A method and apparatus for diagnosing heart rejection is disclosed. Heart rejection is diagnosed based on the pattern of interbeat intervals. The interbeat intervals of the heart are measured shortly after transplant to establish a baseline pattern. The patterns of interbeat intervals from subsequent measurements are compared to the baseline to detect changes from the baseline indicating rejection. The apparatus of the invention measures the interbeat intervals using a Schmidt trigger that detects the upstroke of the QRS and produces a corresponding pulse. The intervals between pulses are timed to produce a series of interbeat interval measurements that are stored and analyzed. Software provides for automated pattern analysis.