Evaluation of spontaneous baroreflex response after 28 days head down tilt bedrest

The spontaneous baroreflex response was evaluated during supine rest and head up tilt (60°) before and immediately after a 28 day 6° HDT bedrest in 6 healthy adult men (age 30–42 years). Sequences of 3 or more beats where RR-interval and systolic blood pressure changed in the same direction were used to evaluate baroreflex response slope (BRS). Prior to bedrest, the mean BRS and RR-interval were 18.0 ± 3.9 ms/mm Hg and 926 ± 61 ms at rest and 10.5 ± 2.5 ms/mm Hg and 772 ± 63 ms during the first 10 min of 60° tilt. Following bedrest, these values changed to 15.6 ± 2.7 ms/mm Hg and 780 ± 53 ms at rest, and to 6.5 ± 1.2 ms/mm Hg and 636 ± 44 ms during tilt. Thus, (1) the spontaneous baroreflex can be evaluated in human subjects during experiments of orthostatic stress; (2) the baroreflex slope was reduced on going from supine to the head up tilt position; and (3) 28 days of bedrest reduced the spontaneous baroreflex slope.