Lecture 5: Physiological Basis of the ECG

Question 1

How many electrodes in an electrocardiogram; how many readings generated?

Answer 1

• 9 electrodes

- Generate 12 readings

Question 2

What is produced on an ECG if there is a difference in voltage between two electrodes?

Answer 2

A deflection

Question 3

What is the ECG measuring?

Answer 3

Extracellular potential

Question 4

What 2 factors are necessary to cause a deflection?

Answer 4

1) Part of the cardiac excitable tissue is at a different membrane potential than the rest of the heart
2) Current flow can occurs between those two regions

Question 5

What will occur if only the atria and ventricles are of differing potentials?

Answer 5

Does not a cause a deflection

Question 6

Why does the ECG not report differences between the atria and ventricles?

Answer 6

Due to the fibrous cartilage between the atria and ventricles

Question 7

Which direction does lead I look at the heart from?

Answer 7

Left to right (left arm (+) to right arm (-))

Question 8

SA node causes atria to depolarize from _____ to _____?

Answer 8

Right to Left

Question 9

What is occurring during the PR interval?

Answer 9

AV node is delaying the signal

Question 10

AP phase 2 delays repolarization of ventricles, and is seen as what segment?

Answer 10

ST segment

Question 11

Ventricles repolarize from ______ to ______; seen as which wave?

Answer 11

Left to right; T wave

Question 12

Which phase of the atrial AP is represented by the P wave?

Answer 12

Phase 0

Question 13

Why is no signal seen at the end of the P wave?

Answer 13

All of the atrial cells are effectively the same polarity, all depolarized, no voltage difference.

Question 14

Which phase of the ventricular AP is represented by the QRS complex?

Answer 14

Phase 0

Question 15

Which phase of the ventricular AP is represented by the T wave?

Answer 15

Phase 3 or repolarization

Question 16

What does a segment represent on the ECG?

Answer 16

Duration of single event

Question 17

What does an interval represent on the ECG?

Answer 17

Duration of several events

Question 18

What is the PR segment?

Answer 18

End of atrial depolarization until beginning QRS

Question 19

What is the ST segment?

Answer 19

End of QRS until beginning of T wave

Question 20

What is the PR interval?

Answer 20

Beginning of P wave until beginning of QRS

Question 21

What is the QT interval?

Answer 21

Beginning of QRS until end of T wave

Question 22

What would cause a prolonged QT interval?

Answer 22

A prolonged phase 2, caused by Ca2+ and K+(b).

Question 23

When determining rate what does the time between the dark lines (bolder) represent?

Answer 23

.2 sec

Question 24

When determining rate what does the time between the light lines represent?

Answer 24

.04 sec

Question 25

Why use 12 leads?

Answer 25

Each lead displays voltage differences in the myocardium seen from different perspectives by different electrodes

Question 26

Which limbs are involved in an aVF?

Answer 26

(RA+LA) to LL (+)

Question 27

Which limbs are involved in aVR?

Answer 27

(LL+LA) to RA (+)

Question 28

Which limbs are involved in aVL?

Answer 28

(LL+RA) to LA

Question 29

How many vectors for chest or precordial leads?

Answer 29

V1-V6

Question 30

What makes up Einthoven's Triangle?

Answer 30

Lead I: RA (-) to LA (+) Lead II: RA (-) to LL (+) Lead III: LA (-) to LL (+)

Question 31

How do augmented limb leads work?

Answer 31

One of the limb leads is assigned a positive polarity and the remaining 2 electrodes are averaged and assigned a negative polarity

Question 32

Chest or precordial leads are not assigned an axis, they are instead assigned?

Answer 32

To a certain region of heart and each acts as a positive electrode

Question 33

Which leads predominantly represent the inferior region of the heart?

Answer 33

II, III, aVF

Question 34

Which leads predominantly represent the septal region of the heart?

Answer 34

V1 and V2

Question 35

Which leads predominantly represent the anterior region of the heart?

Answer 35

V2, V3, and V4

Question 36

Which leads predominantly represent the lateral region of the heart?

Answer 36

I, aVL, V4, V5 and V6

Question 37

What is the mean electrical axis?

Answer 37

Average direction of spreading AP in ventricles

Question 38

What is used for determining the mean electrical axis?

Answer 38

Average amplitude of QRS in I and aVF

Question 39

If both lead I and aVF are positive what does this signify?

Answer 39

A normal mean electrical axis

Question 40

If lead I is negative, what does this signify?

Answer 40

A right axis deviation, regardless if aVF is either positive or negative.

Question 41

If lead I is positive, but aVF is negative what does this signify?

Answer 41

A left axis deviation

Question 42

The P wave begins ________contraction of the atria.

Answer 42

Before

Question 43

QRS begins ______ ventricular contraction

Answer 43

Before

Question 44

T wave begins ________ ventricular relaxation

Answer 44

Before