CCI Certified Cardiographic Technician (CCT) Practice Exam

Phase 0 is the rapid upstroke of the cardiac action potential. In fast-response cardiac cells like atrial and ventricular myocytes, the resting membrane potential sits around -90 mV. When these cells are excited, fast voltage-gated sodium channels open and a large inward Na+ current drives the membrane potential quickly from about -90 mV toward +20 mV in a very short time. This swift depolarization is what defines Phase 0. After this sharp rise, the action potential proceeds to Phase 1 (brief repolarization), Phase 2 (plateau due to calcium influx balanced by potassium efflux), Phase 3 (repolarization as potassium exits), and Phase 4 (resting potential ready for the next impulse). In nodal tissue, the upstroke can be calcium-driven, but for the typical fast fibers in exams, the initial rapid depolarization is due to sodium entry during Phase 0.