Chad Carvin, who made a heartwarming comeback this year after being sidelined the Olympic year with a heart problem, will miss the World Championships because of a nagging back problem. Carvin, 23, who was named USS Swimmer of the Year for 1997, qualified for the championships in the 200, 400 and 1500 frees and 800 relay. He won four events (200, 400 and 1500 frees, 400 IM) at the U.S. spring nationals and two (400 and 1500 free) at the summer nationals in 1997 and earned the men's Kiputh Award as the high scorer at both meets. The former University of Arizona star has seen several doctors and received different diagnoses. If his back doesn't improve, he plans to retire.