Evolution of T13 parameter (graph A) and corresponding arterial pressure pulse line shapes (10-pulse averages) for subject #5, 31 y. f, over the course of an entire LBNP session. The narrowing of the arterial pulse is indicated by the downward (with time) pointing arrows that identify the changing temporal position of component pulses #2 and #3 relative to the primary systolic pulse #1. In this subject the heart rate changed significantly. Note the different rate of change in the T13 interval and the heart rate. Of interest also is the massive rebound that is observed in the #2 component pulse amplitude after venting, which subsided within minutes.