Heart Lecture 4: Cardiac Pacemaker Mechanisms Flashcards
What determines ventricular rate?
AV refractory period
What is normal duration range for QT interval?
250-400ms
How does hypokalemia lead to prolonged QT?
decreases K+ permeability making it hard to leave the cell, therefore prolonging AP
Why is sinus node inherent pacemaker?
fastest beating rate
Hierarchy or pacemaking:
1) SA (fastest)
2) atrial and ventricular subsidiary/latent pacemakers
3) AV nodal/His bundle
4) bundle branches
5) Purkinje fibers (slowest)
Diastolic depolarization
generates automaticity of SA node (phase 4 of action potential)
What current is special to pacemaking tissue?
IF = Funny current (hyperpolarization active inward current)
Funny because it is the only channel in the heart that is activated on HYPERpolarization, not depolarization
What does IF carry?
sodium
What activates IF?
hyperpolarization
Block IF channel and what happens?
Lose pacemaker activity in sinus node by ~30%
Name 4 mechanisms that contribute to pacemaking in the SA node:
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
Purkinje fibers latent pacemaker activated by what:
1) IF current
2) deactivation of K+ current
Main way to modulate heart rate
enhance or depress slope of diastolic depolarization (gets to threshold later)
Name for most negative point in the cycle of SLOW AP
maximum diastolic potential
ACH does two things
1) decrease slope of diastolic depolarization
2) depress max diastolic voltage
both slow down rate
Need to have what stimulation when heart rate goes up
sympathetic stimulation of AV node (otherwise accelerated heart rate wouldn’t go through to ventricles)
4 mechanisms responsible for changes in heart rate
1) change in slope of diastolic depolarization
2) change in max diastolic potential
3) change in threshold
4) pacemaker shifts
What does overdrive suppression refer to?
How the sinus node suppresses electrical activity of other pacemakers
What is the danger of overdrive suppression?
Stopping SA stimulation results in temporary suppression of pacemaker activity
What takes over when SA node cuts out?
latent pacemakers (takes about 10-20 seconds for them to pick up)
What effect does ACh have on pacemaking?
inhibits it by making resting membrane potential more negative (hyperpolarizes it)
What effect does ACh have on K+ permeability?
increases it (more K+ leaves the cell, making the interior more negative)
What effect does ACh have on Ca++ current?
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)
What effect does ACh have on the slope of diastolic depolarization?
decreases it (by hyperpolarizing max diastolic potential)
What does sinus arrhythmia refer to?
The change in rhythm that comes from vagal tone as we breathe (decreased heart rate in expiration and increased in inspiration)
Effects of inspiration on cycle
DECREASES cycle length (shorter) and INCREASES heart rate
inhibition of parasymp vagal activity
Effects of expiration on cycle
INCREASES cycle length (longer) and DECREASES hear rate
stimulation of parasymp activity
Why do aerobically trained individuals have lower resting heart rates?
increased vagal tone
What effect does norepinephrine have on Ca++ current?
INCREASES slow inward L-type Ca++ current and IF current by increasing cAMP synthesis
What effect does norepinephrine have on the slope of diastolic depolarization?
INCREASES it
What effect does NE have on K+ permeability?
none
What effect does NE have on max diastolic potential?
none
How do you decipher SVT vs sinus tachycardia?
clinical context
SVT = pathological
sinus tachycardia = during exercise
