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I am working on implementing a guideline for chainging IV infusion sets and have run into something that confuses me.  In the INS Infusion Therapy in Clinical Practice book, you know, our big blue bible, on page 398 states "...that continuous peripheral and central primary sets and secondary administration sets should be changed every 72 hours.  However, failure to maintain an ongoing phlebitis rate of 5% or less, or any increased rate of catheter-related bacteremia, requires a return to 48-hour administration set change." 

Here is my question.  What has phlebitis got to do with the length of time that tubing has hung?  I thought the reason that we changed tubing periodically was to avoid catheter related blood stream infrections.  What is the link with phlebitis?