Cardiac Rhythm

Phase 0 (Rapid Upstroke)

Massive sodium inflow. Activated by -65 mV threshold

Phase 1 (Early Repolarization)

K+ efflux and cessation of Na+ inflow

Phase 2 (Plateau)

Diminution of most current flows

Phase 3 (Late Repolarization)

Inactivation of Ca++ flows and re-activation of Ka+ efflux

Phase 4 (Diastolic Depolarization)

Cardiac Conduction

SA node is supplied by the RCA and located at the junction of the RA and the SVC. Three conduction pathways connect the SA and AV nodes – anterior (Bachmann’s bundle), middle, and posterior internodal systems. The AV node, supplied by the RCA, is also in the RA, near the coronary sinus. The Bundle of His exits the AV node, traveling along the membranous septum, after which it divides into the left bundle (then anterior and posterior) and right bundle, both of which ultimately subdivide into the Purkinje network.