Minimum sea-level pressure and maximum wind speed intensity relationship in Hurricane Katrina (2005)
Generally the relationship is the lower the air pressure the more intense the hurricane. For example during the same hurricane season as Katrina (2005) which also happened to be a record breaking season for the number of hurricanes recorded in the Atlantic basin, hurricane Wilma was recorded as being the most intense hurricane ever to form in the Atlantic. It reached a minimum air pressure of 882 millibar which was a record low. The corresponding maximum wind speed during this period of intensification was 185 mph. Hurricane Katrina was also a category 5 hurricane and reached a minimum air pressure of 902 millibar with maximum wind speeds at this pressure of 175 mph. Katrina of course striking where it did caused far more destruction even though it was the less intense hurricane of the two:

http://en.wikipedia.org/wiki/Hurricane_Wilma

http://en.wikipedia.org/wiki/Hurricane_Katrina