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

Abstract

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.

Reproducibility of the heart rate variability responses to graded lower body negative pressure

“…In the present study, the maximal negative pressure tolerated varied less than 9% over the two trials which was similar to the 8% seen over their four trials. Others found LTI to vary $7% from the first to third LBNP trial but not to differ between the first two trials (Howden et al 2001). We are uncertain why LTI increased in our subjects.…”

“…While these responses are widely regarded as reproducible, the evidence supporting this assertion has been largely anecdotal. Only three published studies were found which assessed LBNP tolerance (Convertino et al 1986;Howden et al 2001;Lightfoot et al 1991). Moreover, there exist few published evaluations of the reproducibility of the cardiovascular responses and these were restricted to HR and blood pressure (Hoffler et al 1990;Howden et al 2001;Lightfoot et al 1991).…”

“…The low frequency (LF) component of the spectrum was between 0.04-0.15 Hz and the high frequency (HF) component was between 0.15-0.40 Hz (Task Force 1996). Data were presented both in absolute units and normalized units which adjust for changes in total power that occur with LBNP (Pagani et al 1986;Task Force 1996) Statistical analyses LBNP tolerance was quantified by the LBNP tolerance index (LTI; Howden et al 2001). For statistical analyses, mean autonomic and cardiovascular responses for the last 5 min (minutes 2-6) of each level of LBNP common to all subjects ( £ )60 mmHg) were determined.…”

Delaying orthostatic syncope with mental challenge: A pilot study

“…Some investigators reported that subjects adapt to repeated LBNP sessions with increased LBNP tolerance on the third successive exposure [23]. Ector et al [24], who observed an increase in tolerance times following 1 week of tilt training in patients, also confirmed this.…”

“…Conversely, applying graded orthostatic stress to the same subjects four times, separated by 72 h [25] or 2 weeks [10] demonstrated no differences in orthostatic tolerance across trials. Since no objective end point exists for an LBNP tolerance, it is possible that some differences between studies can be attributed to ambiguity associated with LBNP tolerance end point determination [23] or a lay-off effect when tests are more than 1 week apart. For example, aircrews that have not flown at high G for a week develop a reduction in G tolerance on return to flying [26].…”