Measuring postural-related changes of spontaneous baroreflex sensitivity after repeated long-duration diving: Frequency domain approaches

Sustained water immersion is thought to modulate orthostatic tolerance to an extent dependent on the duration and repetition over consecutive days of the diving sessions. We tested this hypothesis investigating in ten healthy subjects the potential changes in the cardiovascular response to head-up tilt induced by single and multiple resting air dives. Parametric cross-spectral analysis of spontaneous RR interval and systolic arterial pressure variability was performed in three experimental sessions: before diving (BD), after single 6-hour dive (ASD), and after multiple 6-hour dives (AMD, 5 consecutive days with 18-hour surface interval). From this analysis, baroreflex sensitivity (BRS) was computed as spectral power ratio (ceBRS), non-causal transfer function gain (tfBRS) and causal transfer function gain (gamma BRS) evaluated at low frequency (0.04-0.14 Hz) in the supine position (su) as well as in the standing upright position in the early tilt (et) and late tilt (It) epochs. We found that, while ceBRS decreased significantly in et and It compared to su during all sessions, tfBRS and gamma BRS decreased during ASD and AMD but not during BD; moreover gamma BRS evidenced a progressive decrease from BD to ASD and to AMD in both et and It epochs. These results indicate the necessity of following a causal approach for the estimation of BRS in the frequency domain, and suggest a progressive impairment of the baroreflex response to postural stress after single and multiple dives, which may reflect symptoms of increasing orthostatic intolerance. (C) 2013 Elsevier B.V. All rights reserved.