RIGHT BUNDLE-BRANCH BLOCK MASKS EXERCISE-INDUCED ST SEGMENT DEPRESSION

We observed intermittent right bundle branch block alternating with no rmal conduction during early exercise, early recovery or both in 6 pat ients undergoing exercise test. Four patients developed ST segment dep ression. During exercise or recovery, ST segment depression appeared o n normally conducted beats at a lower heart rate when ST segment was i soelectric on right bundle branch blocked beats. ST segment depression on bundle branch blocked beats appeared at a higher heart rate, indic ating that right bundle branch block masks ST segment depression. In t he remaining 2 patients, ST segment remained isoelectric throughout th e test. Since conduction towards peak exercise was uniformly right bun dle branch blocked, isoelectric ST segment might represent a false-neg ative test.