Call me dumb if you like but could you provide an example for the surgery mechanic?
The patient can go one Time unit plus his Constitution bonus before suffering Constitution damage from a successful check. At each time unit the surgeon makes a check at the base DC +1 per Time Unit that has passed. Success does 1 temporary Constitution damage if the time is past patient tolerance.
Failure on the Surgery check forces the patient to make a Fortitude save at the Surgery DC +1 per Time Unit has passed. The Patient tolerance has no impact at this point. If the save is made the patient suffers he takes 1d4 Constitution damage. If the save fails he takes 2d4 temporary Constitution damage. In either case for every 5 points of temporary Constitution damage the patient needs to make another Fortitude save at DC 15 or 1 point of the Constitution loss is permanent.
If the Constitution ever reaches 0 the patient dies. Due to the consequences of the check, the surgeon cannot take 20.
So say for example I have Constitution 12. And I'm going under the knife for...
Moderate Surgery; DC: 25; Time Unit: 1 hour
So I can go for 2 hours under the knife without worrying (1 + Con modifier of 1, x Time unit of 2 hours).
Each hour the surgeon makes a test at an increasing difficulty. IE:
1st hour - DC 25 (or is it 26? Has the first time unit passed or not?)
2nd hour - DC 26
3rd hour - DC 27 - from now on even if it is successful I take 1 temporay consitution damage.
4th hour - DC 28
5th hour - DC 29
etc...
If the surgeon ever fails a roll you automatically suffer at least 1d4 temp constitution damage, and could suffer 2d4.
For every 5 points you lose during surgery there is a chance 1 point is permanent.
I think all thats clear....
My BIG question is how do you know when the surgery is over? As far as I can see I can't find any rules for how long the actual surgery takes.
By the looks of things you need at least a 7th level Dedicated Hero surgeon with Treat Injury Skill Emphasis and Healing Knack to even set a broken leg. As you will be wanting them to Take 10 on the roll, and even then you might not make it off the operating table if it takes more than an hour. You might want to drop all the difficulties a bit.