If you are damaged while using an object (such as opening a door, pulling a lever, etc) one of two checks are made.
Concentration check: Skill+d20 vs DC=damage taken
Strength check: Mod+10+d20 vs DC=damage taken
Whichever check has the higher modifier before the d20 roll is used. Eg. a barbarian with 10 Concentration (from his Con mod) and a +20 Strength mod would always receive the Strength check, whereas a sorcerer with 45 Concentration and a +4 Strength mod would always receive the Concentration check.
If the check is passed, the object manipulation continues uninterrupted.
A separate check must be made for each source of damage suffered during the use of the object.
Successful attacks which cause loss of control of your character will always interrupt object use.
For example, if you were pulling a lever and fail your save against a Stunning Blow that dealt only 5 damage, even if you passed your Strength check, you would still be stunned and the lever would not be pulled.
Alternate option:
When manipulating an object, you can ignore interruption from sources of damage that deal equal to or less than (Character level/5)*Constitution Modifier.
This option is more friendly to any class/race combination. Everyone is expected to have decent Constitution and everyone will cap out at level 20.