Systolic goes up and diastolic goes down with widening pulse pressure. With increased ICP it's an ominous sign. Earliest sign is changes in LOC. There is a triad (Cushing's?) of signs of increased ICP: widening pulse pressure, rise in systolic BP, bradycardia. With these manifestations, the pressure increases on the brain stem. One of the functions in the brainstem is cerebral auto-regulation. When this malfunctions and BP goes up this increases the cerebral blood volume which leads to increased estravassation edema in the brain. Your result is increased ICP. If the opposite occurs with a decrease in BP, you can still wind up with the same result. If BP decreases so does the cerebral blood volume. This leads to hypoxia and hypercarbia and they also increase ICP.
Now as far as time goes, I don't remember there being a timetable as what to expect. What they did tell us was about a volume pressure curve. Will try to cut and paste: no soap! Will try later.
Anyway, the gist of this curve was measuring the units of volume against ICP. You want to keep the ICP low (about 15?) With there being 1-2 mL too much of volume the patient's ICP stays low (around 10), but when you're getting to 3 the compensation is beginning to fail and it jumps to 15 with just one more mL. When it gets to 4mL it jumps to 25 mmHg. One thing they really emphasized with us is that as a nurse you don't know where the patient is on this curve and to do NOTHING that would increase ICP.
The other thing I found was on ICP monitoring. This measurement has a time factor. If there are waves measuring 50 mmHg, but they last just a short time (a minute or two), they are considered a warning sign. When they get between 50-100 mmHg and stay there: the pts ICP is up. I think the other reason there are no hard and fast time standards post-injury, is that ICP can increase a few days after the injury related to what they call "secondary insult" from the edema caused by the injury rather than the injury itself.
Hope this helps and sorry it was so long. Can you tell we were just tested on this. Repost this in a few weeks and I'll be a lot more fuzzy.
Have a good night!