Which of the following medications increases heart rate by accelerating the SA node discharge rate and blocking the vagus nerve?
a. Digitalis
b. Atropine
c. Amiodarone
d. Beta-blocker
B
Atropine is a vagolytic drug that is used to increase the heart rate. Vago refers to the vagus nerves (right and left), which are the main nerves of the parasympathetic division of the autonomic nervous system. Lytic refers to "lyse," which means "to interfere with." Atropine works by blocking acetylcholine at the endings of the vagus nerves. The vagus nerves innervate the heart at the SA and AV nodes. Thus, atropine is most effective for narrow-QRS bradycardia. By blocking the effects of acetylcholine, atropine allows more activity from the sympathetic division of the autonomic nervous system. As a result, the rate at which the SA node can fire is increased. Areas of the heart that are not innervated or that are minimally innervated by the vagus nerves (e.g., the ventricles) will not respond to atropine. Thus, atropine is usually ineffective for the treatment of wide-QRS bradycardia. Atropine also increases the rate at which an impulse is conducted through the AV node. It has little or no effect on the force of contraction.
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