Which class of antiarrhythmic drugs affect the SA and AV nodes?

• A. Class I
• B. Class II
• C. Class III
• D. Class IV

Answer: (B)

Blocking sympathetic stimulation to the heart slows the automaticity of the SA node and the conduction through the AV node. Class II antiarrhythmics are beta-blockers, and they achieve this by blocking beta-adrenergic receptors, which lowers cAMP in nodal cells. This reduces the slope of phase 4 in the SA node and decreases calcium-dependent conduction in the AV node, leading to a slower heart rate and slower AV conduction (often seen as a longer PR interval). This rate-control effect is especially useful for tachyarrhythmias with rapid ventricular response, such as atrial fibrillation.

While calcium channel blockers (Class IV) also slow AV nodal conduction, the classic nodal effect taught for this class is through beta-blockade. Sodium channel blockers (Class I) mainly affect depolarization and conduction in ventricular tissue, and potassium channel blockers (Class III) impact repolarization rather than nodal conduction.