This 62-year-old white female had been in good health for most of her life, with the exception of a history of congenital heart disease. In 1975, at the age of 55, she was found to have an ostium primum atrial septal defect, as well as a cleft mitral valve which was mildly regurgitant. Second-degree AV block was present. In late 1975 she underwent surgical repair of the atrial septal defect and had a permanent pacemaker installed. She felt reasonably well until 1980, two years prior to admission, when she had the onset of palpitations during sleep. She would awaken with a sensation of forceful pounding in the chest. Recurrent nightmares became a problem. There was a thirty-year history of snoring, although her family had not noticed any periods of apnea, choking, or gasping for air. She did state that she often abruptly awakened from sleep, although she was not aware of the reason. In addition, the patient complained of severe headaches which usually occurred in the morning upon arising from bed. The patient stated that she was always able to fall asleep easily. However, excessive daytime sleepiness or falling asleep at inappropriate times had not occurred. There were no other significant complaints.