Ch268 Electrocardiography Flashcards by Mae Matira

Graphic recording of electric potentials generated by the heart; noninvasive, inexpensive and highly versatile test

Electrocardiogram (ECG or EKG)

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What do ECG leads display?

Instantaneous differences in potential between the electrodes

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The initiating event for cardiac contraction

Depolarization

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3 components that produce the electric currents through the heart

Cardiac pacemaker cells
Specialized conduction tissue
Heart muscle

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A collection of pacemaker cells where depolarization stimulus for normal hearbeat originates

Sinoatrial (SA) node or sinus node

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T or F: Depolarization wavefronts spread through the ventricular wall from the epicardium to the endocardium triggering the ventricular contraction.

False

Endocardium to epicardium

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ECG waveform representing atrial depolarization

P wave

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ECG waveform representing ventricular depolarization

QRS complex

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ECG waveform representing ventricular repolarization

ST-T-U complex

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Junction between the end of QRS complex and beginning of ST segment

J point

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What conditions can J point become apparent?

Acute pericarditis

Atrial infarction

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Phase in the cardiac action potential corresponding to the onset of QRS

Phase 0 (rapid upstroke)

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Phase in cardiac action potential corresponding to the isoelectric ST segment

Phase 2 (plateau)

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Phase in cardiac action potential corresponding to the inscription of the T wave

Phase 3 (active repolarization)

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Effects of hyperkalemia and flecainide on cardiac action potential

Decrease slope of Phase 0
Increase QRS duration
Impairs Na influx

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Effects of amiodarone and hypocalcemia on cardiac action potential

Prolong Phase 2

2. Increase QT interval

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Effect of digitalis or hypercalcemia on cardiac action potential

Shortens ventricular repolarization (Phase 2)

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Four major ECG intervals

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Compute the heart rate using the RR interval

Divide the number of large squares between consecutive R waves into 300 OR

Divide the number of small squares into 1500

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PR interval measures the time between atrial and ventricular depolarization including the physiologic delay imposed by stimulation cells in AV junction area, has the normal value of:

120-200ms

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QRS interval reflects the duration of ventricular depolarization with normal value of

100-110ms or less

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T or F: QT interval is directly proportional to the heart rate

False

QT interval varies inversely with heart rate

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Can be calculated as QT/square root of RR

Corrected QT interval (QTc)

Normal value: =0.44s

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What wave corresponds to a negative initial QRS deflection

Q wave

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First positive deflection in QRS complex

R wave

The negative deflection after an R wave in QRS complex

S wave

An entirely negative QRS complex

QS wave

Leads that record potentials transmitted onto the frontal plane

Limb leads (6)

Leads that record potentials transmitted onto the horizontal plane

Chest (precordial) leads (6)

Diagram representing the spatial orientation and polarity of the six FRONTAL plane leads

Hexaxial diagram

Anatomical positions of leads V7, V8, V9

V7 midaxillary line V8 posterior axillary line V9 posterior scapular line All in line with V4 (5th intercostal space)

Left axis deviation on hexaxial diagram

-90 to -30 degrees

Right axis deviation on hexaxial diagram

+100 to +180 degrees

Normal axis on hexaxial diagram

-30 to +100 degrees

Implications of left axis deviation

1. Normal variant 2. Left ventricular hypertrophy 3. Block in anterior fascicle of left bundle system (left anterior fascicular block/hemiblock) 4. Inferior MI

Implications of right axis deviation

1. Normal variant (children and young adults) 2. Due to reversal of left and right arm electrodes 3. Right ventricular overload (acute or chronic) 4. Infarction of lateral wall of left ventricle 5. Dextrocardia 6. Left pneumothorax 7. Left posterior fascicular block

T or F: The normal U wave is small, rounded deflection (=1mm) that has same polarity as the T wave

True

Drugs that abnormally increases U wave amplitude

1. Dofetilide 2. Amiodarone 3. Sotalol 4. Quinidine Condition: Hypokalemia

Very prominent U waves are a marker of increased susceptibility to this type of ventricular tachycardia

Torsades de pointes

Increased P wave amplitude (peaked P waves) also referred as P-pulmonale has normal value of

>/= 2.5mm

Pattern that produces a biphasic P wave in V1 with broad negative component or a broad often notched P wave in one or more limb leads

P-mitrale Typically from left atrial overload

Condition produced due to sustained, severe pressure load characterized by relatively tall R wave in lead V1 (R >/= S wave), usually with right axis deviation

Right VENTRICULAR hypertrophy

Pattern attributed to repolarization abnormalities in acutely or chronically overloaded muscle described as ST depression and T-wave inversion in the right-to-midprecordial leads

Formerly called right ventricular "strain"

T or F: Acute cor pulmonale due to pulmonary embolism can present with normal ECG

True

The most common arrhythmia

Sinus tachycardia

ECG finding in this condition is associated with small R waves in right-to-midprecordial leads (slow R-wave progression) due in part to downward displacement of diaphragm and heart in obstructive lung disease

Chronic cor pulmonale

Sokolow-Lyon criteria to assess left ventricular hypertrophy

SV1 + (RV5 or RV6) >35mm

T or F: Prominent precordial voltages may occur as a normal variant in the elderly.

False Athletic or young individuals

Sensitivity of conventional voltage criteria for LVH is decreased in these type of population

Obese persons and smokers

QRS interval in COMPLETE bundle branch blocks

>/= 120 ms

QRS interval in INCOMPLETE bundle branch blocks

100-120 ms

RBBB are more common in subjects without structural heart disease but may also occur with heart disease such as

Congenital (ASD) | Acquired (e.g. valvular, ischemic)

Often a marker of one of four underlying conditions associated with increased risk of CV morbidity and mortality rates

LBBB

4 conditions associated with increased risk of CV morbidity and mortality

1. Coronary heart disease 2. Hypertensive heart disease 3. Aortic valve disease 4. Cardiomyopathy

T or F: Bundle branch blocks can be rate-related such that is can occur when the heart rate exceeds some critical value

True

T or F: Bundle branch blocks can be intermittent

True

Primary repolarization abnormalities (ST-T wave changes) can occur in these conditions

1. Ischemia 2. Electrolyte imbalance 3. Digitalis

Most common cause of marked left axis deviation in adults

Left ANTERIOR fascicular block (QRS axis more negative than -45 degrees)

T or F: Left POSTERIOR fascicular block (QRS axis more rightward than +110-120 degrees) is extremely rare and requires exclusion of other factors causing right axis deviation

True

Which has higher risk of progression to high-degree AV block: A. Chronic bifascicular block in asymptomatic individual B. New bifascicular block with acute MI

B. New fascicular block with acute MI

Alternation of right and left bundle branch block is a sign of what disease?

Trifascicular disease

Factors that slows down the ventricular conduction

1. Hyperkalemia | 2. Drugs (Class I antiarrhythmic agents, TCA, Phenothiazines)

Diagnostic triad of Wolff-Parkinson-White

1. Wide QRS complex 2. Relatively short PR interval 3. Slurring of the initial part of QRS (Delta wave)

Prolongation of QRS duration due to preexcitation of the ventricles via a bypass tract is seen in this condition

Wolff-Parkinson-White (WPW)

Cornerstone in diagnosis of acute and chronic ischemic heart disease

ECG

Effect of severe, acute ischemia on resting membrane potential and action potential

Lowers resting membrane potential | Shortens duration of AP

ST changes seen in: A. Transmural ischemia B. Ischemia in subendocardium

A. ST elevations, Hyperacute T waves | B. ST depression

Posterior wall ischemia can be indirectly recognized due to this reciprocal changes

ST depression in leads V1 to V3

REVERSIBLE TRANSMURAL ischemia due to coronary vasospasm in these conditions may cause transient ST SEGMENT ELEVATIONS WITHOUT development of Q WAVES

1. Prinzmetal's variant angina | 2. Tako-tsubo "stress" cardiomyopathy syndrome

Pattern in severe anterior wall ischemia causing prominent T wave inversions in precordial leads that is associated with high grade stenosis of left anterior descending coronary artery

Wellens T waves

T or F: Transmural infarcts may occur without Q waves and subendocardial infarcts sometimes may be associated with Q waves.

True

Differential diagnosis of ST segment elevations

1. Ischemia / MI 2. Acute pericarditis 3. Normal variants 4. LVH/LBBB 5. Others: Acute pulmo embolism, Brugada patterns, Hypercalcemia, Hyperkalemia, Hypothermia, Myocarditis, Tumor invading left ventricle, Trauma to ventricles, Class 1C antiarrhythmic drugs

ECG finding seen in hypothermia

J wave / Osborn wave Distinctive convex elevation of J point

Conditions that may cause ST segment depression mimicking subendocardial ischemia

1. Digoxin 2. Ventricular hypertrophy 3. Hypokalemia

May cause prominent T wave inversions

1. Ventricular hypertrophy 2. Cardiomyopathies 3. Myocarditis 4. Cerebrovascular injury (bleed)

This condition causes cardiac arrest with a slow sinusoidal type of mechanism ("sine-wave" pattern) followed by asystole

Hyperkalemia

This condition prolongs ventricular repolarization often with prominent U waves

Hypokalemia

Class IA, TCA, phenothiazines and Class III antiarrhythmics produce these ECG changes

Prolongation of QT interval They increase the duration of ventricular action potential

ECG pattern that may occur with intracranial bleeds particularly subarachnoid hemorrhage

"CVA T-wave" pattern Deep wide T-wave inversions

ECG changes in hypocalcemia vs hypercalcemia

Hypo: Prolongs QT interval (ST portion) Hyper: Shortens QT interval (Also digitalis)

Characteristic finding on ECG due to digitalis

"scooping" of the ST-T wave complex (digitalis effect)

Defined as peak-to-trough QRS amplitudes of =5mm in the six limb leads and/or =10mm in the chest leads

Low QRS voltage

Conditions that produce low QRS voltage

1. Pericardial effusion 2. Pleural effusion 3. COPD 4. Infiltrative cardiomyopathies 5. Anasarca

A beat-to-beat alternation in one or more components of the ECG signal

Electrical alternans

ECG findings that are relatively specific sign of pericardial effusion with cardiac tamponade

Total electrical alternans (P-QRST-T) with sinus tachycardia

Sign of electrical instability and may precede ventricular tachyarrhythmias

Pure repolarization (ST-T or U wave) alternans

14 points that should be analyzed in every ECG

1. Standardization (calibration) and technical features 2. Rhythm 3. Heart Rate 4. PR interval/AV conduction 5. QRS interval 6. QT/QTc intervals 7. Mean QRS electrical axis 8. P waves 9. QRS voltages 10. Precordial R-wave progression 11. Abnormal Q waves 12. ST segments 13. T waves 14. U waves

T or F: Computerized interpretation should not be accepted without careful clinician review.

True