What is the significance of the T wave on an ECG?

The T wave on an ECG is significant because it represents ventricular repolarization. This phase occurs after the ventricles have contracted and pumped blood out of the heart. During ventricular repolarization, the myocardial cells recover from depolarization and prepare for the next contraction. The T wave is crucial for understanding the heart's electrical activity, as it indicates the process of the ventricles returning to their resting state, which is essential for maintaining a normal rhythm and workload on the heart.

In the context of the other options, atrial depolarization is represented by the P wave, and the firing of the SA node is also associated with this initial phase. The isoelectric line represents a baseline where no electrical activity is occurring, and it does not correlate with any specific cardiac event like repolarization does. Thus, the correct answer about the T wave signifies a critical aspect of the cardiac cycle.