What is the total duration of the SA node action potential, and what heart rate does this correspond to?

What is the total duration of the SA node action potential, and what heart rate does this correspond to?

• A. 0.8 seconds; about 75 bpm
• B. 0.08 seconds; about 750 bpm
• C. 1.6 seconds; about 37 bpm
• D. 0.4 seconds; about 150 bpm

Answer: (A)

The question is testing how heart rate relates to the timing of the SA node’s electrical activity. The heart beats at a rate determined by how long it takes the SA node to generate successive impulses—a cycle length. If the complete cycle from one SA node action potential to the next lasts about 0.8 seconds, the heart rate is 60 divided by 0.8, which is roughly 75 beats per minute. That resting rate fits normal physiology, since the SA node ordinarily sets a pace around 60–100 bpm. Why the other options don’t fit: a duration of 0.08 seconds would imply about 750 bpm, which is far beyond physiological limits; 1.6 seconds would correspond to about 37 bpm, which is too slow for a normal resting heart rate; 0.4 seconds would be about 150 bpm, which represents a high heart rate not typical at rest. So the 0.8-second cycle length yields a sensible, normal resting heart rate of about 75 bpm.

The question is testing how heart rate relates to the timing of the SA node’s electrical activity. The heart beats at a rate determined by how long it takes the SA node to generate successive impulses—a cycle length. If the complete cycle from one SA node action potential to the next lasts about 0.8 seconds, the heart rate is 60 divided by 0.8, which is roughly 75 beats per minute. That resting rate fits normal physiology, since the SA node ordinarily sets a pace around 60–100 bpm.

Why the other options don’t fit: a duration of 0.08 seconds would imply about 750 bpm, which is far beyond physiological limits; 1.6 seconds would correspond to about 37 bpm, which is too slow for a normal resting heart rate; 0.4 seconds would be about 150 bpm, which represents a high heart rate not typical at rest. So the 0.8-second cycle length yields a sensible, normal resting heart rate of about 75 bpm.