A 78-year-old man is brought to hospital with an episode of dizziness. He was well until the last 6 months, since when he has had some falls, irregularly. On some occasions he lost consciousness and is unsure how long he has been unconscious. On a few occasions he has fallen, grazing his knees and on others he has felt dizzy and has had to sit down but has not lost consciousness. These episodes usually happened on exertion, but once or twice they have occurred while sitting down. He recovers over 10–15 min after each episode. Once, his wife was with him when he blacked out. Worried, she called an ambulance. He looked so pale and still that she thought that he had died. He was taken to hospital, by which time he had recovered completely and was discharged and told that he had a normal electrocardiogram (ECG) and chest X-ray. On examination He is pale with a blood pressure of 93/63 mm Hg. The pulse rate is 35/min, regular. There are no heart murmurs. The jugular veno