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Background: Aim of this study was to test the hypothesis that after presyncope, some blood hormone pools increase while others decrease., Materials and Methods: In twelve healthy male adults, we determined plasma volume changes with plasma mass densitometry and hormone levels. The following were compared: supine rest, presyncope and 20-min post-presyncopal supine rest. We determined plasma renin activity (PRA), aldosterone, adrenocorticotropic hormone (ACTH), adrenomedullin and vasopressin (AVP) from venous blood samples., Results: Using passive 4-min 70° head-up tilt followed by 4-min sequences of additional lower body negative pressure of increasing intensity (15 mmHg steps), presyncope occurred after 11·6 ± 2·8 min, at which time plasma volume was reduced by 15·5 ± 7·4%, aldosterone increased by 37%, ACTH by 75%, PRA by 187% and AVP about 16-fold in average (all P < 0·01); no significant changes in adrenomedullin were seen. Twenty-min post-presyncope, ACTH increased above presyncopal levels (+36%, P < 0·05), aldosterone by 35% (P = 0·07). PRA (-47%, P < 0·01) and AVP (-84%, P < 0·05) decreased below presyncopal but were still above supine control (P < 0·01); similarly, plasma density fell by 2·17 ± 0·97 g L(-1) below presyncopal (P < 0·01), but above supine control (P < 0·05), indicating rapid recovery (83% of initial plasma volume)., Conclusions: We conclude that during the 20-min supine post-syncopal period, plasma volume, PRA and AVP return closer to baseline but aldosterone and ACTH continue increasing. The magnitude of observed concentration changes cannot be explained by haemoconcentration/haemodilution, rather it appears that the observed changes are indicative of hormone-specific endocrine activation patterns in the recovery phase., (© 2011 The Authors. European Journal of Clinical Investigation © 2011 Stichting European Society for Clinical Investigation Journal Foundation.)

Hypothesis: Measurements of plasma volume (PV) and its changes (delta%PV) by Evans blue (EB) dye are presumed to be valid only in fasting subjects. In addition, delta%PVEB with acute altitude exposure has not been compared with other methods employing the concentration or dilution of naturally occurring blood (hematocrit (Hct), hemoglobin (Hb)) and plasma (density, proteins) components, but should be similar if capillary permeability and the sampled vein/whole body Hct ratio remain unchanged., Methods: PVEB was determined in six subjects while fasting or eating on different days, with injection and sampling in the same arm, 4-h extrapolation to time zero and correcting readings with the 620-740 A method. For 93 experiments at altitude, delta%PVEB was obtained similarly from a 3-h extrapolation near the end of a 12-h chamber exposure to 426 mm Hg (-4,880 m =16,000 ft) and at the same time on the preceding control day., Results: Mean PVEB with and without eating was not significantly different (SE of absolute difference = +/- 2.8%). The EB decay curves had significantly more scatter with eating than fasting. The fasting vs. non-fasting values for the single 20-min post-injection point also gave a close comparison (r = +0.97). At altitude the loss in PV measured with EB was significantly greater (delta%PVEB = -6.3%) than losses estimated from Hct-Hb (-2.9%), plasma protein (-3.7%), and plasma density (-3.9%). The expected larger PV loss in subjects tolerant to altitude sickness compared with intolerant ones was most clearly shown by delta%PVEB (8.8%)., Conclusions: Obtaining more samples can offset reproducibility lost by eating. The delta%PVEB were largest and nearest to values previously reported at altitude, perhaps because the single baseline and altitude samples utilized by the other methods are more sensitive to subtle, transient fluctuations in body water and vasomotor tone associated with apprehension, vomiting, fluid intake, and regional vasodilation and constriction.

Purpose: To study hormonal factors that may account for the dissociation between beverage-induced plasma sodium p[Na+] and osmotic p[Osm] concentrations that appear to refute the high theoretical correlation between p[Na+] and p[Osm]., Methods: Ten men (24 +/- SD 3 yr of age) sat reclining (head up) for 12 h in a chamber (21-23 degrees C dry bulb, 25-33% relative humidity) at 2800 m (9184 ft, 539 mm Hg) altitude (ALT), and at 321 m (1053 ft, 732 mm Hg) on the ground (GND). During 1000-1030 hours they consumed 3 fluids (12 ml x kg(-1),X = 948 ml x d(-1)) with large differences in sodium and osmotic contents: AstroAde (AA) with 185 mEq x L(-1) Na+ and 283 mOsm x kg(-1), Performance 1 (Shaklee) (P1) with 22 mEq x L(-1) Na+ and 365 mOsm kg(-1), or H2O at ALT; and only H2O on the GND., Results: After drinking: plasma volume (PV) increased at 1200 hours by 8.3% (p < 0.05) with AA but was not significantly (NS) changed in the other sessions (Xdelta = +0.9%, range -0.9 to 2.8%); p[Na+] and p[Osm] were unchanged. Urinary rates and free-water clearances were attenuated with AA and P1 vs. those with H2O. Correlations between and among p[Na+] and p[Osm] suggest that the pNa+ ion is more tightly controlled than pOsm; and that there was no clear hormonal response that could account for this dissociation from theoretical considerations., Conclusions: There is significant dissociation between plasma sodium and osmotic concentrations after fluid intake. Induction and maintenance of hypervolemia requires increased (near isotonic) drink Na+ osmols rather than increased non-ionic osmols.

Background: The purpose for this study was to evaluate various carbohydrate (CHO)-electrolyte fluid formulations for consumption by astronauts to maintain or restore their plasma volume (PV) and total body water (TBW) during and after extravehicular activity (exercise experiment, EE) and for a few hours before reentry and immediately after landing (rest experiment RE)., Hypothesis: That fluid formulation electrolyte content would be more important than osmotic (Osm) content for increasing or maintaining PV during the RE and EE., Methods: In the RE, 5 healthy men (23-44 yr), previously dehydrated for 24 h, drank 6 fluid formulations (Water, 19.6 Na, 157 Na, 19.6 Na + glucose, and the prepared drinks Performances and Power)--one each at weekly intervals, and then sat for 70 min. In the EE, four healthy 24-h dehydrated men (30-46 yr) exercised for 70 min supine on a cycle ergometer (load = 71 +/- 1% peak VO2)., Results: Rest: Subjects who consumed formulations with total Osm concentrations nearer the normal range (157 Na - 270 mOsm x kg(-1), Performance with 19.6 mEq x L(-1) Na - 380 mOsm, and to some extent Power with 23.5 mEq x L(-1) Na - 390 mOsm) had the greater increases in PV; intake of drink 157 Na, with the largest Na content, induced the greatest hypervolemia of 7.6% (p < 0.05). The various additional ions, in addition to 19.6 Na, probably contributed to the 4.6% (p < 0.05) hypervolemia with Performance. Water was not effective. Exercise: Stabilization of PV between 15-70 min was not related to drink total CHO, Na or Osm content. Performance and 157 Na were no more effective than 19.6 Na or 19.6 Na + glu for PV stabilization. Water was the least effective. Regulatory mechanisms controlling PV during exercise appear to be independent of oral fluid formulation Osm-electrolyte content., Conclusions: Drink cation (sodium) content is more important that its total osmotic content for increasing plasma volume at rest. Fluid formulations with greater hypervolemic action in resting subjects may not be as effective during exercise; therefore different formulations for use during exercise appear to be necessary.

The present study determines the effect of repeated 70 degrees head-up tilt (HUT) on plasma volume (PV) shifts by measuring blood density (BD), plasma density (PD), and hematocrit (Hct). Eight men (18-26 yr) underwent a predrink period with two supine (P1 and P3) and two HUT (P2 and P4) phases of 45 min each. At the end of P4 they drank 10 ml/kg body wt of isotonic (290 mosmol/kg) sodium chloride (Iso) or hypotonic (< 10 mosmol/kg) unsweetened tea (Hypo) or nothing [control (Con)]. The following periods continued the supine (P5, P7)/upright (P6) sequence. BD and PD were measured from ear lobe blood; they were different (P < 0.05) between Con, Hypo, and Iso P6 and P7. The density of fluid that moved between intra- and extravascular compartments was 1,008.2 +/- 0.4 g/l and did not differ with test situations. In Con (P3, P5, P7), supine PV steadily decreased compared with P1 (P < 0.05). PV in P1, P2, and P3 of all treatments averaged 120 +/- 1, 101 +/- 1, and 115 +/- 1%, respectively, of PV in P4. Tilt-induced PV shifts ranged from -9.7 to -16.7% compared with PV during the respective previous phases. After drinking, PV increased (P < 0.05) above Con values at the end of P7 by 12.9% with Iso and by 6.6% with Hypo. Progressive hemoconcentration occurred in the nondrink supine periods; isotonic saline ingestion increased supine PV to Con level but did not stop or reverse the decrease of upright hemoconcentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Postural blood volume changes aggravate the regulation of arterial blood pressure and perfusion vis-a-vis the hydrostatic effects of orthostasis, ie, blood pooling below the hydrostatic indifferent points and reduced cardiac preload. Corresponding problems surface with extended passive standing, particularly in highly trained, dehydrated, or otherwise compromised subjects, or after long-lasting immobilization, as with space flight.

At the beginning and with ceasing of suction which is exerted upon the lower parts of the body, we have recently discovered transient hemodilution/hemoconcentration effects by use of the high-precision so-called mechanical oscillator technique. Since transient effects are useful for biological systems analysis, we addressed the question if such transients in blood and plasma mass density also occur with and immediately after water immersion in humans. Water immersion (WI) has been used as an analog of microgravity conditions. The effects on blood volume depend on the test conditions, particularly body position before immersion, and the time of measurement. Consequently, hemodilution has been found as an early effect of WI in many but not all studies. If test persons are immersed after sitting or standing on air, WI counteracts postural hemoconcentration and consistently produces a decrease in hematocrit, hemoglobin, and plasma protein concentration and, hence, plasma and blood mass density. In this paper we present evidence that, like with termination of lower body suction, there might be transient hemoconcentration in the first minutes of thermoneutral WI in a sitting position in men, but no evidence could be obtained for any counterdirected transient hemodilution when test subjects are passively brought out (in sitting position) from the water, as was found with starting lower body suction (in supine position).

Abstract: Blood pressure stability may be jeopardized in astronauts experiencing orthostatic stress. There is disagreement about cardiovascular and endocrine stress responses that emerge when a critical (presyncopal) state is reached. We studied hemodynamic and neurohormonal changes as induced by an orthostatic stress paradigm (head-up tilt combined with lower body negative pressure) that leads to a syncopal endpoint. From supine control to presyncope, heart rate increased by 78% and thoracic impedance by 12%. There was a 49% fall in stroke volume index, 19% in mean arterial blood pressure, 14% in total peripheral resistance index and 11% in plasma volume. Plasma norepinephrine rose by 107, epinephrine by 491, plasma renin activity by 167, and cortisol by 25%. Hemodynamic and hormonal changes of clearly different magnitude emerge in presyncope as compared to supine rest. Additional studies are warranted to reveal the exact time course of orthostatic changes up to syncopal levels. [Copyright &y& Elsevier]