ANN ARBOR — Health care prices rose nationally just 1.1 percent from April 2012 to April 2013, the lowest annual increase since 1997, and a sharp 0.5 percentage point drop from the rate of increase reported in March from a year earlier. The 12-month moving average price increase, at 1.8 percent, is the lowest since 1.7 percent recorded in September 1998. National health expenditures, which reflect both prices and utilization, grew 4.2 percent, remaining in the vicinity of the record low levels seen annually since 2009.