activity that is directed up, down, left or
right
• Precordial leads are unipolar & measure cardiac
activity moving anteriorly or posteriorly
• ECG helps in diagnosis of certain heart
conditions (myocardial infarction, cardiac
ischemia
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Precordial Lead Placement
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Basic ECG Waves
• Atrial depolarization is seen as P wave
• Ventricular depolarization is seen as QRS
complex
• Normal QRS complex is not wider than 3 mm
(0.12 second)
• Wave of ventricular repolarization is seen as T
wave
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Basic ECG Waves (cont.)
• PR interval is time from start of atrial contraction
to start of ventricular contraction (normally not
>0.20 second)
• Long PR intervals = heart block
• ST segment is time from end of ventricular
depolarization to start of ventricular
repolarization (normally isoelectric)
• Elevated or depressed ST segments = ischemia
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Where can atrial repolarization be seen on an
ECG tracing?
A. During the ST segment
B. It is obscured by the electrical activity of the
ventricles.
C. It is shown during atrial depolarization (P wave)
D. It is obscured by the QT interval
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Basic ECG Waves (cont.)
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ECG Paper & Measurement
• Grid-like boxes
 Horizontal axis defines time
 Vertical axis defines voltage
• Large box (5 mm) = 0.20 sec; small box (1 mm)
= 0.04 sec
• 1 mV will cause an upward deflection of 10
boxes representing depolarization of ventricles
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ECG Paper
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Interpreting the Electrocardiogram
1. Identify atrial & ventricular rates; normally, the
same & 60 to 100/min.
2. Measure PR interval; normally, <0.20 second
3. Evaluate QRS complex; should be no longer
than 0.12 second
4. Evaluate T wave; normally, should be upright &
rounded; inversion = ischemia
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Steps to Follow (cont.)
5. Evaluate ST segment; normally, flat; significant
elevation or depression = ischemia
6. Assess RR interval to evaluate regularity of
rhythm
7. Identify mean QRS axis by finding limb lead
with most voltage; if this lead has positive QRS
complex, axis is very close to where this lead is
labeled on hexaxial reference circle
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Hexaxial Reference
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The QRS complex of an ECG falls on a dark
vertical line on the ECG paper. The next QRS
complexes fall on every third dark line (15 mm
apart). What is the approximate ventricular rate?
A. 100 beats/min
B. 150 beats/min
C. 200 beats/min
D. 250 beats/min
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All of these are possible conditions that may result
from a patient’s QRS complexes measuring at
0.20 second, except:
A. An impulse-conducting system abnormality
within the ventricles
B. A drop in cardiac output and blood pressure
C. Patient may experience fainting spells
D. An impulse-conducting system abnormality
within the atria
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Axis Evaluation
• Helps determine general direction of current flow
during ventricular depolarization to detect
ventricular hypertrophy
• Normally, mean axis is between 0 & +90
degrees
• Right-axis deviation (+90 to +180 degrees) is
consistent with right ventricular hypertrophy
• Left-axis deviation (between +90 and 90
degrees) is consistent with left ventricular
hypertrophy
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A patient’s mean QRS axis is +122 degrees on a
hexaxial reference circle with a negative Lead I.
This would suggest:
A. Left-axis deviation
B. Cor pulmonale
C. Left ventricular hypertrophy
D. Myocardial infarction
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Normal Sinus Rhythm
• Has an upright P wave identical throughout strip
• PR interval is <0.20 second
• QRS complexes are identical & no longer than
0.12 second
• ST segment is flat
• RR interval is regular & heart rate is 60 to
100/min
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Normal Sinus Rhythm (cont.)
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Sinus Tachycardia
• Recognized when heart rate exceeds 100/min at
rest
• Each QRS complex is preceded by P wave
• Common finding in patients with acute illness &
may be caused by pain, anxiety, fever,
hypovolemia, & hypoxemia
• May also be caused by certain medications,
such as bronchodilators
• Underlying cause must be treated
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Sinus Tachycardia (cont.)
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Sinus Bradycardia
•
•
•
•
Heart rate is less than 60/min
Each QRS is preceded by P wave
PR interval & QRS complex are normal
Of concern only when causing clinical problems,
such hypotension & syncope
• Atropine is effective treatment
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Sinus Bradycardia (cont.)
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First-Degree Heart Block
• PR interval exceeds 0.20 second
• Each QRS complex is preceded by P wave
• Cardiac impulse from SA node is delayed in
passing through AV node or bundle of His
• Typically, RR intervals are regular
• May occur after MI or with use of beta-blockers
• Treatment may not be needed if patient
asymptomatic
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First-Degree Heart Block (cont.)
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A patient is having symptoms of hypotension and
occasional syncope. His ECG shows a heart rate
of less than 60 beats/min. The tracing shows a
QRS complex preceded by a P wave. What
medication would be recommended to treat his
condition?
A. Atropine
B. Beta blockers
C. Digoxin
D. Calcium channel blockers
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Second-Degree Heart Block
• Comes in 2 types:
 Type I (Wenckebach or Mobitz type I) block;
recognized when PR interval gets progressively
longer until one does not pass on to ventricles
 Type II (Mobitz type II) is:
• Less common
• Occurs with more serious problems, such as MI
• Seen as series of nonconducted P waves followed by P wave
that conducts to ventricles
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Second-Degree Heart Block (cont.)
• Treatment for type I not usually needed
• Treatment for type II often requires
medication, such as atropine & possibly
pacemaker
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Second-Degree Heart Block (cont.)
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Third-Degree Heart Block
• Occurs when conduction system between atria &
ventricles is completely blocked
• Atria & ventricles are paced independently &
there is no relationship between P waves & QRS
complexes
• PP intervals & RR intervals remain regular with
no correlation with one another
• Treatment includes medications to speed up
ventricles & placement of pacemaker
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Third-Degree Heart Block (cont.)
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Atrial Flutter
• Represents rapid depolarization of atria from
ectopic focus (250 to 350/min)
• Causes characteristic sawtooth pattern;
numerous P waves are present for each QRS
• Caused by wide variety of disorders, such as
rheumatic heart disease, coronary heart
disease, renal failure, stress, & hypoxemia
• Treated with medications & cardioversion
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Atrial Flutter (cont.)
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Atrial Fibrillation
• Results when atrial muscle quivers in irregular
pattern; no true P waves are seen
• Ventricular rate may be slow & irregular
• Atrial fibrillation causes cardiac output to drop &
may lead to thrombi in atria due to blood
stagnation
• Cardioversion used as treatment in most cases
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Atrial Fibrillation (cont.)
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A patient with a history of coronary heart disease,
arrives in the ER, and immediately has an ECG done
that indicates atrial flutter with a 1:3 conduction ratio.
When should this patient receive cardioversion to treat
this condition?
A. Immediately upon arrival to the ER
B. At the same time he receives his medication
C. Once the medication has slowed down the
arrhythmia
D. After admission to the ICU
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Premature Ventricular Contractions
(PVCs)
• Occurs when ectopic beats originate in
ventricles
• PVCs are commonly result of hypoxia,
electrolyte imbalances, & acid-base disorders
• QRS complex is wide & has no preceding P
wave
• Frequent PVCs call for treatment of underlying
cause (lidocaine offers temporary solution in
some cases)
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Premature Ventricular Contractions
(cont.)
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Ventricular Tachycardia (V-tach)
• Represent run of 3 or more PVCs in a row
• Easy to recognize as series of wide QRS
complexes with no preceding P wave
• Ventricular rate is usually 100 to 250/min
• V-tach represents serious arrhythmia often
progressing to V-fib if untreated
• Treatment includes cardioversion & medications
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Ventricular Tachycardia (cont.)
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Ventricular Fibrillation (VF)
• Represents most life-threatening arrhythmia
• Defined as erratic quivering of ventricular
muscle mass
• Causes cardiac output to drop to zero
• ECG shows grossly irregular fluctuations with
zigzag pattern
• Treatment includes rapid defibrillation, CPR,
oxygen, & antiarrhythmic medications
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Ventricular Fibrillation (cont.)
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Pulseless Electrical Activity (PEA)
• Very rare but serious arrhythmia
• Characterized by ECG pattern that does not
generate pulse
• Generally, does not occurs without precipitating
event, such as tension pneumothorax, MI, drug
overdose)
• Treatment involves emergency life support &
immediate reversal of cause
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