Heart 4 Flashcards
Hierarchy of pacemaker activity
Pacemakers in the heart are arranged anatomically according to the decreasing inherent beating rate
Which nodes have the highest beating rates?
SA node is the primary pacemaker and has the fastest inherent beat
Atria and ventricles have their ectopic pacemakers that are slower
What are the four mechanisms that underlie the SA node pacemaker activity during diastolic depolarization?
- T-type calcium current
- Hyperpolarization-activated inward current (If)
- Deactivation of potassium current (Ik)
- Inward sodium/calcium exchange current activated by intracellular SR calcium release
What is the main mechanism for diastolic depolarization in the Purkinje fibers?
The “funny current” (If) with some help from the deactivation of the potassium currents
Four mechanisms responsible for changes in heart rate
- Change in the slope of diastolic depolarization
- Change int he maximum diastolic potential
- Change in threshold
- Pacemaker shifts
Explain how pacemaker shifts can cause a change in heart rate.
Changes in pacemaker site can cause abrupt changes in heart rate because of the hierarchy of pacemaker activities
What happens when a pacemaker is stimulated at a frequency higher than its intrinsic frequency?
Stopping the stimulation results in a temporary suppression of pacemaker activity
Overdrive suppression
The SA node “overdrives” all subsidiary pacemakers in the heart and suppresses the ectopic pacemaker activity
Clinical applications for overdrive suppression
- SA nodal or AV nodal block (second or third degree heart block)
- Stopping artificial pacemakers
- Sick sinus syndrome
ACh inhibits what to types of currents?
Slow inward L-type calcium current and “funny” current by inhibiting cAMP
What does ACh do to the slope of diastolic depolarization?
Decreases the slope and also hyperpolarizes maximum diastolic potential
What is normal sinus arrhythmia constantly caused by?
Normal variability in the pacemaker cycle length due to respiratory changes in parasympathetic nerve activity to the SA node
When is sinus arrhythmia more pronounced?
In aerobically trained individuals
Inspiration causes . . .
Decrease in cycle length (increase in heart rate) by inhibition of parasympathetic nerve activity
Expiration causes . . .
Increase in cycle length (decrease in heart rate) caused by stimulation of parasympathetic nerve activity
