Fluid Shifts with Acute Exposure to Normobaric and Hypobaric Hypoxia

Abstract

Purpose To determine the body fluid shifts that occur with exposure to normobaric hypoxia (NH) and hypobaric hypoxia (HH) compared to normobaric normoxia (NN). Methods Eighteen males and females (male=10, female=8, age 35±7 yrs) completed three trials on back to back days in a randomized order. NN was conducted at sea level, NH was conducted at sea level but breathing a gas mixture to simulate 4,617 m of elevation, and HH was conducted at 4,617 m of terrestrial elevation. The time and rate of hypoxia exposure was consistent between NN and HH (0 m to 4,617 m in 90 minutes). Finger-stick blood samples were taken after the 90 minute intervention and analyzed for hemoglobin and hematocrit to calculate plasma volume shifts and urine was collected to measure urine specific gravity. After 10 minutes of supine rest, body water was analyzed via BIA. Results Plasma volume change was not altered by NH relative to NN (-3.77±8.36%, p = 0.073), but lower in HH relative to NN (-16.610±7.84%, p<0.001). Urine specific gravity was similar between all trials (p=0.120, main effect). Total body water was not different between NN (43.8±7.6 L) and NH (43.7±7.7 L, p=0.492), but was lower in HH (43.0±7.6 L) than NN (p<0.001) and NH (p<0.001). Extracellular fluid was not different between NN (16.35±2.85 L) and NH (16.32±2.84 L, p=0.696), but was lower in HH (16.08±2.78 L) than NN (p<0.001) and NH (p=0.006). Intracellular Fluid was not different between NN (27.41±4.81 L) and NH (27.33±4.89 L, p=0.420) but was lower in HH (26.97±4.80 L) than NN (p<0.001) and NH (p=0.016). Conclusion Body fluid shifts are stimulated by an alteration of barometric pressure and not by hypoxia. Caution should be made when applying data from studies conducted in NH to HH.