Which cardiac impulse is typically least effective in causing full contraction of the ventricles?

The least effective cardiac impulse in causing full contraction of the ventricles is associated with a premature ventricular contraction (PVC). A PVC occurs when an ectopic focus in the ventricles depolarizes before the normal electrical impulse reaches them. This leads to an early contraction of the ventricle, but the preceding atrial contraction does not occur, which means that the filled volume of blood in the ventricle is reduced during this early contraction.

Because the ventricles do not have adequate time to fill with blood before the contraction, and the coordinated effort between atrial and ventricular contractions is disrupted, the overall stroke volume is decreased. Therefore, while a PVC can initiate a contraction, it is not effective in contributing to the overall, synchronized function of the heart, resulting in reduced efficiency of ventricular contraction and decreased cardiac output.

In contrast, impulses like those from the normal sinus node or atrial contractions typically lead to coordinated and effective heartbeats that ensure full contraction of the ventricles following proper filling.