A 74-year-old man is referred after three falls over two months. He describes a brief feeling of faintness after rising from a chair, without spinning sensation, palpitations or loss of consciousness; neurological examination is normal. His blood pressure is 108/68 mmHg lying and 82/50 mmHg after standing for two minutes, with a pulse increase from sixty-eight to seventy-four per minute. Which one of the following is the most likely diagnosis?