Transverse bulkheads were a technology developed by Lieutenant Tom Paris in an alternate 2374, Bulkheads were utilized by the ancient steamship RMS Titanic, the honeycomb-like set of bulkheads of which would lower into place in case of a major hull breach, theoretically saving the ship even if half of it were filled with water.
In light of continued Krenim attack, Tom Paris set up emergency force fields between all decks and every section, the force fields were designed to protect most of the crew in the event of a cataclysmic breach. By Day 180, the transverse force fields were off-line, preventing any chance of The Doctor using them to confine Captain Kathryn Janeway after she refused to be relieved of duty.
The timeline in which these events occurred was later negated when a temporal incursion erased the Krenim weapon ship from history. (VOY: "Year of Hell", "Year of Hell, Part II")