Learning Objectives:
1) Describe the difference between normal and abnormal automaticity and list the factors that can
increase automaticity.
1. Normal automaticity: the property of some cardiac myocytes to undergo spontaneous
depolarization, initiating an electrical impulse
a. This is the consequence of phase 4 depolarization
b. Rate of automaticity depends on:
i.
Rate of phase 4 depolarization - faster phase 4 = steeper curve = more frequent
AP
ii.
Maximum negative membrane potential - the potential at the transition from
phase 3 → 4
iii.
Threshold potential - the potential at the transition from phase 4 → 0
c. Normally occurs within specialized pacemaker cells w/n the sinus (SA) node, as well as
w/n latent pacemakers in the atria, AV junction, and ventricles
d.
i.
If sinus node fails the others are backup - they’re called escape rhythms
e.
2. Abnormal automaticity: an increased automaticity the occurs only w/n non-pacemaker cells
3. Factors that increase automaticity:
2) Understand the factors that underlie reentrant circuits which allows for the development of latent
pacemakers
1. Re-entrant circuits: occurs when an electrical impulse doesn’t cease at the end of one cardiac
cycle, but rather persists and re-excites the heart as part of a self-propagating mechanism
a. Re-entry is responsible for majority of clinically relevant tachyarrhythmias
2. 2 categories of re-entry based on whether the circuit is large enough to be mapped via catheter:
a. Macroreentry - circuit is large enough to be mapped (ex. Atrial flutter, AVRT)
b. Microreentry - circuit is too small to be mapped (ex. Atrial fibrillation, intraatrial
reentrant tachycardia)
c. AVNRT is a mix of both, often falls into micro, but there are cases of macro so it’s just on
its own
3. Can also be described using a different dichotomy - not as useful in clinical settings
a. Anatomic reentry - circuit encircles an anatomic obstacle (ex. VT around a post-infarct
scar)
b. Functional reentry - circuit depends upon the intrinsic heterogeneity in the
electrophysiologic properties of the myocardium (ex. Atrial fibrillation)
4. Factors underlying these circuits:
a.
b.
3) Discuss the mechanisms underlying early and late afterdepolarizations.
1. Triggered activity occurs when depolarizing oscillation of the membrane potential occur during
or after an action potential, in which one oscillation is strong enough to trigger a new action
potential
a. Oscillations are called after-depolarizations
b.