Hilton, F, T Lightfoot, C Tankersley, W Ehrlich, and S Fortney. Leg circumference dynamics during repeated lower body negative pressure . Federation Proc. 47(6): A1313, 1988.
To examine one potential mechanism for the adaptation seen with repeated exposure to lower body negative pressure (LBNP), leg circumference (LC) changes were monitored during presyncopal symptom-limited (PSL) LBNP. Six healthy, young men completed on exposure to PSL LBNP per day for 7 consecutive days. The exposure protocol consisted of 5 min at 0 mmHg, then 1 min each at -8 and -16 mm Hg, followed by 3 min at -30 mmHg, then in 10 mmHg decrements at 5 min intervals until tolerance was reached. A significant increase in LBNP tolerance was demonstrated by a 32% rise in the mean Luft's cumulative stress index from day 1 to day 7, an increase from -70 to -80 mmHg in LBNP toelrated and prolonged exposure duration by 5 min. The blood pressure and heart rate measured 1 min before exposure termination did not change significantly with repeated exposure. It would seem that the LBNP-induced reduction of blood pressure was attenuated. The index of edema formation (change in LC from rest to 1 min after release of LBNP) increased significantly reaching a plateau over the last 3 days; this was independent of exposure duration. The LC measured at -50 mmHg peaked significantly on day 5. These findings were consistent with they hypothesis that reflexely increased vessel tone in the vasculatrue of the lower body would tend to decrease the rate of blood pooling but at the same time increase the rate of edema formation. This increased tone may be responsible for the increased tolerance.