Role of Hypobaria in Fluid Balance Response to Hypoxia

Abstract

To estimate the separate and combined effects of reduced P(B) and O2 levels on body fluid balance and regulating hormones, measurements were made during reduced PB (altitude, ALT; P(B) = 432 mm Hg, F(I(O2)) = 0.207), reduced inspired O2 concentration (normobaric hypoxia, HYX; P(B) = 614 mm Hg, F(I(O2)) = 0.142), and lowered ambient pressure without hypoxia (normoxic hypobaria HYB; P(B) = 434 mm Hg, F(I(O2)) = 0.296). Nine fit and healthy young men were exposed to these conditions for 10 h in a decompression chamber. Lake Louise AMS scores, urine collections, and blood samples were obtained every 3 h, with recovery measurements 2 h after exposure. AMS was significantly greater during ALT than HYX, as previously reported (J. Appl. Physiol. 81:1908-1910. 1996), because the combination of reduced P(B) and P(O2) over the 10 h favored fluid retention by reducing urine volume, while plasma volume (PV) remained higher than during HYX. At ALT the plasma Na+ fell significantly at 6 h, probably from dilution of extracellular fluid, and antidiuretic hormone (ADH) was highest (p = 0.006 versus HYB). The PV, urine flow, free water clearance, and plasma renin activity (PRA) rose significantly during recovery from ALT as AMS symptoms subsided, suggesting increased intravascular fluid and reduced adrenergic tone. During HYB, the plasma aldosterone (ALDO) and K+ levels were significantly elevated, and PRA was highest and ADH lowest, without fluid retention. During HYX, fluid balance was similar to HYB, but PV and ALDO were significantly lower, and ALDO increased significantly in recovery from HYX. The fluid retention at ALT in AMS-susceptible subjects appears related to a synergistic interaction involving reduced P(B) and ADH and ALDO.

“Live high–train low” using normobaric hypoxia: a double-blinded, placebo-controlled study

“…However, this is based on the assumption that the response to altitude is solely dependent on O 2 partial pressure and unrelated to barometric pressure. This has been questioned recently (12), and several studies have reported slight differences between the effects of normobaric and hypobaric hypoxia (37,46,54). Most importantly, in the present context, the reduction in CaO 2 seems to be more marked in hypobaric than in equivalent normobaric hypoxia (54).…”

Point: Counterpoint: Hypobaric hypoxia induces/does not induce different responses from normobaric hypoxia

“…Change in fluid balance has been reported to be different between HH and NH (18). Compared with HH, NH exposure results in a negative fluid balance, as indicated by a significant decrease in plasma volume.…”

“…Most previous studies have shown that the severity of AMS symptoms were higher in HH than in NH (6,18,20). The potential direct mechanisms (density of breathing gas; N 2 movement or direct influence on CNS) have been discussed (6) but other factors (e.g., hemodynamic factors) require further investigation (18).…”

Renal Function and Fluid Homeostasis

“…These responses occur within the fi rst hour and persist for 1-2 days. Diuresis and natriuresis occur with subsequent new and greater hypoxic challenges [ 3 ]. Antidiuresis, particularly associated with ADH release, causes the expected urinary concentration.…”