When the heart rate becomes exceedingly high:

• A. Cardiac output decreases.
• B. Ventricular filling time decreases.
• C. Atrial contraction strength increases.
• D. Myocardial oxygen demand decreases.

Answer: (B)

When the heart rate climbs to an extreme level, the ventricles have less time to fill between beats because diastole is shortened. That means ventricular filling time decreases. With less filling, preload drops and stroke volume falls, which can reduce overall cardiac output when the rate is very high. This direct effect on how long the ventricles fill is the key reason why the correct choice is that ventricular filling time decreases. Note that myocardial oxygen demand actually increases with higher rate, and the other effects described are not reliably produced by a very fast heart rate.