Cardiac Pacemaker Mechanisms (L8)

Question 1

What is the Hierarchy of Pacemaker Activity

Answer 1

SA node, then latent atrial pacemakers, then AV node/His bundle (junctional), then bundle branches, then Purkinje fibers

Question 2

How can you determine, from an EKG recording, whether the SA node or a junctional or ventricular ectopic pacemaker is exciting the heart?

Answer 2

If its the SA node, then you would have a normal atrial depolarization (p-wave). If its junctional, meaning the AV node/His bundles, then the p-wave would be missing (A-fib). If its a ventricular ectopic pacemaker, its not coming through the SA or AV node, so the EKG would have a slurred QRS complex, if its pacing normally, or it could go into V-fib, in which case, get the paddles and start praying

Question 3

What are the 4 mechanisms that underlie the SA node pacemaker activity?

Answer 3

1) T-type Ca2+ current
2) Hyperpolarization-activated inward current (If)
3) Deactivation of K+ current (Ik)
4) Inward Na/Ca exchange current activated by intracellular SR Ca2+ release

Question 4

What is the funny current?

Answer 4

The pacemaker channel, activated by hyperpolarization. Found in all pacemaker tissue, so well the cell repolarizes, it turns on this channel, leaks some Na in, and helps cause depolarization. It is regulated by kinases

Question 5

Diastolic depolarization is regulated by what two things?

Answer 5

1) SA node pacemaker activity

2) Purkinje fibers, primarily due to If (funny) current and deactivation of K+ current

Question 6

What are the mechanisms responsible for changes in heart rate?

Answer 6

1) Changes in the slope of diastolic depolarization
2) Changes in maximum diastolic potential
3) Change in threshold
4) Pacemaker shifts, changes in pacemaker site can cause abrupt changes in heart rate because of the hierarchy of pacemaker activity

Question 7

How would a change in heart rate appear on an EKG?

Answer 7

Basically everything would speed up. So the interval between beats (from the previous t-wave to the next p-wave) would shorten. Even though the heart is still being depolarized and repolarized, the EKG could show missed P-wave or such because, once you get to a high enough heart rate, everything starts to speed up on the EKG (idk if this is right guys, you might want to double check.)