find what fills T-log

Hi,

I have below queries:

Sometimes our Transaction log gets filled and I need to shrink the T-log. 

So first I check logspace using "dbcc sqlperf (logspace)

Next I check if there are any inactive vlfs present in T-log using "Select name,log_resue_Wait,log_resuse_wait_desc from sys.databases"

if the status is NOTHING for T-log in question, I use dbcc shrinkfile (TEST_log,1024).

But now if there is no space in T-log, how can I get the top transactions which use T-log and how can I get the end user (application) username who is running that Transaction,so that I can check with him, because I can not go and kill the transaction

Regards