Right bundle branch block masks exercise-induced ST segment depression

We observed intermittent right bundle branch block alternating with normal conduction during early exercise, early recovery or both in 6 patients undergoing exercise test. Four patients developed ST segment depression. During exercise or recovery, ST segment depression appeared on normally conducted beats at a lower heart rate when ST segment was isoelectric on right bundle branch blocked beats. ST segment depression on bundle branch blocked beats appeared at a higher heart rate, indicating that right bundle branch block masks ST segment depression. In the remaining 2 patients, ST segment remained isoelectric throughout the test. Since conduction towards peak exercise was uniformly right bundle branch blocked, isoelectric ST segment might represent a false-negative test.