Positive chronotropic agents include which of the following?

Positive chronotropic agents include which of the following?

• A. Factors that increase heart rate, such as sympathetic stimulation and hormones like norepinephrine and epinephrine.
• B. Factors that decrease heart rate.
• C. Factors that increase stroke volume.
• D. Factors that reduce conduction velocity.

Answer: (A)

Positive chronotropic effects raise heart rate by increasing the automatic firing rate of the SA node. The classic drivers are sympathetic stimulation and hormones like norepinephrine and epinephrine. When the sympathetic system activates the heart, it releases norepinephrine that binds to β1-adrenergic receptors in the SA node. This triggers a cascade that increases cyclic AMP, which then enhances the activity of channels responsible for the pacemaker potential (notably the funny current I_f and L-type Ca2+ channels). The result is a steeper phase 4 slope, a faster reach to threshold, and a higher heart rate. Epinephrine from the adrenal medulla adds a similar effect, producing a systemic bump in heart rate. This is in contrast to actions that decrease heart rate, increase stroke volume, or slow conduction velocity, which do not represent positive chronotropic effects.

Positive chronotropic effects raise heart rate by increasing the automatic firing rate of the SA node. The classic drivers are sympathetic stimulation and hormones like norepinephrine and epinephrine. When the sympathetic system activates the heart, it releases norepinephrine that binds to β1-adrenergic receptors in the SA node. This triggers a cascade that increases cyclic AMP, which then enhances the activity of channels responsible for the pacemaker potential (notably the funny current I_f and L-type Ca2+ channels). The result is a steeper phase 4 slope, a faster reach to threshold, and a higher heart rate. Epinephrine from the adrenal medulla adds a similar effect, producing a systemic bump in heart rate.

This is in contrast to actions that decrease heart rate, increase stroke volume, or slow conduction velocity, which do not represent positive chronotropic effects.