SA Node Intrinsic Rate — FRCA Primary MCQ

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EasyPhysiology - CardiovascularSA Node Intrinsic RateFRCA Primary

In a healthy resting adult, what is the conventional range for the rate of impulses generated by the sinoatrial node during sinus rhythm?

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Correct answer: C — 60–100 bpm

Explanation lettering: C = shown as A · A = shown as B · E = shown as C · B = shown as D · D = shown as E

E is correct. The conventional adult sinus-node discharge range at rest is 60–100 bpm. The SA node normally governs cardiac rhythm because its pacemaker cells have the fastest spontaneous phase 4 depolarisation within the cardiac conduction system. A rate of 40–60 bpm is characteristic of a junctional escape pacemaker arising around the AV junction, while 20–40 bpm is typical of a distal His–Purkinje or ventricular escape rhythm. Rates of 100–120 bpm and 120–150 bpm represent tachycardic sinus rates rather than the conventional resting range. The term “intrinsic heart rate” should be used cautiously: when defined experimentally after complete sympathetic and parasympathetic blockade, it is age-dependent and often lies near 100 bpm.

Reference: Lancashire Teaching Hospitals NHS Foundation Trust, Cardiac Catheterisation Suite Student Placement Information: Basic Cardiac Knowledge, Electrical Conduction section, 2021. https://healthacademy.lancsteachinghospitals.nhs.uk/app/uploads/2021/11/Cardiac-Catheter-Lab.pdf