Final answer:
The most significant finding to determine whether mannitol is producing its therapeutic effect for a client after intracranial surgery is increased urine output, as mannitol is an osmotic diuretic designed to reduce brain swelling by promoting water loss.
Step-by-step explanation:
A client receiving an I.V. infusion of mannitol (Osmitrol) after intracranial surgery is being administered this to reduce brain swelling. Mannitol is an osmotic diuretic, and its therapeutic effect is characterized by promoting water loss by osmosis. Thus, to determine whether mannitol is producing its therapeutic effect following surgery, the most significant finding would be increased urine output. This indicates that the mannitol is effectively reducing intracranial pressure by drawing excess fluid out of the brain into the bloodstream, which is then excreted as urine.
Other options, such as elevated blood pressure, decreased level of consciousness (LOC), or a decreased heart rate, are not direct indicators of mannitol's efficacy as an osmotic diuretic. Elevated blood pressure would be a concern and could indicate other complications, while a decreased LOC might suggest worsening brain swelling or other neurological issues, both outcomes mannitol aims to prevent. Decreased heart rate has limited direct connection to the action of mannitol and intracranial pressure changes.