You can set a "data breakpoint" after you started a debugging session. To declare a data breakpoint, set a normal breakpoint somewhere in the part of your code that can see the variable or data address in question, some time before the problem occurs. Start the debugger and wait for it to break at that position. Then select the following menu entry:
"Debug --> New Breakpoint --> New Data Breakpoint ..."
There you can enter the address of your variable hr, and tell the debugger to break when the value stored at that address changes.
This applies to VS 2010, english version.