For this month of December, we will continue with our theme of micro-anatomy of nerves but emphasize the very related topic of why continuous peripheral nerve blocks (CPNB) fail.  Secondary block failure is a very prevalent issue and, IMHO, one of the main reasons why CPNB’s have been decreasing in popularity.  The secondary block is when the primary block, where a larger volume of a high concentration local anesthetic (LA), had worn off and the low volume of a low…