The reproducibility of tolerance to lower-body negative pressure and its quantification.

The reproducibility of tolerance to lower-body negative pressure (LBNP) has not been assessed sufficiently. Furthermore, there has been confusion concerning the most appropriate index by which LBNP tolerance can be quantified. The purpose of this study was to assess the degree of reproducibility in presyncopal-symptom-limited LBNP (LBNPtol), using an LBNP chamber. Twenty physically active subjects [median age (range) 21 (18-27) years] underwent three successive LBNPtol tests with 72-120 h between each test. LBNPtol was quantified using the LBNP tolerance index (LTI; delta mmHg.min), cumulative stress index (CSI; mmHg.min), duration of negative pressure (DNP) and maximum magnitude of negative pressure (MNP). Heart rate (fc), systolic (SBP) and diastolic (DBP) blood pressures from the three repeated tests were compared during a control period. The changes from control to maximum response (fc, SBP, DBP) during LBNP were also compared, and percentage changes in estimated blood volume were measured. There were no statistical differences between any of these comparisons (P > 0.05). LTI and CSI were greater in the third test when compared to the first two tests (P < 0.05). The values for DNP and MNP were not statistically different between tests (P > 0.05). Measures of LTI and CSI showed an acceptable level of reproducibility for the first two repeated tests. However, there was an increase in LBNPtol on the third successive exposure to LBNP. These findings have shown that it is possible to achieve reproducible measures of tolerance to LBNP when using a custom-built chamber. This only applies to a test-retest procedure. Furthermore, these data also suggest that DNP and MNP do not adequately reflect the differences shown in LBNP tolerance when using LTI and CSI as measures.