Acute, high altitude exposure induces a large variety of adaptive mechanisms in the nonadapted human body. Currently, the main research focus has been on the physiology and pathophysiology of the cardiovascular, cerebral, and pulmonary systems, including maladaptation, in acute mountain sickness [
Most information about acute hypoxia and hemostatic changes has been obtained by studies focusing on long-haul flights and travel thrombosis. However, these data are inconsistent [
Even studies focusing on high altitude are nonuniform. Maher et al. [
At high and extreme altitudes, subjects are exposed to a variety of factors which could influence the hemostatic system (e.g., cold, dehydration, polyglobulia, immobility during periods of bad weather, and exhaustive physical exercise). Since decades, thrombotic and thromboembolic events have been described in climbers [
Hypoxic chamber studies appear to be an effective and valid method to investigate acute mountain sickness (AMS), since AMS not only manifests in nonacclimatized trekkers and mountaineers who rapidly ascent to altitudes above 2,500 m [
To evaluate the effects on procoagulants by acute and chronic hypoxia [
We hypothesized that a 12-hour sojourn in a hypoxic chamber corresponding to 4,500 m would provoke the activation of hemostasis in nonacclimatized healthy volunteers. In addition, we speculated that this coagulation activation is more pronounced in volunteers who develop AMS during hypoxia as compared to those who do not.
The present study was part of a large, simulated, high altitude project performed in Innsbruck, Austria. Parts of the project have already been published [
All participants gave their written informed consent prior to participation in the study. The study was carried out in conformity with the ethical standards laid down in the 2008 Declaration of Helsinki and was approved by the Ethics Committee of the Medical University of Innsbruck (program code: UN4522, session: 306/4.11).
Participants were passively exposed to a FiO2 of 12.6% (corresponding to a simulated altitude hypoxia of 4,500 m at 590 m, PiO2 = 83.9 mmHg) for 12 hours. Room temperature and humidity were kept constant at 22–24°C and 23–27%, respectively. Prior to entering the hypoxic chamber, participants were examined, including a medical routine check. During the stay in the chamber, food (e.g., brown bread, cheese, boiled ham, cucumber, banana, apple, cookies, and chocolate) and drinks (water and apple juice) were provided ad libitum. Most of the time, participants stayed seated, but some activities (e.g., standing, walking, and stretching) were also performed. Recumbent position or sleeping was not allowed.
To assess the prevalence and severity of AMS [
Arterial oxygen saturation (SpO2) and heart rate were measured using pulse oximetry (Onyx II 9550, NONIN, Plymouth, MI, USA) after 0.5, 3, 6, 9, and 12 hours in the chamber.
Venous blood samples were taken immediately before and at the end of the 12-hour hypoxic exposure or when participants left the chamber at an earlier time point. Blood processing was done immediately thereafter in the hemostasis laboratory.
Blood was drawn in ethylenediaminetetraacetic acid-containing tubes (Sarstedt, Nümbrecht, Germany) for platelet counts and in sodium citrate-containing tubes (Sarstedt) for coagulation assays. Platelet counts were determined using the Sysmex XE 5000 hematology analyzer (Sysmex Corporation, Kobe, Japan).
Platelet-poor plasma for the determination of plasma coagulation tests was obtained by centrifugation at 2,100 ×g for 15 min and another centrifugation at 10,000 ×g for 5 min.
Parameters of aPTT, PT, and Clauss fibrinogen assay results [
In ROTEM, the resistance of a rotated pin in a stationary cuvette, filled with citrated whole blood, is measured after coagulation activation with different reagents. For additional monitoring of routine coagulation tests in subjects, the following parameters were automatically detected by the ROTEM analysis software based on thromboelastograms: clotting time (CT), clot formation time (CFT), alpha angle, maximum clot firmness (MCF) after 15 min in ExTEM (activation of coagulation via tissue factor) and INTEM (activation of coagulation via the contact phase), and MCF after 15 min in FIBTEM (activation of coagulation via tissue factor and cytochalasin D for the inhibition of platelets). Although the ROTEM system was mainly established and used for the control and differential diagnosis of hemostatic disorders within the context of acute bleeding, recent literature has also suggested a possible role for the ROTEM system in testing for hypercoagulable states [
Thrombin generation analysis was performed with platelet poor plasma using the Innovance ETP assay (Siemens) on a BCS XP automated coagulation analyzer (Siemens). After the activation of coagulation with synthetic phospholipids [
All participants who endured the simulated exposure to high altitude for 12 hours without suffering severe symptoms were included in the statistical analysis. Others were counted as dropouts. Box plots were used to determine whether data distributions were symmetrical. Parametric tests were used for normally distributed data, and nonparametric tests were used for skew data. Results from categorical variables are reported as proportions, and continuous variables are reported as means ± standard deviation (SD). Comparisons of parameters before and after simulated altitude exposure were made using the Student’s
Changes over time in heart rate and SpO2 values, as well as location of changes over time, were calculated using linear models for repeated measurements. Significant values were adapted using Bonferroni’s correction. Two-tailed
All statistical calculations were made using IBM SPSS Statistics, version 20.0 (Chicago, IL, USA).
No serious or unexpected adverse events were observed during the chamber stay.
In total, 37 participants were included in the statistical analysis (Table
Anthropometric data and baseline characteristics.
All ( |
Male ( |
Female ( |
AMS− ( |
AMS+ ( |
|
---|---|---|---|---|---|
Age [y] | 25.9 ± 5.6 | 25.3 ± 5.2 | 26.7 ± 6.1 | 26.4 ± 3.9 | 25.6 ± 6.5 |
Body height [cm] | 174 ± 9 | 179 ± 8 | 168 ± 5 | 178 ± 10 | 172 ± 7 |
Body weight [kg] | 67 ± 11 | 73 ± 9 | 60 ± 8 | 69.7 ± 13.2 | 66.0 ± 9.3 |
BMI [kg/m2] | 22.0 ± 2.3 | 22.7 ± 2.2 | 21.2 ± 2.2 | 21.8 ± 2.8 | 22.2 ± 1.9 |
Baseline characteristics of included participants after classification of those without AMS (AMS−) and those suffering from AMS symptoms (AMS+). Data are shown as mean values ± standard deviation.
Both groups (AMS−/AMS+) were homogeneous, and no significant differences were shown for age (
One participant who discontinued chamber exposure after 5 hours and 40 min was classified as a dropout, because he showed no symptoms of AMS and terminated for unknown reasons. Five subjects were excluded as dropouts as a result of preanalytical errors such as insufficient blood sample volumes or missing AMS data. Due to severe symptoms, nine participants (three male and six female subjects) left the chamber before the end of the 12 hours (mean LLS =
Whole-group data showed that both HR and SpO2 significantly changed over time (HR from
HR was significantly higher after 3 hours of hypoxic exposure compared to preexposure values (
All baseline data of the aPTT and PT analysis were within the reference range. There were no changes in aPTT for the whole group or for AMS+ and AMS− subjects during hypoxic exposure. PT was increased after the chamber sojourn in the AMS− group only. A comparison of the differences (data before and after the chamber session) of the two populations resulted in a significant ΔPT (
Pooled data did not show any changes in platelet counts or fibrinogen, and baseline data stayed within reference ranges (Table
Levels of aPTT, PT, platelet count, fibrinogen, SpO2, and heart rate.
ALL | AMS+ | AMS− | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Before | After |
|
|
Before | After |
|
|
Before | After |
| |
aPTT (26–37 sec) | 37 | 31.5 ± 3.7 | 30.8 ± 3.4 | 0.091 | 23 | 30.9 ± 3.5 | 30.1 ± 3.0 | 0.100 | 14 | 32.5 ± 3.9 | 31.9 ± 3.7 | 0.489 |
PT (70–130%) | 37 | 91.7 ± 10.3 | 94.3 ± 11.2 | 0.058 | 23 | 93.3 ± 8.8 | 93.7 ± 8.5 | 0.748 | 14 | 89.1 ± 12.3 | 95.2 ± 15.1 |
0.029 |
Platelet count (150–380 G/L) | 37 | 226.0 ± 43.6 | 232.3 ± 61.4 | 0.340 | 23 | 233.5 ± 36.7 | 232.3 ± 64.1 | 0.908 | 14 | 213.8 ± 52.2 | 232.4 ± 58.9 |
0.000 |
Fibrinogen (210–400 mg/dL) | 37 | 233.1 ± 36.9 | 238.6 ± 41.0 | 0.169 | 23 | 238.5 ± 38.8 | 246.2 ± 45.3 | 0.104 | 14 | 224.1 ± 33.0 | 226.2 ± 30.3 | 0.787 |
SpO2 (92–98%) | 37 | 98.1 ± 1.2 | 85.1 ± 6.2 | 0.000 |
23 | 98.4 ± 1.0 | 85.3 ± 6.9 | 0.000 | 14 | 97.5 ± 1.5 | 84.1 ± 6.2 | 0.000 |
Heart rate (72–77 bpm) | 37 | 76 ± 12 | 84 ± 15 | 0.008 |
23 | 79.8 ± 14.3 | 86.3 ± 16.4 | 0.011 | 14 | 72.6 ± 7.0 | 81.1 ± 12.0 | 0.042 |
aPTT, PT, platelet count, and plasma fibrinogen concentration for all participants (ALL) and for volunteers with (AMS+) and without (AMS−) AMS before and after hypoxia. Data are shown as mean values ± standard deviation.
All baseline data from the thrombin generation analysis were within the reference range (Table
Thrombin generation.
ALL | AMS+ | AMS− | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Before | After |
|
|
Before | After |
|
|
Before | After |
| |
ETP AUC (76–107%) | 37 | 89.2 ± 10.3 | 89.9 ± 11.4 | 0.430 | 23 | 90.1 ± 11.0 | 90.8 ± 11.5 | 0.474 | 14 | 87.9 ± 9.4 | 88.3 ± 11.5 | 0.734 |
|
37 | 94.0 ± 11.9 | 100.2 ± 16.7 | 0.019 |
23 | 95.4 ± 12.4 | 101.7 ± 17.1 | 0.083 | 14 | 91.7 ± 11.0 | 97.8 ± 16.5 | 0.122 |
|
37 | 23.9 ± 4.9 | 23.3 ± 3.8 | 0.540 | 23 | 24.2 ± 5.9 | 23.8 ± 4.4 | 0.802 | 14 | 23.4 ± 2.4 | 22.5 ± 2.4 | 0.143 |
|
37 | 66.5 ± 16.9 | 60.5 ± 7.7 | n.a. | 23 | 67.6 ± 18.2 | 59.7 ± 5.5 | n.a. | 14 | 64.7 ± 15.1 | 61.8 ± 9.8 | 0.465 |
n.a.: not applicable.
Endogenous thrombin potential (ETP AUC), maximum concentration of thrombin (
ROTEM baseline data measurements changed within reference ranges (Table
ROTEM measurements.
ALL | AMS+ | AMS− | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Before | After |
|
|
Before | After |
|
|
Before | After |
| |
CT InTEM (134–218 sec) | 37 | 156.2 ± 12.8 | 149.9 ± 16.6 |
0.012 | 23 | 155.7 ± 13.2 | 148.6 ± 14.4 |
0.011 | 14 | 157.0 ± 12.6 | 152.1 ± 20.0 | 0.327 |
CT ExTEM (42–78 sec) | 37 | 48.0 ± 7.2 | 49.8 ± 9.8 | 0.329 | 23 | 48.7 ± 7.9 | 50.2 ± 11.6 | 0.596 | 14 | 46.7 ± 5.9 | 49.1 ± 6.2 | 0.283 |
CFT InTEM (52–116 sec) | 37 | 85.5 ± 17.2 | 83.5 ± 20.5 | 0.406 | 23 | 80.0 ± 16.1 | 81.5 ± 18.6 | 0.552 | 14 | 94.6 ± 15.5 | 86.8 ± 23.7 | 0.103 |
CFT ExTEM (53–144 sec) | 37 | 107.6 ± 22.3 | 107.1 ± 28.4 | 0.884 | 23 | 99.4 ± 18.2 | 103.6 ± 24.7 | 0.325 | 14 | 121.2 ± 22.3 | 112.9 ± 33.8 | 0.237 |
MCF InTEM (47–69 mm) | 37 | 54.8 ± 4.2 | 55.8 ± 4.3 |
0.039 | 23 | 55.7 ± 4.4 | 55.8 ± 4.5 | 0.749 | 14 | 53.5 ± 3.7 | 55.7 ± 4.1 |
0.005 |
MCF ExTEM (48–70 mm) | 37 | 56.9 ± 4.5 | 56.6 ± 5.1 | 0.677 | 23 | 58.2 ± 4.2 | 57.0 ± 5.0 | 0.088 | 14 | 54.8 ± 4.2 | 56.1 ± 5.6 | 0.163 |
MCF FibTEM (7–21 mm) | 37 | 11.6 ± 2.4 | 12.4 ± 2.8 |
0.035 | 23 | 12.2 ± 2.4 | 12.8 ± 2.9 | 0.307 | 14 | 10.6 ± 2.3 | 11.9 ± 2.6 |
0.008 |
ROTEM analysis before and after chamber exposure for all volunteers (ALL) and according to absence (AMS−) or presence (AMS+) of AMS. ROTEM reference values are given in parentheses. Data are shown as mean values ± standard deviation.
The CT InTEM analysis showed a significant shortening in AMS+ subjects (ΔCT = −
When assessing the relationship between LLS max and the laboratory parameters, a Spearman correlation coefficient of 0.329 (
The aim of the study at hand was to investigate possible hypoxia-induced changes in hemostasis in primary healthy volunteers during short-term (12 hours) exposure in a normobaric hypoxic chamber. Furthermore, it was hypothesized that subjects developing AMS would exhibit an activation of coagulation. However, during the 12-hour hypoxic exposure, only a few, small changes in the routine as well as in the specialized parameters of coagulation and fibrinolysis could be detected. Moreover, no significant differences in key parameters between volunteers who developed AMS and those who did not were measured.
The present chamber study simulated an altitude of 4,500 m, which was sufficient to provoke AMS even within 12 hours. The overall prevalence of AMS was 62.2%, which proved that our setting was adequate not only to investigate possible coagulation alterations for the whole group of participants but also to detect group differences (AMS+ versus AMS−). There are different approaches to investigate AMS and its consequences in controlled settings. Commonly used methods are chamber decompression to generate hypobaric hypoxia or adjustments for oxygen levels for normobaric hypoxia [
Our unacclimatized participants showed reduced peripheral capillary SpO2 and increased HR in the chamber. These results are in accordance with others who reported an activation of the sympathetic nervous system during acute exposure to high altitude, which was evident not only during environmental exposure but also in hypobaric chambers [
Currently, the majority of publications related to hemostatic alterations in hypoxia have not been based on high altitude but on travel medicine, that is, travel-related thromboembolism. Studies were performed either under simulated moderate hypoxic conditions or during situations of long-distance travel (flights or bus travel). The corresponding data are conflicting and results vary from unchanged coagulation [
Data on hemostasis during real ambient hypoxia at high altitudes is scarce and more or less inconsistent. Only a few studies that focused on coagulation changes within a few hours of hypoxia are available. After a 22-hour ascent to 4,559 m Bärtsch et al. reported only a slight increase in PF1+2 with no evidence of significant thrombin or fibrin formation [
By pooling all data of the participants, independent of developing AMS, only a few significant changes in the measured hemostatic parameters were detected. In detail, in the thrombin generation analysis
In order to detect possible effects of hypoxia on AMS genesis, subgroup analyses were performed. In the AMS+ group, no changes in the standard coagulation tests aPTT, PT, platelet count, or fibrinogen were detected. In the AMS− group, PT was shortened and platelet count was increased after hypoxia. ETP AUC remained unchanged in both groups during chamber exposure. Subgroup results of the ROTEM parameters showed shortening of CT InTEM in the AMS+ population and an increase of MCF in the InTEM and FibTEM analysis. Comparing the absolute changes between both groups, no stringent evidence for significant or relevant differences between AMS+ versus AMS− exists.
In case of a strong association between changes in hemostasis and the development of AMS, significant correlations of the maximum LLS during hypoxia with the measured coagulation parameters should have been obtained. However, only a few of the evaluated parameters showed a significant association with maximum LLS (PT, aPTT, CFT InTEM, MCF InTEM, and MCF ExTEM), and correlation coefficients were moderate to low (<0.45) in all cases. This result might indicate the lack of a pathophysiological and clinical relationship between the development of AMS and hypercoagulation.
Limitations of the present study include the relatively small number of participants (
In short, the hypothesis of a procoagulant effect of acute hypoxia in healthy individuals was not supported by the present study, since all data remained within normal reference ranges. Furthermore, a clinically relevant alteration of hemostasis in subjects suffering from AMS was not detected during exposure to hypoxia. Therefore, the authors conclude that there is no association in the development of AMS and hypercoagulability.
With respect to high altitude medicine, more studies need to be performed applying new hemostaseological methods that indicate in vivo thrombin formation during longer lasting high altitude exposure.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Marc Schaber was supported by a grant of the Austrian Society for Mountain Medicine (ÖGAHM). The authors thank the subjects who participated in their study and helped to expand the knowledge about high altitude medicine. The project was financially funded by the Oesterreichische Nationalbank.