The number of times in an hour when a sleeping person either stops breathing completely or has limited airflow. Each episode must last at least 10 sec. The AHI is one indicator of obstructive sleep apnea, although it is recognized as an imperfect diagnostic tool. An AHI of 30 or more events in an hour indicates severe sleep apnea; 15 to 29 events suggests moderate apnea; and 5 to 14 events indicates mild apnea.