Which statement describes the mechanism of Type 1A antiarrhythmics as described in the material?

Which statement describes the mechanism of Type 1A antiarrhythmics as described in the material?

• A. Increase the refractory period of the heart
• B. Decrease the refractory period of the heart
• C. Block Na+ channels in skeletal muscle
• D. Block Ca2+ channels

Answer: (A)

Type 1A antiarrhythmics work by moderately blocking cardiac Na+ channels and, importantly, also blocking some K+ channels. This combination slows the initial depolarization (phase 0) and, as a result, lengthens the action potential duration (phase 3). Prolonging the action potential also extends the effective refractory period of cardiac tissue, which helps suppress reentrant circuits and ectopic pacemaker activity. That’s why increasing the refractory period is the correct description. They don’t primarily decrease the refractory period; they actually lengthen it. They do not act mainly on Ca2+ channels—that would be a different class of antiarrhythmics. And while Na+ channels exist in skeletal muscle, the clinically relevant mechanism here targets cardiac Na+ channels, not skeletal muscle.

Type 1A antiarrhythmics work by moderately blocking cardiac Na+ channels and, importantly, also blocking some K+ channels. This combination slows the initial depolarization (phase 0) and, as a result, lengthens the action potential duration (phase 3). Prolonging the action potential also extends the effective refractory period of cardiac tissue, which helps suppress reentrant circuits and ectopic pacemaker activity. That’s why increasing the refractory period is the correct description.

They don’t primarily decrease the refractory period; they actually lengthen it. They do not act mainly on Ca2+ channels—that would be a different class of antiarrhythmics. And while Na+ channels exist in skeletal muscle, the clinically relevant mechanism here targets cardiac Na+ channels, not skeletal muscle.