class II and class IV antiarrhythmics are effective in slowing conduction in the SA and AV nodes. This is because their end result in SA and AV action potential generation is the same. Both classes prevent calcium influx and slow phase 0. PKA which is activated via beta adrenergic receptor activation phosphorylates L-Ca2+ leading to Ca2+ influx and phase 0, so beta blockers prevent this event while Ca2+ blockers directly inhibit L-Ca2+