If former vice president Richard B. Cheney's experience is similar to that of other patients who have heart pumps implanted, he has a better than 50-50 chance of surviving two years. The device, which takes over the work of the heart's main pumping chamber, should lessen discomfort and allow Cheney to do activities as strenuous as riding a bicycle. But it is far from a miracle cure for end-stage congestive heart failure, the condition from which he apparently suffers. Cheney, 69, had a left ventricular assist device implanted last week at Inova Fairfax Heart and Vascular Institute. In a statement Wednesday, Cheney said he was "entering a new phase of the disease . . . and decided to take advantage of one of the new technologies available."