Acclimatization to altitude is a relatively slow process, usually attained by staying several days or weeks at progressively higher altitudes. Exposure to hypoxia in hypobaric chambers has been used as an alternative procedure to induce acclimation to altitude (13,14), and also to improve performance at sea level by training at different simulated altitudes (18-20).
The effect of hypoxia on the blood oxygen transport capacity, the erythropoietic response and the increase in Hb affinity for oxygen have also been studied (5,8,15). Most studies have revealed effective adaptations to chronic hypoxia, but acclimatization imposes considerable limitations on normal living conditions for several weeks and a reduction in the training level determined by the hypoxic conditions.
Athletes training at altitude work less intensely, which reduces the benefits obtained from altitude acclimatization. The most effective technique may be to live at altitude and train at sea level (10), but this approach may not be ideal for practical reasons.
In a previous study in the INEFC-UB hypobaric chamber (2), short-term intermittent hypoxia (equivalent to 4000-5500 m) combined with low-intensity exercise compatible with normal life at sea level, effectively induced acclimation in a group of six elite climbers (UPC-Everest '95). Adaptations included improvement of aerobic endurance and activation of erythropoiesis.
The aim of the present study was to characterize the effect of a very short (9 d) intermittent exposure to moderate hypoxia in a hypobaric chamber, on aerobic performance capacity at sea level and on the erythropoietic response and also to compare the effect of hypobaric hypoxia alone and combined with low-intensity exercise.
MATERIALS AND METHODS
A group of 17 members of three high-altitude expeditions (Cota 7000, Akon-cauak, and UME '97) to Aconcagua (6962 m), 14 men (mean age 28 ± 5 yr, mean weight 73 ± 5 kg, and height 177 ± 5 cm) and 3 women (mean age 27 ± 5 yr, mean weight 63 ± 13 kg and height 167 ± 3 cm) were exposed to intermittent hypoxia in the INEFC-UB hypobaric chamber. All subjects were informed about the objective and methods of the study. The investigation was made with their written consent and according to the recommendations of the Declaration of Helsinki.
Simulated altitude increased progressively from one day to the next from 4000 m (462 Torr; PO2 = 97 Torr) to 5500 m (379 Torr; PO2 = 79 Torr) over 9 d, from 3 to 5 h·d−1. Subjects were randomly assigned to two groups. One group (N = 7, 1 woman and 6 men; HE group) combined passive exposure to hypoxia with low-intensity exercise on a cycle ergometer (Monark AB model, Varberg, Sweden). The daily exercise protocol was 3-5 bouts of exercise, lasting 30-75 min every session at a predetermined heart rate (HR) (ranging from 120 to 130 beats·min−1, corresponding to an individual 100-W workload at simulated altitude equivalent of 5500 m). The second group (N = 10, 2 women and 8 men; H group) was only exposed to hypoxia in resting conditions. Table 1 presents the details of the experimental acclimation protocol in the hypobaric chamber.
Before and after exposure to the hypobaric hypoxia program, medical status, and performance capacity were evaluated. Medical evaluation included clinical history, physical characteristics, and cardiovascular and respiratory parameters. Individual performance capacity was established by means of a maximal incremental treadmill test at sea level with continuous "breath by breath" gas analysis (CPX II, Medical Graphics, St. Paul, MN) and capillary blood lactate determinations every 3 min. A modified Bruce exercise protocol was used, adding to the classical incremental procedure intervals of 30 s between loads to allow for blood sampling. HR was also monitored throughout the test. Blood lactate concentration was determined by an enzymatic method based on lactate dehydrogenase (LDH) activity (Lactate-Test, Boehringer-Mannheim, Germany) and quantified by spectrophotometry.
Complete hematological and hemorheological profiles were determined in venous blood samples. Blood withdrawal was performed (10 mL collected from the antecubital vein) before and after the maximal incremental treadmill test at sea level and also after returning from the expedition, from 10 to 12 d after reaching the summit (about 1 wk after the end of the expedition). All venous blood samples were taken without stasis using plastic syringes. Samples were immediately placed on ice in EDTA (hematology) and lithium heparin (hemorheology) tubes, where they were kept until assayed.
Hematocrit (Hct) or packed cell volume (PCV) measurements were made after centrifugation of capillary samples (Hemofuge Heraeus Sepatech, Germany) for 5 min at 12,000 g and were expressed as percentages. Red blood cells counts were determined using an automated cytological cell counter (Coulter Counter Model ZF, UK). Total Hb concentration was determined by the Drabkin's method (4). The absorbance at 540 nm was determined with a Spectronic 2000 spectrophotometer (Bausch & Lomb, Germany). A standard curve was prepared using a dilution of standard Hb. The reticulocytes were identified with a cresyl brilliant blue stain (16).
Venous blood was centrifuged at 3000 g for 10 min and the separated plasma was kept in Eppendorf tubes. Plasma osmolality was measured with a micro-osmometer (Advanced Instruments, 3MO model, U.S.).
The hemorheological profile was also determined, including plasma and blood viscosity, using a cone-plate microviscosimeter (Brookfield Digital Viscometer, Brookfield Engineering Laboratories, U.S.).
The lactate threshold was detected using a log-log transformation procedure, in order to determine the two breaking points of each lactate-load curves (7). In this method, three segments can be detected corresponding to initial levels, transition, and accumulation of blood lactate during exercise.
Statistical analysis included paired t-tests, one way ANOVA, and Wilcoxon matched-pairs signed ranks test, as performed by the SPSS statistical package (SPSS, Chicago, IL).
No significant differences were observed between the two groups of subjects (HE vs H), in any of the parameters studied: hematological values (Table 2), performance, functional adaptations, or hemorheological profile (data not shown). Accordingly, from now on, groups HE and H are considered as a single group. Values corresponding to the whole group (N = 17) are presented in the rest of figures and tables.
Maximal exercise test. After the acclimation program in the hypobaric chamber, a significant increase in maximal exercise time (mean diff. = +3.9%; P < 0.01) and maximal pulmonary ventilation (mean diff. = +5.5%; P < 0.03) was observed in the maximal incremental test at sea level. No significant differences were observed in maximal oxygen uptake or other maximal ergospirometric parameters (Table 3).
After the acclimation period all individual lactate-velocity curves shifted to the right. A significant increase of the mean lactate threshold (P < 0.05) was observed, as indicated by a reduction in the lactate values for the same workload (2.13 ± 0.34 to 1.66 ± 0.07 mmol·L−1 in step 3, and 5.13 ± 0.95 to 4.49 ± 1.23 mmol·L−1 in step 4) (Fig. 1).
Hematological parameters. The values of the hematological parameters are shown in Figure 2. Hematological changes after exposure to hypobaric hypoxia were characterized by a significant increase in Hct [PCV] (mean diff. = +11.1%; P < 0.01), RBC count (+12.1%; P < 0.01), reticulocyte count (+54%; P < 0.01), and Hb concentration (+17.7%; P < 0.01). Nonsignificant differences in the hematological indexes (MCV, MCHC, and MCH) were found after the acclimation program. Hb concentration (mean diff. = +11.0%; P < 0.01) and reticulocyte count (mean diff. = +52%; P < 0.01) were significantly higher after the expedition than postacclimation, whereas PCV and RBC count were at similar level (Fig. 2). However, MCHC and MCH increased after the climbers returned from Aconcagua.
Plasma osmolality (mean diff. = +5.4%) significantly increased after the acclimation period. Nevertheless, no significant differences were observed after the expedition.
Hemorheological profile. No significant differences were found in the hemorheological profile at shear rates between 2.25 and 450 s−1(Fig. 3), although a slight non-significant increment in blood and plasma viscosity were observed after the acclimation period.
Very short intermittent exposure to hypoxia in a hypobaric chamber at moderate simulated altitude (4000 up to 5500 m) induced acclimation and improved aerobic performance capacity in healthy subjects.
During exercise, the oxygen requirements of the muscle increase dramatically. In hypoxic conditions, the oxygen availability decreases, which is the main cause for lower physical performance capacity (3,17,24). The most important adaptations to altitude (acclimatization) are those concerning the oxygen transport system (i.e., the respiratory and circulatory systems, as well as the response of hematopoietic tissues). Oxygen diffusion in the alveoli is not hindered by altitude, neither is the oxygen transport capacity of the heart (6,17,22).
In the present study, the acclimation program in the hypobaric chamber led to a significant increase in maximal exercise time, although no significant differences were observed in maximal oxygen uptake. This can be explained by the increase in the lactate threshold, which has been consistently related to the aerobic endurance capacity in both normoxic (11) and hypoxic conditions (9). In lowlanders acutely exposed to moderate altitude, the arterial blood lactate concentration rises during exercise similar to exercise at sea level, but this lactate response is progressively blunted during short-term acclimatization at altitude (1,12). We have observed a progressive shift to the right of lactate curves in elite swimmers training at moderate altitude (1850 m) over 5 wk (Rodríguez, unpublished results) and in elite climbers after acclimation in a hypobaric chamber for 18 d at 4000-5000 m (2). In the present experiment, the increase in maximal exercise time can be attributed to lower lactate accumulation during exercise, with the subsequent enhancement of aerobic endurance capacity.
The increase in maximal pulmonary ventilation observed after the acclimation program is similar to that found by other authors (14), and can be related to the significant longer duration of the exercise test.
The most remarkable adaptations to the acclimation program are probably those observed in the hematological profile. They were characterized by a significant increase in the erythrocytic mass, as evidenced by a significant rise in PCV, red blood count, reticulocyte count, and Hb concentration (mean diff. = 2.52 g·dL−1 = 17.7%). These adaptations can be clearly associated with an enhancement of blood oxygen transport capacity. These adaptations may have continued during the expedition to high altitude, as the Hb concentration and reticulocytes were significantly higher after returning from the expedition than postacclimation, although PCV and RBC count did not change. The hematological changes are consistent with those observed in our previous study (2) in which the intermittent exposure period was much longer in duration (18 d). Consequently, it seems to be demonstrated that very short-term intermittent exposure to hypoxia can also stimulate erythropoiesis to the same extent as to short-term or permanent exposure.
Concerning the differences observed in plasma osmolality, previous studies have found different sodium plasma concentrations when comparing high- and lowlanders. This finding could be related with antidiuretic hormone (ADH) modulation, as it has been proved that hypoxia stimulates ADH release, and as a result, increases plasma osmolality (25). Although increase in plasma osmolality was indeed observed in the present study after the acclimation period in the chamber, it does not completely explain the remarkable rise in PCV and Hb concentration. After correcting for the effect of a rise in plasma osmolality according to Van Beaumont et al. (21), both changes were still significant, thus indicating that a slight hemoconcentration does not explain the increased PVC and RBC count.
As a consequence of the rise in the erythrocytic mass, one would expect an increase in blood viscosity, but the hemorheological characteristics did not significantly change in our observations after the acclimation period, although a positive trend was observed. Moreover, the hemorheological profile observed after the expedition to Aconcagua was almost identical to that observed before the acclimation period. The presence of compensatory mechanisms, possibly related with erythrocyte aggregability (clearly seen at low shear rates) and also with erythrocyte deformability, which could eventually prevent negative effects evoked by an increase in blood viscosity can be hypothesized. This has particular relevance when Hb concentrations are in excess of 18 g·dL−1, because blood viscosity increases markedly over these values (23).
Interestingly, after the return from the expedition, hematological changes indicated that the erythrocytic mass had slightly decreased (lower PCV and RBC count values), whereas Hb concentration and reticulocytes showed a further increase. This adaptation would improve the blood oxygen transport capacity thus avoiding a further increase in blood viscosity.
Because no significant differences were observed between the two groups of subjects (HE vs H), nor in performance, functional adaptations, or hematological-hemorheological profile, hypobaric hypoxia alone seems responsible for all these adaptations.
On the other hand, none of the subjects reported remarkable clinical acute mountain sickness or physical deterioration symptoms, and only very mild headache and tympanic pain were observed in a few cases among some of the subjects. None of these symptoms caused the interruption of the hypoxic exposure session, or required medical treatment.
From these results, it was concluded that very short-term (9 d) intermittent exposure (3-5 h·d−1) to moderate hypoxia (462-369 Torr) in a hypobaric chamber activates the erythropoietic response and improves the aerobic performance capacity in healthy subjects.
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