Chapter 11 Interpretation of Electrocardiogram Tracings

Introduction

• RTs observing the onset of an ischemic cardiac event may be the first link in the chain of survival

• Early recognition of a serious cardiac problem may minimize cardiac damage or prevent death

Introduction

• ECG reflects electrical activity of the heart• 12-lead ECGs provide 12 different views

of that activity

• Diagnostic tool to detect abnormalities such as:• Myocardial infarctions• Ventricular hypertrophy• Dysrhythmias

When Should an ECG Be Obtained?

• Obtain an ECG when there are signs/symptoms of acute or chronic cardiac disorders• CHF• Angina• Acute myocardial infarction

• Prior to surgery as a screening tool

Cardiac Anatomy and Physiology

• The heart is composed of 4 chambers

• 2 upper chambers: atria• 2 lower chambers: ventricles

• Heart divided down the middle to form the right and left side

Cardiac Anatomy and Physiology• Right atria and ventricle receive venous

blood and circulate it to the lungs for gas exchange

• Left atria and ventricle receive oxygenated blood from lungs and circulate it to entire body

Conduction Pathway in the Heart

• Electrical activity starts in the sinoatrial (SA) node because it has the greatest level of automaticity

• Signal travels throughout atria and via internodal pathways to atrioventricular (AV) node

Conduction Pathway

• Signal delayed in AV node to allow atrial contraction prior to ventricular contraction

• Then travels through right and left bundle branches and to Purkinje fibers of the ventricles

Basic ECG Waves

•Depolarization: sudden loss of negative charge in polarized cells when stimulated

•Repolarization: return of the negative charge within the cell

Basic ECG waves

•ECG tracings record the waves of depolarization and repolarization traveling across the myocardium

•Each wave can be identified and analyzed

ECG Waves

• P wave: atrial depolarization

• PR interval: time for impulse to travel to ventricles:

• Normally 0.12 to 0.20 second• Allows atrial contraction and “priming of

the pump” to precede ventricular contraction

Basic ECG Waves

• QRS complex: ventricular depolarization• Normally occurs in <0.12 second

• ST segment: between QRS and T wave

• T wave: ventricular repolarization

Basic ECG Waves

Evaluating heart rate (HR)

• If HR regular, count number of large boxes (0.2 sec) between QRS complexes and divide this number into 300

• 300: number of large boxes occurring in 1 minute• 1 box between QRS would be rate of 300• 2 boxes between would be rate of 150

etc.

Evaluating HR

• If HR irregular, average obtained by counting QRS complexes in a 6-sec strip times 10

ECG Leads

• 12-lead ECG provides different views of same event• 6 limb leads: • I, II, III, aVR, aVL, aVF

• View heart on frontal plane used to determine axis

• 6 chest leads:• V1, V2, V3, V4, V5, V6 • View the heart on the horizontal plane• Overlie the RV (V1, V2), ventricular septum (V3,

V4) and LV (V5, V6 )

Steps of ECG Interpretation

• First evaluate patient’s overall condition• Clinical signs and symptoms may aid in

identifying the dysrhythmia• Interpretation can be done on three levels• Ventricular response• Origin of the impulse• Electrophysiology• Ectopic beats or rhythms, escape beats or

rhythms• AV blocks or bundle branch blocks

Steps of ECG Interpretation

Use a systematic method:1. Identify the heart rate2. Evaluate the rhythm3. Note the presence of P waves4. Measure the PR interval5. Measure the width of the QRS complex6. Inspect the ST segment in all leads7. Identify the mean QRS axis8. Assess the waveform morphology9. Evaluate the Q wave10.Look for signs of chamber enlargement

Sinus Bradycardia•Meets all the criteria for NSR but is too

slow• Rate: less than 60 beats/min• Rhythm: regular• P waves: normal and followed by a QRS

complex• PR interval: 0.12 to 0.2 second • QRS: less than 0.12 second in width

Sinus Tachycardia

•Meets all criteria for NSR but is too fast• Rate: 100 to 150 beats/min• Rhythm: regular• P waves: normal and followed by a QRS

complex• PR interval: 0.12 to 0.2 second • QRS: less than 0.12 second in width

Atrial Flutter• Distinct rapid sawtooth pattern between

normal QRS• Rate: atrial rates 180 to 400; ventricular

rate is slower• Rhythm: regular• P waves: sawtooth and uniform• PRI: not measurable• QRS: less than 0.12 second in width

Atrial Fibrillation• Characterized by chaotic baseline between

QRSs• Rate: variable (count QRSs in 6-second

strip)• Rhythm: irregularly irregular• P waves: fibrillatory waves that all vary• PRI: not measurable• QRS: less than 0.12 second in width

Premature Ventricular Contractions• Underlying rhythm is interrupted by wide

QRS (>0.12 s) not preceded by a P wave, with an inverted T• Rate: that of the underlying rhythm• Rhythm: regular rhythm is interrupted by

PVC• P waves: not associated with the PVC• PR interval: not measurable• QRS: >0.12 second, premature, abnormal

configuration, followed by compensatory pause

PVC

Ventricular Tachycardia•Wide QRSs occurring rapidly without

P waves•Rate: 140 to 300 beats/min•Rhythm: regular •P waves: not associated with the

QRS complexes•PR interval: not measurable•QRS: abnormal and >0.12 second

V-Tach

Ventricular Fibrillation

• Chaotic, characterized by wavy irregular pattern• Rate: none• Rhythm: irregular, chaotic waves• P waves: none• PRI: none• QRS: none or sporadic low-amplitude

waves

V-Fib

Asystole• Characterized by a straight or almost flat

line• Rate: none• Rhythm: none• P waves: none• PRI: none• QRS: none

AV Heart Block

• General term: problems conducting impulses from the atrial to the ventricles• Blocks can occur at the AV node, bundle of

His, or the bundle branches• Complete heart block may be associated

with hypotension•Milder forms of heart block often cause no

symptoms

First-Degree AV Block• Rate: underlying rhythm rate• Rhythm: regular • P waves: normal each preceding a QRS

complex• PRI: >0.2 second* (Key Feature)• QRS: < 0.12 second in width

Second-Degree AV Block Type I (Wenckebach)

• Recurrent lengthening PRI followed by a dropped QRS

• Rate: varies, ventricular rate less than atrial rate• Rhythm: irregular • P waves: normal not always followed by

a QRS• PR interval: varies, lengthens, then none

conducted • QRS: < 0.12 second in width

Second-Degree AV Block Type I (Wenckebach)

Second-Degree AV Block Type II

• Characterized by nonconducted P waves followed by a P wave that is conducted thus has an associated QRS• Rate: varies, ventricular rate less than atrial

rate• Rhythm: atrial rate is regular, ventricular rate

may be regular or irregular • P waves: normal not always followed by a QRS• PR interval: normal or prolonged but constant• QRS: < 0.12 second in width

Second-Degree AV Block Type II

Third-Degree AV Block (Complete Heart Block)

• No association between P waves and QRS complexes• Rate: slow, ventricular rate less than atrial

rate• Rhythm: atrial and ventricular rates are

regular • P waves: normal not always followed by a

QRS• PRI: varies, no relationship to QRS complexes• QRS: generally but not always >0.12 second

Evidence of Cardiac Ischemia, Injury, or Infarction

• Cardiac ischemia• Indicated by depressed ST segment (≥1

mm below baseline) or inversion of the T waves• Injury is potentially reversible at this

point

Evidence of Cardiac Ischemia, Injury, or Infarction

• Acute myocardial injury• Noted by elevated ST segment changes

over the affected myocardium• Generally indicates acute myocardial

infarction• ST segment returns to baseline with

restored perfusion

Evidence of Cardiac Ischemia, Injury, or Infarction

•Myocardial infarction• Noted early by elevated ST segments and

T wave changes• Once fully evolved pathologic Q waves

appear• Appear hours to days following AMI• Generally remain for the duration of

patient’s life

Assessing Chest Pain• Assess S/S by asking the patient:• O: onset of pain• P: provoked by …• Q: quality of pain• R: radiation of pain to …• S: severity of pain between 0 and 10• T: time frame of symptoms (acute or

chronic)• U: what do You perceive as wrong?• AHA says suspect AMI if nonresponsive to

nitroglycerin

ECG Patterns with Chronic Lung Disease

•Multiple ECG changes with severe COPD• Right axis deviation is common• P waves larger due to right atrial

enlargement

• Leads II, III, and aVF

• Prominent and negative P wave in lead I•May have changes associated with cor

pulmonale• Increase R-wave size on leads V1, V2, and V3

• Reduced size of QRS in leads I, II, III, V5, and V6

Summary

• An ECG provides a picture of heart’s electrical activity • ECG can identify the condition of the

heart’s electrical conduction system• The ECG can also identify abnormal

rhythms that may be of little consequence or very serious and life threatening• ECG does not measure pumping ability of

the heart