Vaughan Williams Class 2 effects include which of the following on the cardiac action potential?

Vaughan Williams Class 2 effects include which of the following on the cardiac action potential?

• A. Slows nodal action potential; Depresses phase 4
• B. Lengthens phase 0 and ERP
• C. Increases depolarization threshold
• D. Blocks calcium channels

Answer: (A)

Class II antiarrhythmic drugs are beta-adrenergic blockers. In the cardiac nodal tissues (SA and AV nodes), the pace of automaticity is set by the slope of phase 4. Beta-blockade reduces sympathetic stimulation, lowers cAMP, and decreases calcium entry, which together slow the rate of spontaneous depolarization. This depresses the slope of phase 4 and slows nodal firing, leading to a slower nodal action potential. It also slows AV nodal conduction, which is the hallmark effect of these drugs. The other options don’t fit this mechanism. Lengthening phase 0 and the effective refractory period is not the primary action of beta-blockers (that’s more characteristic of other classes that alter repolarization). Increasing the depolarization threshold isn’t the typical direct effect of Class II agents, and outright blocking calcium channels describes calcium channel blockers (Class IV), not beta-blockers.

Class II antiarrhythmic drugs are beta-adrenergic blockers. In the cardiac nodal tissues (SA and AV nodes), the pace of automaticity is set by the slope of phase 4. Beta-blockade reduces sympathetic stimulation, lowers cAMP, and decreases calcium entry, which together slow the rate of spontaneous depolarization. This depresses the slope of phase 4 and slows nodal firing, leading to a slower nodal action potential. It also slows AV nodal conduction, which is the hallmark effect of these drugs.

The other options don’t fit this mechanism. Lengthening phase 0 and the effective refractory period is not the primary action of beta-blockers (that’s more characteristic of other classes that alter repolarization). Increasing the depolarization threshold isn’t the typical direct effect of Class II agents, and outright blocking calcium channels describes calcium channel blockers (Class IV), not beta-blockers.