Dirk van Zyl, 68, who lived a record 23 years after one of the first heart transplants. Van Zyl received a new heart in 1971 in the sixth transplant operation performed by the pioneering surgeon, Christiaan Barnard. During the surgery, Van Zyl's ailing heart stopped beating, prompting the anesthetist to say he was dead. But Barnard revived him and completed the transplant. Van Zyl received the heart of a man who died in a fall from a tree. Although chances of long-term survival after such operations have remained about 50-50, Van Zyl continually encouraged patients who were offered a new heart. "If you don't go for the transplant," he said in 1992, "there is no chance." About 3,500 heart transplants are now performed each year, mostly in the United States. The survival rate for the first year exceeds 90%. On Wednesday in Cape Town, South Africa, after a stroke.