Theoretical analysis of the mechanisms of a gender differentiation in the propensity for orthostatic intolerance after spaceflight

<p>Abstract</p> <p>Background</p> <p>A tendency to develop reentry orthostasis after a prolonged exposure to microgravity is a common problem among astronauts. The problem is 5 times more prevalent in female astronauts as compared to their male counterparts. The mechanisms responsible for this gender differentiation are poorly understood despite many detailed and complex investigations directed toward an analysis of the physiologic control systems involved.</p> <p>Methods</p> <p>In this study, a series of computer simulation studies using a mathematical model of cardiovascular functioning were performed to examine the proposed hypothesis that this phenomenon could be explained by basic physical forces acting through the simple common anatomic differences between men and women. In the computer simulations, the circulatory components and hydrostatic gradients of the model were allowed to adapt to the physical constraints of microgravity. After a simulated period of one month, the model was returned to the conditions of earth's gravity and the standard postflight tilt test protocol was performed while the model output depicting the typical vital signs was monitored.</p> <p>Conclusions</p> <p>The analysis demonstrated that a 15% lowering of the longitudinal center of gravity in the anatomic structure of the model was all that was necessary to prevent the physiologic compensatory mechanisms from overcoming the propensity for reentry orthostasis leading to syncope.</p

Investigation of in vivo measurement of cerebral cytochrome-c-oxidase redox changes using near-infrared spectroscopy in patients with orthostatic hypotension

We have previously used a continuous four wavelength near infrared spectrometer to measure changes in the cerebral concentrations of oxy- (Δ[HbO2] and deoxy- haemoglobin (Δ[HHb]) during head-up tilt in patients with primary autonomic failure. The measured changes in light attenuation also allow calculation of changes in the concentration of oxidised cytochrome c oxidase (Δ[oxCCO]), and this paper analyses the Δ[oxCCO] during the severe episodes of orthostatic hypotension produced by this experimental protocol. We studied 12 patients during a passive change in position from supine to a 60º head-up tilt. The challenge caused a reduction in mean blood pressure of 59.93 (±26.12) mmHg (Mean (±SD), p<0.0001), which was associated with a reduction in the total concentration of haemoglobin (Δ[HbT]= Δ[HbO2]+Δ[HHb]) of 5.02 (±3.81) μM (p<0.0001) and a reduction in the haemoglobin difference concentration (Δ[Hbdiff]= Δ[HbO2]-Δ[HHb]) of 14.4 (±6.73) μM (p<0.0001). We observed a wide range of responses in Δ[oxCCO]. 6 patients demonstrated a drop in Δ[oxCCO] (0.17 ±0.15μM ); 4 patients demonstrated no change (0.01 ±0.12 μM ) and 2 patients showed an increase in Δ[oxCCO] (0.21 ±0.01 μM ). Investigation of the association between the changes in concentrations of haemoglobin species and the Δ[oxCCO] for each patient show a range of relationships. This suggests that a simple mechanism for crosstalk, which might produce artefactual changes in [oxCCO], is not present between the haemoglobin and the oxCCO NIRS signals. Further investigation is required to determine the clinical significance of the changes in [oxCCO]

Aerospace medicine and biology: A cumulative index to a continuing bibliography (supplement 384)

This publication is a cumulative index to the abstracts contained in Supplements 372 through 383 of Aerospace Medicine and Biology: A Continuing Bibliography. It includes seven indexes: subject, personal author, corporate source, foreign technology, contract number, report number, and accession number

A cumulative index to the 1976 issues of a continuing bibliography on Aerospace Medicine and Biology

This publication is a cumulative index to the abstracts contained in Supplements 151 through 162 of Aerospace Medicine and Biology: A continuing bibliography. It includes three indexes - subject, personal author, and corporate source