Which statement is true regarding secondary pacemaker sites?

Which statement is true regarding secondary pacemaker sites?

• A. They have a higher intrinsic rate than the SA node
• B. They have a lower intrinsic rate than the SA node
• C. They have the same intrinsic rate as the SA node
• D. They have no intrinsic rate

Answer: (B)

Secondary pacemaker sites have automaticity, but their intrinsic firing rate is slower than the SA node. The SA node is the fastest natural pacemaker, typically around 60–100 beats per minute. If the SA node fails or its impulse doesn’t reach the ventricles, a backup pacemaker (such as the AV node or Purkinje tissue) can take over, but its intrinsic rate is lower—AV node about 40–60 bpm, Purkinje around 20–40 bpm. This is why the true statement is that secondary pacemaker sites have a lower intrinsic rate than the SA node. That slower rate explains the slower escape rhythms that occur when the primary pacemaker is suppressed or blocked. The other possibilities don’t fit because these tissues do not normally run faster than the SA node, nor do they lack an intrinsic rate.

Secondary pacemaker sites have automaticity, but their intrinsic firing rate is slower than the SA node. The SA node is the fastest natural pacemaker, typically around 60–100 beats per minute. If the SA node fails or its impulse doesn’t reach the ventricles, a backup pacemaker (such as the AV node or Purkinje tissue) can take over, but its intrinsic rate is lower—AV node about 40–60 bpm, Purkinje around 20–40 bpm. This is why the true statement is that secondary pacemaker sites have a lower intrinsic rate than the SA node. That slower rate explains the slower escape rhythms that occur when the primary pacemaker is suppressed or blocked. The other possibilities don’t fit because these tissues do not normally run faster than the SA node, nor do they lack an intrinsic rate.