Which drug class slows conduction through the heart via a negative dromotropic effect?

Which drug class slows conduction through the heart via a negative dromotropic effect?

• A. Calcium channel blockers
• B. Beta blockers
• C. Sodium channel blockers
• D. Potassium channel blockers

Answer: (C)

Negative dromotropic effect means slower conduction of impulses through the heart. Blocking fast sodium channels—the ones that drive the rapid upstroke (phase 0) of the action potential in atrial and ventricular muscle and in the His-Purkinje system—reduces the slope of depolarization. With a slower upstroke, the impulse travels more slowly, which is exactly the slowing of conduction this concept refers to. This is what sodium channel blockers (Class I antiarrhythmics) do: they slow conduction velocity across the myocardium by limiting Na+ influx during depolarization. On an ECG, you’d expect signs of slower conduction, such as potential QRS broadening with greater block in ventricular tissue. While calcium channel blockers and beta blockers also slow conduction—especially through the AV node—by different mechanisms, the direct slowing of conduction in atrial/ventricular myocardium due to sodium channel blockade best exemplifies a negative dromotropic effect. Potassium channel blockers, by contrast, mainly prolong repolarization and refractoriness rather than slowing conduction.

Negative dromotropic effect means slower conduction of impulses through the heart. Blocking fast sodium channels—the ones that drive the rapid upstroke (phase 0) of the action potential in atrial and ventricular muscle and in the His-Purkinje system—reduces the slope of depolarization. With a slower upstroke, the impulse travels more slowly, which is exactly the slowing of conduction this concept refers to. This is what sodium channel blockers (Class I antiarrhythmics) do: they slow conduction velocity across the myocardium by limiting Na+ influx during depolarization. On an ECG, you’d expect signs of slower conduction, such as potential QRS broadening with greater block in ventricular tissue. While calcium channel blockers and beta blockers also slow conduction—especially through the AV node—by different mechanisms, the direct slowing of conduction in atrial/ventricular myocardium due to sodium channel blockade best exemplifies a negative dromotropic effect. Potassium channel blockers, by contrast, mainly prolong repolarization and refractoriness rather than slowing conduction.