7: Cardiac Conduction And APs

Question 1

Spread of AP through heart

Answer 1

SA node -> AV node -> Purkinje fibers -> Bundle of His -> R and L bundle branch -> L further divides into smaller Purkinje fibers

Question 2

Which side does the AP go through first, right or left?

Answer 2

right - in atria and ventricles

Question 3

Which layer of the heart receives the AP first

Answer 3

Endocardium (before epicardium)

Question 4

What parts of the heart can have pacemaker qualities if the conditions are right?

Answer 4

1. SA node
2. AV nodE
3. Purkinje fibers
4. Bundle of His
5. RBB and LBB

Question 5

What quality of a heart cell allows higher velocity conduction of AP? Which cell types are fastest vs slowest?

Answer 5

Larger diameter - Purkinje > myocytes > AV node

Question 6

Where do fast vs slow cardiac APs occur?

Answer 6

Fast: myocytes, Purkinje fibers
Slow: SA and AV nodes

Question 7

What do INa and Ito stand for?

Answer 7

INa: Na conductance
Ito: outward K current

Question 8

What causes cells undergoing slow APs to act as pacemakers?

Answer 8

Slow influx of Na through funny voltage-gated Na channels that open upon complete repol of membrane

Question 9

Chronotropic

Answer 9

Affects HR

Question 10

Dromotropic

Answer 10

Affects conduction speed

Question 11

Inotropic

Answer 11

Affects contractility

Question 12

Refractory period

Answer 12

When AP cannot be generated / is more difficult

Question 13

Why are refractory periods longer in the heart? Two reasons

Answer 13

1. Aid in rhythmic contraction

2. Protect against arrhthmias

Question 14

Two types of refractory periods in the heart

Answer 14

1. Absolute (ARP): No AP can be generated

2. Relative (RRP): AP requires much greater stimulus and/or will have abnormal conduction

Question 15

What causes absolute vs relative refractory periods?

Answer 15

Absolute: all voltage gated Na channels are inactivated
Relative: many voltage gated Na channels have reset, but not all

Question 16

Supranormal period (SNP)

Answer 16

Cell more excitable than normal, but AP will be abnormal

Question 17

What two things occur in the phases of the cardiac AP due to hyperkalemia?

Answer 17

Slows phase 0 + speeds up phase 3 -> messes up refractory period and shrinks the AP

Question 18

Interval vs segment in ECGs

Answer 18

Interval: includes entires waves involved
Segment: starts at end of a wave -> ends at beginning of a wave

Question 19

What interval on ECG is used to determine HR

Answer 19

RR interval

Question 20

Bradycardia vs tachycardia numbers

Answer 20

Brady: <60 bpm
Tachy: >100 bpm

Question 21

PR interval of a first degree AV block

Answer 21

PR interval > 0.2 ms

Question 22

Long QT syndrome - what is different in the AP phases?

Answer 22

Transition from phase 2 to 3 is lengthened

Question 23

Determining HR on ECG: time between dark vs light lines

Answer 23

Dark lines: 0.2 seconds

Light lines: 0.04 seconds