During automatic supraventricular tachycardia, which phase of the action potential is elevated?

During automatic supraventricular tachycardia, which phase of the action potential is elevated?

• A. Phase 0
• B. Phase 4
• C. Phase 2
• D. Phase 3

Answer: (B)

In automatic tachycardia, the driving factor is increased automaticity of the pacemaker tissue, which comes from the diastolic depolarization phase. This is phase 4 of the cardiac action potential. When the slope of phase 4 is elevated, the membrane potential climbs to the threshold more quickly, causing impulses to fire sooner and at a faster rate. The other phases represent rapid depolarization (phase 0), the plateau (phase 2), and repolarization (phase 3) in working myocardium and are not the source of automatic firing. So the elevated phase is phase 4.

In automatic tachycardia, the driving factor is increased automaticity of the pacemaker tissue, which comes from the diastolic depolarization phase. This is phase 4 of the cardiac action potential. When the slope of phase 4 is elevated, the membrane potential climbs to the threshold more quickly, causing impulses to fire sooner and at a faster rate. The other phases represent rapid depolarization (phase 0), the plateau (phase 2), and repolarization (phase 3) in working myocardium and are not the source of automatic firing. So the elevated phase is phase 4.