Phase 0 Sodium Influx — FRCA Primary MCQ
Instant feedback + full explanation. One question, done properly.
Educational content. Not a substitute for clinical judgement or local policy.
Reveal the answer and explanation
Correct answer: B — Rapid sodium influx through voltage-gated sodium channels
Phase 0 (rapid depolarisation) of the ventricular action potential is caused by rapid influx of Na+ through voltage-gated sodium channels. This produces the sharp upstroke of the action potential reaching approximately +30 mV. The speed of Phase 0 determines conduction velocity. SA and AV node Phase 0 is slower because it depends on Ca2+ (not Na+) influx.
Reference: Power Kam 2018 Principles of Physiology for the Anaesthetist