16. Physio Basis of ECG

Question 1

how many electrodes are placed on the skin and how many readings are produces

Answer 1

9 electrodes
12 readings

***measures extracellular potential

Question 2

which phases of the cardiac action cycle produce deflections on the ECG

Answer 2

phase 0 (depolarization) and phase 3 (repolarization)

Question 3

___ causes atria to depolarize from ___ to ___ creating the P wave

Answer 3

• SA node

- from R to L

Question 4

the PR interval represents ___

Answer 4

delay in signal conduction caused by AV node

Question 5

in what direction do ventricles depolarize?

repolarize?

Answer 5

depolarize: from R to L, from base to apex, from interior to exterior
repolarize: L to R, apex to base

Question 6

P wave represents what phase of the AP in atrial muscle?

what ions are most permeable?

Answer 6

• phase 0 (all cells are depolarized by then end of the p wave)
• highly permeable to Na

Question 7

PR segment represents what phase of the AP in atrial muscles?

what ions are most permeable?

Answer 7

• phase 2

- highly permeable to Ca

Question 8

the QRS represents what phase of the AP in ventricular muscles?

what ions are most permeable?

Answer 8

• phase 0
• highly permeable to Na

***also phase 3 for atria (repolarization), so high K permeability

Question 9

ST segment represents what phase of the AP in ventricular muscles?

what ions are most permeable?

Answer 9

• phase 2

- highly permeable to Ca

Question 10

T wave represents what phase of the AP in ventricular muscles?

what ions are most permeable?

Answer 10

• phase 3

- highly permeable to K

Question 11

what is the difference between a segment and an interval

Answer 11

segment: duration of a single event
interval: 2+ events

Question 12

what is the normal duration for the PR interval?

QT interval?

Answer 12

PR: 0.16 s (beginning of p to beginning of q)

QT: 0.35 s (beginning of q to end of T)

Question 13

what is the normal voltage for the QRS complex?

p wave?

t wave?

Answer 13

QRS: 1.0 - 1.5 mv, 2-3 large boxes
(from top of R to bottom of S)

P: 0.1 - 0.3 mv, 1-3 small boxes

T: 0.2 - 0.3 mv, 2-3 small boxes

Question 14

describe the placements for leads I, II and III

Answer 14

I: RA (-) –> LA (+)

II: RA (-) –> LL (+)

III: LA (-) –> LL (+)

Question 15

describe the augmented vectors for AVR, AVL, and AVF

Answer 15

AVR: (LL + LA) –> RA (+)

AVL: (LL + RA) –> LA (+)

AVF: (RA + LA) –> LL (+)

Question 16

which precordial leads are usually negative and which are positive?

Answer 16

-: V1 and V2

+: V4, V5, V6

Question 17

which leads are considered inferior

Answer 17

II, III, AVF

Question 18

which leads are considered anterior

Answer 18

V3 and V4

Question 19

which leads are considered lateral

Answer 19

I, AVL, V5, V6

Question 20

which leads are considered septal

Answer 20

V3, V4

Question 21

which leads show RV

Answer 21

V1, V2

Question 22

what is the mean electrical axis and what does it help determine

Answer 22

• average direction of spreading AP in ventricles
• helps determine morphology of the heart

***uses average QRS amplitude in leads I and AVF

Question 23

how does hypertrophy shift the axis?

infarction?

Answer 23

hypertrophy shifts axis towards hypertrophy

infarction shifts axis away

Question 24

if lead I is negative, what does that say about the heart

Answer 24

RAD, right ventricular hypertrophy

Question 25

if lead I is positive but AVF is negative, what does that ay about the heart?

Answer 25

LAD, left ventricular hypertrophy