Heart Lecture 4: Cardiac Pacemaker Mechanisms

Question 1

What determines ventricular rate?

Answer 1

AV refractory period

Question 2

What is normal duration range for QT interval?

Answer 2

250-400ms

Question 3

How does hypokalemia lead to prolonged QT?

Answer 3

decreases K+ permeability making it hard to leave the cell, therefore prolonging AP

Question 4

Why is sinus node inherent pacemaker?

Answer 4

fastest beating rate

Question 5

Hierarchy or pacemaking:

Answer 5

1) SA (fastest)
2) atrial and ventricular subsidiary/latent pacemakers
3) AV nodal/His bundle
4) bundle branches
5) Purkinje fibers (slowest)

Question 6

Diastolic depolarization

Answer 6

generates automaticity of SA node (phase 4 of action potential)

Question 7

What current is special to pacemaking tissue?

Answer 7

IF = Funny current (hyperpolarization active inward current)

Funny because it is the only channel in the heart that is activated on HYPERpolarization, not depolarization

Question 8

What does IF carry?

Answer 8

sodium

Question 9

What activates IF?

Answer 9

hyperpolarization

Question 10

Block IF channel and what happens?

Answer 10

Lose pacemaker activity in sinus node by ~30%

Question 11

Name 4 mechanisms that contribute to pacemaking in the SA node:

Answer 11

1) T-type Ca++ current (turned on at neg. voltages)
2) hyperpolarization-activated inward current (IF)
3) deactivation of K+ current (leaves pos. charge in cell)
4) inward Na/Ca exchange current activated by intracellular SR Ca++ release

Question 12

Purkinje fibers latent pacemaker activated by what:

Answer 12

1) IF current

2) deactivation of K+ current

Question 13

Main way to modulate heart rate

Answer 13

enhance or depress slope of diastolic depolarization (gets to threshold later)

Question 14

Name for most negative point in the cycle of SLOW AP

Answer 14

maximum diastolic potential

Question 15

ACH does two things

Answer 15

1) decrease slope of diastolic depolarization
2) depress max diastolic voltage

both slow down rate

Question 16

Need to have what stimulation when heart rate goes up

Answer 16

sympathetic stimulation of AV node (otherwise accelerated heart rate wouldn’t go through to ventricles)

Question 17

4 mechanisms responsible for changes in heart rate

Answer 17

1) change in slope of diastolic depolarization
2) change in max diastolic potential
3) change in threshold
4) pacemaker shifts

Question 18

What does overdrive suppression refer to?

Answer 18

How the sinus node suppresses electrical activity of other pacemakers

Question 19

What is the danger of overdrive suppression?

Answer 19

Stopping SA stimulation results in temporary suppression of pacemaker activity

Question 20

What takes over when SA node cuts out?

Answer 20

latent pacemakers (takes about 10-20 seconds for them to pick up)

Question 21

What effect does ACh have on pacemaking?

Answer 21

inhibits it by making resting membrane potential more negative (hyperpolarizes it)

Question 22

What effect does ACh have on K+ permeability?

Answer 22

increases it (more K+ leaves the cell, making the interior more negative)

Question 23

What effect does ACh have on Ca++ current?

Answer 23

inhibits slow inward L-type Ca++ current (which is responsible for upstroke in slow AP tissues) as well as the IF current by inhibiting cAMP synthesis (because ACh inhibits adenylate cyclase activity)

Question 24

What effect does ACh have on the slope of diastolic depolarization?

Answer 24

decreases it (by hyperpolarizing max diastolic potential)

Question 25

What does sinus arrhythmia refer to?

Answer 25

The change in rhythm that comes from vagal tone as we breathe (decreased heart rate in expiration and increased in inspiration)

Question 26

Effects of inspiration on cycle

Answer 26

DECREASES cycle length (shorter) and INCREASES heart rate

inhibition of parasymp vagal activity

Question 27

Effects of expiration on cycle

Answer 27

INCREASES cycle length (longer) and DECREASES hear rate

stimulation of parasymp activity

Question 28

Why do aerobically trained individuals have lower resting heart rates?

Answer 28

increased vagal tone

Question 29

What effect does norepinephrine have on Ca++ current?

Answer 29

INCREASES slow inward L-type Ca++ current and IF current by increasing cAMP synthesis

Question 30

What effect does norepinephrine have on the slope of diastolic depolarization?

Answer 30

INCREASES it

Question 31

What effect does NE have on K+ permeability?

Answer 31

none

Question 32

What effect does NE have on max diastolic potential?

Answer 32

none

Question 33

How do you decipher SVT vs sinus tachycardia?

Answer 33

clinical context
SVT = pathological
sinus tachycardia = during exercise