L8 Cardiac Pacemaker Mechanisms

Question 0

Hierarchy of cardiac pacemaker activity

Answer 0

[[fastest]] SA node -> latent atrial/vent pacemakers -> AV node/His junction -> Septal bundle -> Purkinje fibers [[slowest]]

SA node is the boss that beats at the appropriate rate in the human body. If it goes bad, the next in line is the latent, etc.

Question 1

Automaticity

Answer 1

Definition: the fact that the heart can generate an impulse by itself.

Question 2

How do you determine from an EKG what pacemaker is firing?

Answer 2

The speed of the HR

Question 3

Ionic mechanisms in SA node vs. purkinje fiber pacemaker activity

Answer 3

SA node: multiple mechanisms
1. T-type Ca current
2. hyperpolarization-activated inward Na current from I(f)
I(f) is the only current in all pacemakers, also funny bc activated the opposite way of most, upon hyperpolarization
3. deactivation of I(k)
4. inward Na/Ca exchange current activated by SR Ca release (Na current contributes to formation of next AP)

Purkinje Fibers:
Use only the Na current from I(f) and the deactivation of the I(k) channels to stop K current out

Question 4

How can you ∆ HR?

Answer 4

Change the slope of diastolic depolarization -
===NE acts on this via B adrenergics to increase HR===

Change the threshold potential - making it easier or harder to depol.

Change the diastolic maximum potential - (the most negative point of repolarization before another AP) changes how easy it is to depolarize up to threshold ====Ach decreases this and slows HR===

Question 5

How does a change in HR look like on an EKG?

Answer 5

R-R interval is reduced/increased

Question 6

Overdrive suppression

Answer 6

The idea that when a pacemaker is stimulated faster than its normal rate and then suddenly stopped, it’ll take a little bit of time until another pacemaker jumps in and takes over

Question 7

Ach impact on HR

Answer 7

Vagal stimulation inhibits pacemakers in SA, AV, and atria regions. Increases K permeability,
decreases cAMP,
inhibits L-type Ca current,
decreases slope of diastolic depol.,
hyperpolarizes the maximum diastolic potential

All these things slow heart rate

Question 8

NE impact on HR

Answer 8

NE stimulates ALL pacemakers.
Increases slow inward L-type Ca current,
Increases cAMP
increases slope of diastolic depol.

All give a faster HR

Question 9

Sinus arrhythmia

Answer 9

A normal variability in pacemaker cycle length due to the varied vagal contributions to the SA node with respiration.
Inhale, stretch receptors inhibit vagal stimulation and HR increases/ cycle SHORTENS.
Exhale, opposite happens. cycle LENGTHENS

Good to be in good aerobic health because sinus arrhythmia helps prevent bad arrhythmias

Question 10

Changes in SA node pacemaker rate are shown on the EKG by…

Answer 10

the entire R-R interval would change…

Change in pacemaker location would manifest in an EKG as a sudden difference in cycle length/HR

Question 11

How would Symp nerve activity affect an EKG recording?

Answer 11

Symp affects the HR and slope of diastolic depolarization, which would make the AP quicker to occur