What is the primary reason for administering mannitol IV to a patient?

Mannitol is an osmotic diuretic that is primarily used to decrease intracranial pressure. It works by increasing the osmolarity of the plasma, which draws water out of the brain tissue and into the bloodstream. This process helps to alleviate pressure in the cranial cavity, making it especially useful in conditions such as traumatic brain injury, cerebral edema, or other situations where elevated intracranial pressure is a concern.

When mannitol is administered intravenously, it is rapidly filtered by the kidneys and helps to maintain urine output. This action not only reduces fluid accumulation in the brain but also supports kidney function by promoting diuresis, which can further aid in reducing systemic fluid overload.

The other options listed do not align with the primary use of mannitol. While it can have various physiological effects, its main indication in clinical settings is the management of intracranial pressure rather than fever reduction, heart rate management, or blood volume increase.