Effects of Hyperbaric Exposure on the Cardiovascular System. Role of the Autonomous Nervous System

Introduction Among experienced divers, dive adaptation is seen as a modified pattern of physiological changes. This is reflected, inter alia, in the change in cardiovascular responses, therefore there is need to examine the role of the autonomic nervous system in cardiovascular response modulation after hyperbaric exposure. Material and methods Ten experienced divers took part in the study. The effects of hyperbaric exposure at 30 and 60 meters and interaction (depth x time) were measured. Changes in HR, RRI, CI and HRV values have been taken into analysis. Results Hyperbaric exposure at 30 meters significantly affected HFnu-RRI elevation and decrease of LFnu-RRI (F = 42.92, p <0.00001), without significant affecting the HR, RRI and CI. Exposure to hyperbaric 60 m increased HR and CI (F = 7.64, p = 0.01 and F = 4.89, p = 0.04 respectively) and RRI (F = 7.69, p = 0.01), without significant impact on other variables. The influence of interaction (depth x time) was significant in all measured variables. Conclusions The results indicate that hyperbaric exposure at 60 meters affected HR, RRI, CI parameters, that were not significantly affected by hyperbaric exposure at 30 meters. On the other hand, the exposure at 30 meters showed a significant effect on the LFnu and HFnu HRV, which were not significantly affected by the exposure at 60 meters. Significant effect of time and depth interaction in each of the analyzed variables was observed.

Fracture Mechanics Analysis of Asperity Cracking Due to Repetitive Sliding Contact

Asperity failure due to repetitive sliding is a common process of wear particle formation. Linear elastic fracture mechanics and the finite element method (FEM) were used to analyze asperity cracking due to sliding against another rigid asperity. The maximum ranges of the tensile and shear stress intensity factors (SIFs) were used to determine the crack growth direction and the dominant mode of fracture. Simulations of repetitive sliding showed a strong dependence of the wear particle size and wear rate on the direction and rate of crack growth. The maximum ranges of tensile and shear SIFs were used to determine the dominant mode of crack growth. The effects of asperity interaction depth, sliding friction, initial crack position, crack-face friction, and material properties on crack growth direction, dominant fracture mode, and crack growth rate are discussed in the context of FEM results. It is shown that the asperity interaction depth and sliding friction exhibit the most pronounced effects on the crack growth direction and growth rate. A transition from shear- to tensile-dominant mode of crack growth was observed with the increase of the asperity interaction depth and/or sliding friction coefficient. Crack-face opening, slip, and stick mechanisms are discussed in the light of crack mechanism maps constructed for different asperity interaction depths and sliding friction coefficients.