Which statement describes the mechanism of Type 1B antiarrhythmics?

Which statement describes the mechanism of Type 1B antiarrhythmics?

• A. Increase the refractory period
• B. Decrease the refractory period
• C. Block Na+ channels in cardiac muscle
• D. Block K+ channels

Answer: (B)

Type 1B antiarrhythmics act as sodium channel blockers with rapid kinetics, and their defining effect in ventricular and Purkinje fibers is to shorten the action potential duration and the effective refractory period. They bind preferentially to Na+ channels in the inactivated state and dissociate quickly during repolarization, which means that at faster heart rates or in depolarized (ischemic) tissue, conduction is inhibited without prolonged refractoriness. This shortening of the refractory period is the best description of their mechanism among the options, because it captures the characteristic electrophysiologic change produced by these drugs. While they do block Na+ channels, that attribute is common to all Class I agents; the key feature specific to Type 1B is the reduction (not increase) in the refractory period. Blocking K+ channels would correspond to a Class III effect, not Class I/B.

Type 1B antiarrhythmics act as sodium channel blockers with rapid kinetics, and their defining effect in ventricular and Purkinje fibers is to shorten the action potential duration and the effective refractory period. They bind preferentially to Na+ channels in the inactivated state and dissociate quickly during repolarization, which means that at faster heart rates or in depolarized (ischemic) tissue, conduction is inhibited without prolonged refractoriness. This shortening of the refractory period is the best description of their mechanism among the options, because it captures the characteristic electrophysiologic change produced by these drugs. While they do block Na+ channels, that attribute is common to all Class I agents; the key feature specific to Type 1B is the reduction (not increase) in the refractory period. Blocking K+ channels would correspond to a Class III effect, not Class I/B.