L8 Heart- Cardiac Pacemaker Mechanisms

Question 1

Define automaticity

Answer 1

An intrinsic property of specialized cardiac cells to initiate their own electrical activity (pacemaker activity)

Question 2

Explain the hierarchy of cardiac pacemaker activity throughout the heart

Answer 2

SA node (primary pacemaker, fastest inherent rate) > latent atrial pacemakers > AV nodal/His bundle (junctional) > bundle branches > Purkinje fibers

Question 3

Explain the cellular/ionic mechanism responsible for SA node pacemaker activity.

Answer 3

• T-type Ca current (special Ca channel, turns on at neg. volt., leak + charge in)
• Hyperpolarization-activated inward current, I(f)
• Deactivation of K current (less + charge leaving cell= more negative)
• Inward Na/Ca exchange current activated by intracellular SR Ca release (inflow of Na depolarizes a little)

Question 4

Explain how changes in threshold potential can alter pacemaker activity.

Answer 4

Lower threshold means more negative, you hit it earlier, so HR speeds up

Question 5

Explain how changes in slope of diastolic depolarization can alter pacemaker activity (heart rate)

Answer 5

ACh can change the rate of the SA node, change the slope and make it take longer to reach threshold- slows HR

Question 6

Explain how changes in maximum diastolic potential can alter pacemaker activity (heart rate)

Answer 6

Increasing the max. Diastolic potential= greater hyperpolarization, so you have further to go to reach threshold again= slower HR

Question 7

Devine overdrive suppression

Answer 7

When a PM is stimulated at a frequency higher than it’s intrinsic frequency, stopping the stimulation results in temporary suppression of PM activity
-SA node normally over drives all ectopic PMs, so if SA node or AV node infarct occurs, you get a delay in ventricular pacing until the ectopic pacemakers recover, then start beating at their own frequency (40bpm)

Question 8

What are some clinical applications of overdrive suppression?

Answer 8

SA or AV node block (2 or 3 deg. Heart block)
Stopping artificial PMs
SSS (site of pacemaker keeps jumping around)

Question 9

Define sinus arrhythmia

Answer 9

NORMAL variability in pacemaker cycle length (HR) caused primarily by respirator changes in PS (vagal nerve) activity to the SA node
-Sinus arrhythmia is more pronounced in aerobically trained people b/c of high vagal tone

Question 10

Explain how changes in SA node pacemaker rate manifest on EKG

Answer 10

Increase in rate= AP closer together

Decrease in rate= AP further apart

Question 11

Explain the cellular/ionic mechanism responsible for Purkinje fiber pacemaker activity

Answer 11

Primarily due to I(f) current and deactivation of K current

Question 12

What is the funny current?

Answer 12

I(f)- pacemaker channel, only channel activated by hyperpolarization

• found in all pacemaker tissues (including gut)
• Cell repolarizes, turns on this channel, leaks in some Na, causes some depolarization

Question 13

Explain how pacemaker shifts can change HR.

Answer 13

Changes in PM site can cause abrupt changes in HR b/c of hierarchy of PM activities

Question 14

What are four mechanisms responsible for changes in HR?

Answer 14

Change in slope of diastolic depolarization
Change in maximum diastolic potential
Change in threshold
Pacemaker shifts

Question 15

How does vagal nerve stimulation (PS) affect pacemaker activity?

Answer 15

Inhibits pacemakers within SA node, atria and AV node

• Specifically increases K permeability
• Inhibits cAMP synthesis
• Inhibits cAMP-dependent slow inward L type Ca channel (slows conduction through AV node) and I(f)
• As a result, there are decreases in in slope of diastolic depolarization and hyperpolarization of maximum diastolic potential

Question 16

How do inspiration and expiration affect cycle length and PS nerve activity?

Answer 16

• Inspiration causes decrease in cycle length, increase in HR by inhibition of PS nerve activity
• Expiration causes increase in cycle length, decrease in HR by stimulation of PS nerve activity.

Question 17

How does sympathetic stimulation affect pacemaker activity?

Answer 17

Sympathetic nerves stimulate all cardiac pacemakers

• Increases cAMP synthesis
• Increases slow inward L-type Ca current and I(f) current
• Increases slope of diastolic depolarization