REPRODUCIBILITY OF THE AUTONOMIC AND CARDIOVASCULAR RESPONSES TO MAXIMAL LOWER BODY NEGATIVE PRESSURE

Abstract

Despite widespread acceptance of the reproducibility of the autonomic and cardiovascular responses to lower body negative pressure (LBNP), surprisingly few assessments of this reproducibility have been published. To redress this, 14 active, college age men underwent an orientation exposure and 2 trials of graded LBNP to presyncope or −100 mmHg, separated by 1 week. Heart rate, stroke volume, cardiac output, blood pressure and forearm blood flow were assessed, as was heart rate variability in both time and frequency domains. LBNP tolerance was lower in trial 1 than trial 2 (CSI: 1698 ± 171 vs 1938 ± 139 mmHgmin; LTI: 404 ± 21 vs 437 ± 15 mmHgmin; P < 0.05) but the intraclass correlation coefficient was high (R = 0.80). The responses to LBNP common to all subjects (LBNP ± 60 mmHg and at test termination) did not differ between trials although systolic blood pressure was lower in trial 2 at test termination (104.8 ± 2.6 vs 112.1 ± 2.5 mmHg, P < 0.05). Relatively high reliability estimates were found for the resting data (lowest: R = 0.62 for low frequency/high frequency ratio; highest: R = 0.94 for SDNN). Reliability estimates varied by parameter measured and LBNP stage but were generally similar to that seen at rest. They were lower at test termination [lowest: R = 0.23 for high frequency (abs) power; highest: R = 0.87 for cardiac output]. These data suggest that group mean autonomic and cardiovascular responses to graded LBNP can vary across trials; however, individual responses to submaximal LBNP and LBNP tolerance appear internally consistent.