Vagal activation is an example of which chronotropic effect on heart rate?

• A. Positive chronotropic effect
• B. Negative chronotropic effect
• C. No chronotropic effect
• D. Inotropic chronotropic effect

Answer: (B)

Vagal activation lowers heart rate because it increases parasympathetic influence on the heart, slowing the firing of the SA node and slowing conduction through the AV node. Mechanistically, acetylcholine acting on M2 receptors opens potassium channels and reduces cAMP, which together slow the diastolic depolarization of pacemaker cells and decrease calcium currents. The net result is a slower heart rate, a negative chronotropic effect. This is in contrast to a positive chronotropic effect, which would speed up the heart via sympathetic stimulation; there is no change in rate, or the use of a nonstandard term like “inotropic chronotropic,” which conflates rate with contractile force. So, vagal activation is a negative chronotropic effect.