Mediating effect of onset rate on the relationship between +Gz and LBNP tolerance and cardiovascular reflexes

Research attempting to establish a relationship between human response to lower body negative pressure (LBNP) and +Gz acceleration has in general, disregarded the moderating effects of negative pressure and +Gz onset rates. Many of the past studies designed to examine the relationship between lower body decompression and +Gz acceleration tolerances have also ignored the problem of differences in body position typically encountered during LBNP and +Gz testing (supine LBNP vs. seated +Gz).

There were 17 male subjects (24-34 yr) who were tested in a seated LBNP chamber and received 3 pressure onset rates of 0.067, 0.33, and 2.0 mm Hg x s(-1). Relaxed acceleration tolerance was assessed on a 6.1 -m centrifuge using three gradual onset rates of 0.01, 0.05, and 0.2 +Gz x s(-1). LBNP and +Gz tolerances were subjected to principle components (factor) analysis, and the resulting factors correlated with variables derived from autonomic reflex control tests (Valsalva and carotid-cardiac baroreflex responsiveness) and height.

The factor model suggested a two-dimensional solution consisting of an acceleration factor and an orthostatic factor. The general pattern of the factor loadings indicates the relationship between tolerances of the two forms of orthostatic stress (acceleration and LBNP) is a function of how fast each stress is delivered. The correlation between LBNP tolerance and acceleration tolerance increases as LBNP onset rate is increased, or +Gz onset rate is decreased. Height was highly correlated (-0.71) and carotid-cardiac baroreflex responsiveness moderately correlated (0.54) with general orthostatic tolerance. Valsalva measures of autonomic reflex control had low correlations with general orthostatic tolerance (<0.30).

Although both LBNP and +Gz exposure can lead to eventual loss of consciousness, syncopal events associated with intolerance to either stress are a function of somewhat different cardiovascular mechanisms. There are however, specific situations where LBNP may constitute a viable substitute for +Gz.