chapter 17 interpreting the electrocardiogram. mosby items and derived items © 2009 by mosby, inc.,...

Chapter 17 Chapter 17

Interpreting the Interpreting the ElectrocardiogramElectrocardiogram

Mosby items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc. 2

ObjectivesObjectives

•• Describe the value and limitations of the Describe the value and limitations of the electrocardiogram (ECG).electrocardiogram (ECG).

•• Describe the electrophysiology of cardiac cells.Describe the electrophysiology of cardiac cells.

•• Describe how the cardiac impulse is conducted Describe how the cardiac impulse is conducted through the different structures of the heart. through the different structures of the heart.

•• Recognize various abnormal ECG recordings.Recognize various abnormal ECG recordings.


The ElectrocardiogramThe Electrocardiogram

An ECG is a popular tool because it is inexpensive, An ECG is a popular tool because it is inexpensive, noninvasive, and easy to obtain.noninvasive, and easy to obtain.

Often used to assess the patient suspected of having Often used to assess the patient suspected of having an acute myocardial infarction.an acute myocardial infarction.

Also used as a health screening tool in patients over Also used as a health screening tool in patients over age 40 years.age 40 years.

It cannot predict future heart attacks or detect It cannot predict future heart attacks or detect structural defects (e.g., valve stenosis).structural defects (e.g., valve stenosis).


ElectrophysiologyElectrophysiology

Normally, cardiac cells are polarized with a positive Normally, cardiac cells are polarized with a positive charge on the outside.charge on the outside.

When stimulated, cardiac cells depolarize as sodium When stimulated, cardiac cells depolarize as sodium rushes inside the cells.rushes inside the cells.

Depolarization causes the muscle cells to contract Depolarization causes the muscle cells to contract momentarily.momentarily.

Repolarization reestablishes the electrical imbalance Repolarization reestablishes the electrical imbalance across the cell membrane.across the cell membrane.


Electrophysiology (cont.)Electrophysiology (cont.)

Three different types of cardiac cells are present Three different types of cardiac cells are present in the heartin the heart

1.1. Pacemaker cells (e.g., sinoatrial node)Pacemaker cells (e.g., sinoatrial node)

2.2. Specialized rapidly conducting tissue (e.g., Purkinje Specialized rapidly conducting tissue (e.g., Purkinje fibers)fibers)

3.3. Atrial and ventricular muscle cellsAtrial and ventricular muscle cells

All these cells have the ability to spontaneously All these cells have the ability to spontaneously depolarize (automaticity).depolarize (automaticity).


Electrophysiology (cont.)Electrophysiology (cont.)


The Impulse-Conducting SystemThe Impulse-Conducting System

This system is responsible for initiating the heartbeat This system is responsible for initiating the heartbeat and controlling the heart rate.and controlling the heart rate.

Normally, the SA node has the greatest degree of Normally, the SA node has the greatest degree of automaticity and paces the heart.automaticity and paces the heart.

The AV node serves as the back-up pacemaker when The AV node serves as the back-up pacemaker when the SA node fails.the SA node fails.

After leaving the AV node, the impulse travels After leaving the AV node, the impulse travels through the bundle of HIS, bundle branches, and through the bundle of HIS, bundle branches, and Purkinje fibers. Purkinje fibers.


The Impulse-Conducting System The Impulse-Conducting System (cont.)(cont.)


Basic ECG WavesBasic ECG Waves

Atrial depolarization is seen as the P wave.Atrial depolarization is seen as the P wave.

Ventricular depolarization is seen as the QRS Ventricular depolarization is seen as the QRS complex.complex.

The normal QRS complex is not wider than 3 mm The normal QRS complex is not wider than 3 mm (0.12 second).(0.12 second).

The wave of ventricular repolarization is seen as the The wave of ventricular repolarization is seen as the T wave.T wave.


Basic ECG Waves (cont.)Basic ECG Waves (cont.)

The PR interval is the time from the start of atrial The PR interval is the time from the start of atrial contraction to the start of ventricular contraction contraction to the start of ventricular contraction (normally not >0.20 second).(normally not >0.20 second).

Long PR intervals = heart blockLong PR intervals = heart block

ST segment = the time from the end of ventricular ST segment = the time from the end of ventricular depolarization to the start of ventricular repolarization depolarization to the start of ventricular repolarization (normally isoelectric)(normally isoelectric)

Elevated or depressed ST segments = ischemiaElevated or depressed ST segments = ischemia


Basic ECG Waves (cont.)Basic ECG Waves (cont.)


Axis EvaluationAxis Evaluation

Normally, the mean axis is between 0 and +90 Normally, the mean axis is between 0 and +90 degrees.degrees.

Right-axis deviation (+90 to +180 degrees) is Right-axis deviation (+90 to +180 degrees) is consistent with right ventricular hypertrophy.consistent with right ventricular hypertrophy.

Left-axis deviation (0 to Left-axis deviation (0 to 90 degrees) is consistent 90 degrees) is consistent with left ventricular hypertrophy.with left ventricular hypertrophy.


Steps to FollowSteps to Follow

1.1. Identify the atrial and ventricular rates; normally they Identify the atrial and ventricular rates; normally they are the same and 60 to 100/min.are the same and 60 to 100/min.

2.2. Measure the PR interval; normally this is <0.20 Measure the PR interval; normally this is <0.20 second.second.

3.3. Evaluate the QRS complex; it should be no longer Evaluate the QRS complex; it should be no longer than 0.12 second.than 0.12 second.

4.4. Evaluate the T wave; normally it should be upright Evaluate the T wave; normally it should be upright and rounded; inversion = ischemiaand rounded; inversion = ischemia


Steps to Follow (cont.)Steps to Follow (cont.)

5. Evaluate the ST segment; normally it is flat; elevation 5. Evaluate the ST segment; normally it is flat; elevation or depression = ischemiaor depression = ischemia

6. Assess the RR interval to evaluate the regularity of 6. Assess the RR interval to evaluate the regularity of the rhythm.the rhythm.

7. Identify the mean QRS axis by finding the limb lead 7. Identify the mean QRS axis by finding the limb lead with the most voltage; if this lead has a positive with the most voltage; if this lead has a positive QRS complex the axis is very close to where this QRS complex the axis is very close to where this lead is labeled on the hexaxial reference circle.lead is labeled on the hexaxial reference circle.


Normal Sinus RhythmNormal Sinus Rhythm

Has an upright P wave that is identical throughout the Has an upright P wave that is identical throughout the stripstrip

The PR interval is <0.20 second.The PR interval is <0.20 second.

The QRS complexes are identical and no longer than The QRS complexes are identical and no longer than 0.12 second.0.12 second.

The ST segment is flat.The ST segment is flat.

The RR interval is regular, and the heart rate is 60 to The RR interval is regular, and the heart rate is 60 to 100/min.100/min.


Normal Sinus Rhythm (cont.)Normal Sinus Rhythm (cont.)


Sinus TachycardiaSinus Tachycardia

Recognized when the heart rate exceed 100/min at Recognized when the heart rate exceed 100/min at restrest

Each QRS complex is preceded by a P wave.Each QRS complex is preceded by a P wave.

A common finding in patients with acute illness and A common finding in patients with acute illness and may be caused by pain, anxiety, fever, hypovolemia, may be caused by pain, anxiety, fever, hypovolemia, and hypoxemia.and hypoxemia.

It may also be caused by certain medications such as It may also be caused by certain medications such as bronchodilators.bronchodilators.


Sinus Tachycardia (cont.)Sinus Tachycardia (cont.)


Sinus BradycardiaSinus Bradycardia

Heart rate is less than 60/min.Heart rate is less than 60/min.

Each QRS is preceded by a P wave.Each QRS is preceded by a P wave.

PR interval and QRS complex are normal.PR interval and QRS complex are normal.

Of concern only when it causes clinical problems Of concern only when it causes clinical problems such hypotension and syncopesuch hypotension and syncope

Atropine is an effective treatment.Atropine is an effective treatment.


Sinus Bradycardia (cont.)Sinus Bradycardia (cont.)


First-Degree Heart BlockFirst-Degree Heart Block The PR interval exceeds 0.20 second.The PR interval exceeds 0.20 second.

Each QRS complex is preceded by a P wave.Each QRS complex is preceded by a P wave.

Cardiac impulse is delayed in passing through the AV node Cardiac impulse is delayed in passing through the AV node or bundle of His.or bundle of His.

Typically, the RR intervals are regular.Typically, the RR intervals are regular.

May occur after an MI or with the use of beta-blockersMay occur after an MI or with the use of beta-blockers

Treatment may not be needed.Treatment may not be needed.


First-Degree Heart Block (cont.)First-Degree Heart Block (cont.)


Second-Degree Heart BlockSecond-Degree Heart Block

Comes in two typesComes in two types Type I (Wenckebach or Mobitz type I) block; Type I (Wenckebach or Mobitz type I) block;

recognized when PR interval gets progressively recognized when PR interval gets progressively longer until one does not pass on to the ventricles.longer until one does not pass on to the ventricles.

Type II (Mobitz type II) is less common but occurs Type II (Mobitz type II) is less common but occurs with more serious problems such as an MI.with more serious problems such as an MI.


Second-Degree Heart Block Second-Degree Heart Block (cont.)(cont.)

Treatment of type I not usually needed.Treatment of type I not usually needed.

Treatment of type II often needed and includes Treatment of type II often needed and includes medications such as atropine and possibly a medications such as atropine and possibly a pacemaker.pacemaker.


Third-Degree Heart BlockThird-Degree Heart Block

Occurs when the conduction system between the Occurs when the conduction system between the atria and ventricles is completely blockedatria and ventricles is completely blocked

The atria and ventricles are paced independently and The atria and ventricles are paced independently and there is no relationship between the P waves and the there is no relationship between the P waves and the QRS complexes.QRS complexes.

The PP intervals and RR intervals are regular.The PP intervals and RR intervals are regular.

Treatment includes medications to speed up the Treatment includes medications to speed up the heart and a placement of a pacemaker.heart and a placement of a pacemaker.


Third-Degree Heart Block (cont.)Third-Degree Heart Block (cont.)


Atrial FlutterAtrial Flutter

Represents rapid depolarization of the atria from an Represents rapid depolarization of the atria from an ectopic focus (250 to 350/min)ectopic focus (250 to 350/min)

Causes a characteristic sawtooth pattern; numerous Causes a characteristic sawtooth pattern; numerous P waves are present for each QRSP waves are present for each QRS

Caused by a wide variety of disorders such as Caused by a wide variety of disorders such as rheumatic heart disease, coronary heart disease, rheumatic heart disease, coronary heart disease, renal failure, stress, and hypoxemiarenal failure, stress, and hypoxemia

Treated with medications and cardioversionTreated with medications and cardioversion


Atrial Flutter (cont.)Atrial Flutter (cont.)


Atrial FibrillationAtrial Fibrillation

Results when the atrial muscle quivers in an erratic Results when the atrial muscle quivers in an erratic pattern; no true P waves are seenpattern; no true P waves are seen

The ventricular rate may be slow and irregular.The ventricular rate may be slow and irregular.

Atrial fibrillation causes cardiac output to drop and Atrial fibrillation causes cardiac output to drop and may lead to thrombi in the atria due to blood may lead to thrombi in the atria due to blood stagnation.stagnation.

Cardioversion is used as treatment in most cases.Cardioversion is used as treatment in most cases.


Atrial Fibrillation (cont.)Atrial Fibrillation (cont.)


Premature Ventricular Premature Ventricular ContractionsContractions

Occurs when ectopic beats originate in the ventriclesOccurs when ectopic beats originate in the ventricles

PVCs are commonly the result of hypoxia, electrolyte PVCs are commonly the result of hypoxia, electrolyte imbalances, and acid-base disorders.imbalances, and acid-base disorders.

QRS complex is wide and has no preceding P wave.QRS complex is wide and has no preceding P wave.

Frequent PVCs call for treatment of the underlying Frequent PVCs call for treatment of the underlying cause (lidocaine offers temporary solution in some cause (lidocaine offers temporary solution in some cases).cases).


Premature Ventricular Premature Ventricular Contractions (cont.)Contractions (cont.)


Ventricular TachycardiaVentricular Tachycardia

Represent a run of three or more PVCsRepresent a run of three or more PVCs

Easy to recognize as a series of wide QRS complexes Easy to recognize as a series of wide QRS complexes with no preceding P wavewith no preceding P wave

Ventricular rate is usually 100 to 250/min.Ventricular rate is usually 100 to 250/min.

VT represents a serious arrhythmia that often VT represents a serious arrhythmia that often progresses to V fib if untreated. progresses to V fib if untreated.

Treatment includes cardioversion and medications.Treatment includes cardioversion and medications.


Ventricular Tachycardia (cont.)Ventricular Tachycardia (cont.)


Ventricular FibrillationVentricular Fibrillation

Represents the most life-threatening arrhythmiaRepresents the most life-threatening arrhythmia

Defined as erratic quivering of the ventricular muscle Defined as erratic quivering of the ventricular muscle massmass

Causes cardiac output to drop to zeroCauses cardiac output to drop to zero

The ECG show grossly irregular fluctuations with a The ECG show grossly irregular fluctuations with a zigzag pattern.zigzag pattern.

Treatment includes cardioversion, CPR, oxygen, and Treatment includes cardioversion, CPR, oxygen, and antiarrhythmic medications.antiarrhythmic medications.


Ventricular Fibrillation (cont.)Ventricular Fibrillation (cont.)

chapter 17 interpreting the electrocardiogram. mosby items and derived items © 2009 by mosby, inc.,...

Documents

mosby items

derived items

affiliate of elsevier

electrocardiogram slide

pacemaker cells

cardiac impulse

electrocardiogram ecg

ventricular muscle cells