Which of the following is an example of a functional abnormality affecting conduction?

• A. Ischemic injury related to CAD depolarizes cells and inactivates fast Na+ channels, decreasing slope of phase 0
• B. Structural heart disease with fibrosis causing slow conduction
• C. Conotruncal congenital anomaly
• D. Valve disease causing AV dilation

Answer: (A)

The key idea is that conduction can be slowed by changes in ion channel function rather than by structural changes in the heart tissue. Conduction velocity depends on how steeply the action potential rises, the phase 0 upstroke, which is driven by the fast sodium current through Na+ channels. Ischemia from CAD alters the cellular environment so resting membrane potential becomes less negative and fast Na+ channels become inactivated or unavailable. With fewer usable Na+ channels, the inward current during phase 0 is reduced, the upstroke slope is shallower, and impulse conduction slows. This is a functional alteration because the tissue’s ionic machinery is altered without necessarily changing the tissue’s structure.

In contrast, structural heart disease with fibrosis slows conduction by physically separating cells and creating areas of slow conduction or block, which is a structural substrate. A conotruncal congenital anomaly and valve disease causing AV dilation are also structural issues. They can disrupt conduction through anatomical changes rather than through ion-channel dysfunction.