Which of the following correctly lists the intrinsic spontaneous firing rates of the cardiac conduction system from fastest to slowest?

• A SA node 120 to 140 per minute, AV junction 80 to 100 per minute, His-Purkinje system 60 to 80 per minute
• B SA node 60 to 80 per minute, AV junction 30 to 40 per minute, His-Purkinje system 15 to 20 per minute
• C SA node 80 to 100 per minute, AV junction 60 to 80 per minute, His-Purkinje system 40 to 60 per minute
• D SA node 100 to 120 per minute, AV junction 40 to 60 per minute, His-Purkinje system 20 to 40 per minute ✓

Explanation

The hierarchy of automaticity follows the intrinsic rates: SA node 100 to 120 per minute, AV junctional tissue 40 to 60 per minute, and His-Purkinje fibres 20 to 40 per minute. The faster SA node suppresses the lower pacemakers through overdrive suppression, so the resting rhythm is sinus at 60 to 100 per minute under vagal dominance. Option B reflects the observed resting rates rather than intrinsic pacemaker rates, which is the trap.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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