Which of the following is an example of a positive chronotropic effect?

Which of the following is an example of a positive chronotropic effect?

• A. Norepinephrine release.
• B. Parasympathetic stimulation.
• C. Beta blocker therapy.
• D. Increased afterload.

Answer: (A)

A positive chronotropic effect means the heart rate increases because the SA node fires more rapidly. Norepinephrine release does this by activating beta-1 receptors in the SA node, which raises cAMP, enhances the funny current, and improves calcium handling. All of this speeds up diastolic depolarization so the pacemaker fires more often, increasing heart rate. In contrast, parasympathetic stimulation reduces heart rate (negative chronotropy) by releasing acetylcholine and acting on M2 receptors. Beta blocker therapy blocks those beta receptors, also reducing rate (negative chronotropy). Increased afterload changes the pressure the heart must pump against and can trigger reflex adjustments, but it does not directly increase the SA node’s firing rate, so it isn’t a positive chronotropic effect.

A positive chronotropic effect means the heart rate increases because the SA node fires more rapidly. Norepinephrine release does this by activating beta-1 receptors in the SA node, which raises cAMP, enhances the funny current, and improves calcium handling. All of this speeds up diastolic depolarization so the pacemaker fires more often, increasing heart rate. In contrast, parasympathetic stimulation reduces heart rate (negative chronotropy) by releasing acetylcholine and acting on M2 receptors. Beta blocker therapy blocks those beta receptors, also reducing rate (negative chronotropy). Increased afterload changes the pressure the heart must pump against and can trigger reflex adjustments, but it does not directly increase the SA node’s firing rate, so it isn’t a positive chronotropic effect.