Changes in phase 0 slope will have what effect on conduction velocity?

• A. They always increase velocity
• B. They always decrease velocity
• C. They can alter conduction velocity
• D. They have no effect

Answer: (C)

Phase 0 slope represents how rapidly a cardiac cell depolarizes during the upstroke, driven mainly by the fast inward Na+ current. The speed at which the excitation wave spreads through the heart depends on how quickly each cell reaches threshold and activates its neighbors; a steeper phase 0 slope means a faster upstroke and a quicker activation of adjacent cells, which increases conduction velocity. A flatter slope slows the upstroke and slows the spread of impulse, reducing conduction velocity. But conduction velocity isn’t determined by phase 0 slope alone; factors like sodium-channel availability, cell-to-cell coupling via gap junctions, tissue health, and ischemia can modify CV independently of the slope. In nodal tissue, phase 0 is Ca2+-mediated and the relationship with conduction velocity differs, so the effect isn’t uniform across all cardiac tissues. Thus, changes in phase 0 slope can alter conduction velocity, but the outcome depends on the broader cellular and tissue context.