The Normal ECG/EKG

Question 1

A voltmeter registers the potential difference between the positive and negative leads on the chest

Einthoven’s triangle

Answer 1

Question 2

The ECG is generated by the vectorial movement of electrical currents which form dipoles.

What is the mean electrical axis / mean QRS vector?

Answer 2

Sum of the 3 mean vectors describing the direction of ventricular depolarization for each of the 3 standard leads; determined from the amplitudes of their respective QRS complexes

Ex) In left ventricle depolarizes from the endocardium up to the epicardium

Question 3

The net direction of cardiac depolarizatoin adn repolarization determins whether an ECG deflection goes up or down.

What happens to ECG if..

• Depolarize toward +
• Depolarize away from +
• Repolarize toward +
• Repolarize away from +

Answer 3

The net direction of current flow.

Question 4

Why is the normal T wave upward?

Answer 4

The epicardium repolarizes before the endocardium and goes in the opposite direction from depolarization.

_{Remember: repolarization away from the pos electrode gives an upward deflection of the ECG trace}

Question 5

The amplitude of ECG waves depends on what 3 factors?

Answer 5

1. The number/mass of fibers activated
2. The synchrony which which those fibers fire APs
3. The degree of continuity in the direction of depolarization or repolarization in a given region of the heart (seen in fig)

Question 6

PR interval

Answer 6

The time it takes impulse to pass from atria to ventricles.

Prolonged PR: impulse is conducted more slowly through AV node

Shortened PR: impulse is conducted ove ra shortened abnormal route from atria to ventricles

Question 7

Prolonged QRS complex signifies

Answer 7

Abnormal conduction or delay of conduction through teh ventricles

Question 8

ST segment

Answer 8

Period after ventricular depolarization and before ventricular repolarization.

Elevation or depression of ST indicates ischemia or infarction of heart muscle

Question 9

Inverted T wave (ventricular repolarization) indicates

Answer 9

Net repolarization in the left ventricular wall is taking place in the same direction as depolarization _{(i.e., the wave of repolarization is moving away from the + electrode in Lead I and therefore the wave goes in the downward direction).}

Ischemia

infarction of heart muscles

Hypertrophy

Conduction block

Electrolyte disturbances

All of these slow conduction and/or prolong ventricular depolarization

Question 10

Abnormal or absent P wave implies

Answer 10

Another area of the heart muscle is acting as pacemaker - not the SA node

Question 11

No waves are generated by the activation of

Answer 11

The SA node

The AV node

Bundle of his

Purkinje

All waves are generated by activation of the atria or ventricles.

Question 12

What are some causes of deviation of the normal mean QRS axis? (dont need to memorize)

Answer 12

• Left axis deviation
  
  • Inferior wall MI
  • Left anterior fascicular block
  • Left ventricular hypertrophy
• Right axis deviation
  
  • Right ventricular hypertrophy
  • Acute right heart strain
  • Left posterior fascicular block

Question 13

The normal range of mean QRS vectors are in the shaded area.

Question 14

What are the approximate avg times for each thing on the ECG?

Answer 14

P is .08 sec

ST is .12 sec

Everything else is ~.06 - 0.12