Which Vaughan Williams class slows nodal action potential and depresses phase 4?

Which Vaughan Williams class slows nodal action potential and depresses phase 4?

• A. Class 1a
• B. Class 3
• C. Class 2
• D. Class 4

Answer: (C)

In nodal tissue, the rate at which automatic cells reach the threshold depends on the slope of phase 4, the slow diastolic depolarization. Substances that dampen sympathetic signaling or cAMP levels slow this slope, reducing automaticity and slowing heart rate. Beta-blockers, which comprise Class II antiarrhythmics, do exactly that. By blocking beta-1 receptors, they lower intracellular cAMP, which decreases theIf funny current and calcium entry that drive phase 4, so the SA node fires more slowly and AV nodal conduction is also slowed. This combination—slower nodal automaticity and reduced AV nodal conduction—is the hallmark described, making Class II the best fit. Class III mainly prolongs repolarization (phase 3) through potassium channel blockade, not the depression of phase 4. Class IV calcium channel blockers slow AV nodal conduction by reducing calcium influx and mainly affect the upstroke in nodal tissue rather than depressing the slope of phase 4. Class I agents primarily affect the fast upstroke of non-nodal (atrial/ventricular) tissue.

In nodal tissue, the rate at which automatic cells reach the threshold depends on the slope of phase 4, the slow diastolic depolarization. Substances that dampen sympathetic signaling or cAMP levels slow this slope, reducing automaticity and slowing heart rate. Beta-blockers, which comprise Class II antiarrhythmics, do exactly that. By blocking beta-1 receptors, they lower intracellular cAMP, which decreases theIf funny current and calcium entry that drive phase 4, so the SA node fires more slowly and AV nodal conduction is also slowed. This combination—slower nodal automaticity and reduced AV nodal conduction—is the hallmark described, making Class II the best fit.

Class III mainly prolongs repolarization (phase 3) through potassium channel blockade, not the depression of phase 4. Class IV calcium channel blockers slow AV nodal conduction by reducing calcium influx and mainly affect the upstroke in nodal tissue rather than depressing the slope of phase 4. Class I agents primarily affect the fast upstroke of non-nodal (atrial/ventricular) tissue.