Oxygen Saturation Response to Exercise VO₂ at 2100 m and 4350 m in Women Mountaineering Trainees

Abstract : Human work performances decreases at high altitude (HA).  This decrement does not appear to be similar for every individual, may be due to variety of factors like elevation, mode of induction, work intensity, physical condition and specificity of the subjects.  The purpose of the study was to evaluate the effects of alteration in responses of oxygen saturation (SaO₂) and oxygen consumption (VO₂) to a standard exercise in women mountaineering trainees under hypobaric hypoxia.  Experiments were conducted in 2 groups (10 each) of females and compared the difference in responses of native women of moderate altitude with those of the plains/low altitude.  A standard exercise test (Modified Harvard Step-Test for women) was performed on a 30 cm stool with 24 cycles/min for 5 min, initially at 2100 m and then at 4350 m.  The exercise VO₂ values for plains dwelling women achieved apparently VO_2max level at both altitude locations with significant reduction in SaO₂ during standard exercise.  Exercise VO₂ values decreased on exposure to 4350 m, with further reduction in SaO₂ . Whereas with same work intensity, under same situation the exercise VO₂ values of the moderate altitude women did not appear to have reached VO_2max. They also maintained comparatively higher level of SaO₂. It may be concluded that hypoxic exposure along with mountaineering training, the moderate altitude women maintained a higher level of SaO₂ during standard exercise at both altitude locations, compared to low altitude women who might have lost a compensatory reserve to defend the hypoxic stress to exercise.  Thus, moderate altitude women are proved to be better fit for hypoxic tolerance/HA performance.

 Key words : high altitude, women,VO_2max, oxygen, saturation, modified harvard, step-test

INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

INTRODUCTION

High altitude (HA) poses several operational problems to the sojourners, soldiers as well as mountaineers.  During, ascent to HA, a progressive decrease in atmospheric oxygen pressure severely affect the oxygen gradient between atmospheric air and muscle.  Maximum aerobic capacity is used as a performance index and generally accepted as the best single measure of the functional limit of the combined cardiovascular and respiratory system to delivery adequate amount of oxygen to the active muscles and demonstrate the capability of the muscle to use oxygen.  The decrement of maximal work capacity (VO_2max) of sojourners under HA hypoxic condition is well documented at various altitudes during both acute and prolonged exposure (1-5).  The decrement of oxygen consumption is directly related to the decrease in partial pressure of oxygen at HA (6, 7), even though the rate and magnitude of decrease varies with the elevation, mode of induction, status of hypoxic adaptation, exposure duration, level of activity and sojourner's place (altitude) of birth/residency, etc. (8, 9).  Endurance trained athletes with larger aerobic capacities (>60 ml/kg/min) appear to be selectively affected, demonstrating larger declines in VO_2max at altitude (3,000-4,300 m) compared with the untrained individuals with lower aerobic capacities (10-12).  The completeness of pulmonary gas exchange system may play a vital role in the individual variation of this apparent training status/altitude/ VO_2max relationship (5, 12, 13).  The decrease in arterial oxygen saturation (SaO₂) during maximal exercise at sea level as well as mild and moderate altitudes have been reported (2).  It has been demonstrated that both SaO₂ and the ratio of lung diffusion capacity to VO_2max during maximal exercise is highly correlated to the decline in VO_2max from sea level to altitude (10, 12).  Oxygen hemoglobin dissociation curve is sigmoid in nature, which allows arterial PaO₂ to be reduced during exercise to as low as 75-80 mm Hg with only a small percentage reduction in SaO₂ Once PaO₂ falls below approximately 75 mmHg, a relative small change in PaO₂ begins to have much larger effect on SaO₂. Chapman et al (14) examined the relationship between the degree of arterial oxygen desaturation during maximal exercise at sea level in highly trained athletes and the decline in SaO₂ and VO_2max at a mild (simulated) altitude.  However, no field study appears to have been carried out to evaluate the effects of mountaineering training on the changes in exercise oxygen consumption as well as the changes in SaO₂ to a standard exercise under hypoxic conditions on women residents of moderate altitude and plains.  The mountaineering training course conducted by Himalayan Mountaineering Institute (HMI), Darjeeling, India provided an unique opportunity of carrying out this field study on women during mountaineering training in the Eastern Himalayas.  The purpose of the present study was to determine the response changes in end exercise VO₂ and SaO₂ during standard exercise at 2100 m and 4350 m on 2 groups of women, due to mountaineering training with altitude adaptation, inducted gradually by trekking to 4350 m with a view to evaluate the performance relationship between the native women of moderate altitude and the plains dwelling females.
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METHODS

Experiments were conducted on 20 women trainees, who had come to Himalayan Mountaineering Institute (HMI), Darjeeling (2100m) for a basic mountaineering course.  The subjects were selected randomly from a total strength of 63 trainees.  They all were Indians and belonged to middle class socioeconomic status. Out of these, 10 were residents of moderate altitude (2000-2100 m) and the remaining 10 belonged to the different plain regions (altitude not more than 260m) of India.  Maximum care was taken to match both the groups in respect of age, height and body weight.  After a practice run of the tests the volunteers gave informed consent.  They were also aware of their rights to withdraw from the experiments at any time.  The protocol was approved by the Ethic Committee of the Institute.  All the subjects were medically examined to rule out any systemic illness.  The personal history indicated about their good health, active physique and high motivation.  The ambient temperature at Darjeeling during

The course of study ranged between 7-21 ºC. The study was conducted in to phases : (i) Initial pre-training recordings were carried out at HMI (2100 m), in the temperature maintained (16-20 ºC) MI room at the commencement of the course.  All the pre-climbed tests were completed within three days of arrival of the subjects at HMI. (ii) On induction to 4350 m by trekking the standard exercise test was repeated after three days of sojourn at the base camp of HMI (4350 m), inside the silver hut, where the temperature was maintained between 17-20 ºC.  All the tests were conducted by the same group of observers with same set of equipment in both the situations.  Prior to the tests, the subjects rested in the temperature maintained room for 30 – 35 min.

Pulmonary ventilation (V_E ) and respiratory frequency (Rf) were recorded with the help of ventilation monitor (P.K. Morgan, England) using a low resistance breathing valve fitted with mouth piece and nose clip.  Expired air was analyzed for O₂ and CO₂ content by rapid response Zirconium O₂ analyzer and 901-MK2 CO₂ analyzer (P.K. Morgan, England).  Heart rate was recorded by telemetric method using sports tester (Model PE = 3000, Finland).  Arterial oxygen saturation (SaO₂) was measured with the help of a finger-oximeter (Nellcor, USA) calibrated before and after each study.  The standard exercise test was performed by using a portable wooden stool of 30 cm high with 24 cycles/min for 5 min (modified Harvard Step-Test for women).  The same stool was used in both phases to ensure uniformity in the step-test.  In the absence of any transport it was not possible to carry the heavy bicycle ergometer to 4350 m by porter/yak. Under the circumstances in the field situation, the observers had no choice but to perform the standard exercise test on the wooden stool (modified Harvard Step-Test for women). Harvard Step-Test is a standard method of exercise for assessing physical performance, particularly under field conditions.  Sengupta et al (15) compared Harvard StepTest "scores" of 20 soldiers with their 1 mile running time in the field and had found this is a fairly good method for measuring physical efficiency, which was comparable with field performance.  The O₂ and CO₂ analyzer modules of exercise test assembly (P.K. Morgan, England) were taken to HA separately.  Monitoring of Rf, V_E ,V02, HR and SaO₂ was done initially and thereafter every min during exercise.  V_E was corrected to BTPS and the VO₂ and VCO₂ were corrected to STPD.  The criteria for the assessment of VO_2max includes (a) a heart rate in excess of 90% of age predicted maximum (220 - age), (b) a respiratory exchange ratio of >1.10 and (c) a plateau (<150 ml increase) in VO₂ with the increase in work.  If at least two of the three criteria were met, the highest VO₂ recorded was chosen as the subject's VO_2max.

At Darjeeling, the subjects had to undergo a rigorous training schedule of mountaineering activities for a period of one week from early morning till late evening with intermittent breaks.  After a week's training, they set their journey to the base camp (4350 m).  The trainees were transported (160 km) by bus (day-1) from Darjeeling to Yoksom (2200m).  Next morning (day-2) all the trainees trekked for about 15 km and reached Bhakim (2750 m( in 5-7 h where they stayed for the next day (day-3)).  On day-4, the subjects ascent on foot to Zamlingaon (3660 m) after crossing over a height of 4500 m, covering a distance of 12 km in 5 - 7 h and halted there for the next day (day-5) for acclimatization.  On day 6 the trainees about 14 km in 5-7 h and reached base camp of HMI at Chaurikhang (4350 m), located at the base of the peak Kanchanjangha, in the Eastern Himalayas.  The trekkers were trekking at about 23 km/h for 5-6 h each day, while carrying about 15 kg, load on their body.  During stay at intermittent altitudes of 2750 m and 3660 m as well as at 4350 m the subjects were hiked to higher elevation and brought back to the camp.  They were also engaged in different mountaineering activities for 4 - 5 h each day, which was an integral part of the training schedule for acclimatization.  Thus, the subjects experienced greatest physical strain while trekking to base camp as well as during sojourn.

On day 7-16 all the subjects stayed at 4350 m in the hut made of metal sheets fitted with bunks and used winter clothing.  There they had undergone intense mountaining activities like glacier -marching, ice cutting, peak assault, back packing, repelling, rock climbing etc. and were engaged in snow bound field areas at higher elevations in the forenoon and nearby camp areas in the afternoon.  The maximum and minimum ambient temperatures at the base camp during the period of study were +10 ºC and –7 ºC, respectively.  The clear sunshine and partial cloudiness during the day with occasional snowfall and high velocity wind are the characteristic features of environment there.

Two way classification of analysis of variance technique using Newman Keuls multiple range test has been used for the statistical analysis of the data, to compare the same group at different situations.  Unpaired t-test for the comparison of two different groups for each situation has been used.  Mean ± SD values are presented in the text/tables and P<0.05 has been used as level of significance.
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RESULTS  

Physical characteristics of both the groups are quite similar.  The mean ± S.D value of age and height for moderate altitude women were 20.2 ± 2.6 yrs and 150.5 ± 5.6 cm and for the low altitude women were 21.1 ± 3.4 yrs and 156.5 ± 4.1 cm, respectively.  The body weight showed very negligible fluctuations.  The values being 46.4 ± 6.4 and 46.7 ± 6.3 kg for moderate altitude women and 47.5 ± 5.6 and 47.4 ± 5.4 kg for low altitude women respectively, at 2100 m and 4350 m. The initial resting SaO₂values (98%) were similar for both the groups at 2100 m altitude.  On gradual ascent to 4350 m both the groups showed significant reduction (P<0.001) in SaO_2. The values being 89% and 87.1% respectively, for moderate altitude and plains dwelling women.  The magnitude of all (2%) was comparatively (P<0.05) more for the plains dweller at HA. Data of standard exercise tests for both the groups at 2100 m and 4350 m are presented in Table I. It appears from the responses of different variables that women from the plains have achieved end exercise VO₂ value almost to the level of VO at both the altitude locations, as has been judged by the fact that, subjects of this group met two of the three criteria for assessment of VO_2max. The subjects of this group were exhausted completely by the end of 5 min exercise.  Whereas the moderate altitude women did not appear to have achieved VO_2max under the same circumstances.  The subjective opinion of the volunteers of' this group (moderate altitude women) were that they would have continued the step test exercise for furthermore time to reach maximum VO₂ (VO_2max). It appears, they required higher stimulus for attainment of exercise oxygen consumption to VO_2max level.  During exercise at 4350 m low altitude women showed a significant reduction in exercise VO₂ values compared to that of their own record at 2100 m (42.47 ± 4.12 ml/kg/min Vs 40.04 ± 4.26 ml/kg/min; P<0.05), whereas the native women of moderate altitude did not demonstrate any marked reduction in oxygen consumption (37.44 ± 5.11 ml/kg/min Vs. 36.79 ± 4.66 ml/kg/min) at the end of exercise at 4350 m. The SaO₂ values of low altitude women reached 86.80 ± 1.65 % during standard exercise at 2100 m, as against their resting value of 98% (D 11.2%).

 

Table I click for full view

Table I: Mean ± S.D. values of various physiological parameters of both the groups during standard exercise at 2100 and 4150 m.

This reduction in end exercise SaO₂ value is significantly (P<0.05) more compared to that of the values of moderate altitude women (90.60 ± 0.97 %; D 7.4%). During standard exercise at 4350 m, SaO₂ values decreased further to 71.20 % and 77.3 % respectively for the women of plains and moderate altitude.  This indicates a fall of 26.8 % and 20.7 % respectively, compared to the initial resting value of' 98 % at 2100 m.  When compared with the resting SaO₂ values at 4350 m, the reduction in SaO₂ during exercise were 15.9 % and 11.7 % respectively, for low and moderate altitude women.  Thus the magnitude of fall in SaO₂ during exercise was markedly pronounced in plains dwelling women compared to their counterparts of moderate altitude.  Pulmonary ventilation, respiratory frequency and ventilatory equivalent for oxygen increased significantly at 4350 in, while the exercise HR showed a reduction in both the groups.  The resting HR of both the groups recorded initially at 2100 m showed significantly (P<0.01) elevated value in low altitude women. On induction to 4350 m HR rose similarly for both the groups which were moderate but significant (P<0.01).
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DISCUSSION

The unique feature of the present study was that, the women trainees reached 4350 m gradually by trekking under progessive hypoxia at increasing altitude following staging acclimatization, associated with rigorous mountaineering activities. Immediately on reaching at 4350 m, they were undergoing regular mountaineering activities, without showing, any clinical manifestations of severe fatigue or strain.  None of these women exhibited any symptoms of discomfort or acute altitude sickness.  They all ate well, slept well, worked well with maintenance of body weight.  The maintenance of body weight at HA is an interesting observation which has been described elsewhere.

 

The decrease in SaO₂ on exposure to HA in all the subjects is due to hypoxia.  At HA the reduced oxygen supply to the blood decreases SaO₂ even after an increase of HR. The rise in HR increases cardiac output to compensate for the decrease in blood oxygenation (16, 17), though the magnitude of fall in SaO₂ was comparatively less in moderate altitude women.  This advantage may be primarily due to their place (altitude) of birth/residency.  The factors responsible for this difference between the groups may be due to different climate, physical environment, normal daily routine, social culture, pattern of living as well as status of acclimatization.  The result of the present study indicate that the oxygen consumption of the plains dwelling females born and raised at altitude not more than 260 m) to a standard exercise at 2100 m, has apparently reached the level of  VO_2max, with marked reduction in SaO₂ (D 11.2%). As expected with same schedule of exercise, at 4350 m, the exercise VO₂ decreased with further reduction in SaO₂values. On the contrary, with same level of work intensity at same situation (2100 m) the oxygen consumption of the native women of moderate altitude (born and raised at 2000 – 2100 m) did not appear to have achieved VO_2max with maintenance of higher level of exercise SaO_2. On induction to 4350 m by trekking resulting gradual acclimatization, no marked change was observed in exercise VO₂ compared to their initial performance at 2100 m.  The magnitude of fall in SaO₂ during standard exercise at 4350 m was also less compared to that of the values of low altitude women.  The observation of the better maintenance of SaO₂ during standard exercise at both the study locations by the native women of' moderate altitude compared to their counterparts, suggests that the low altitude women are not able to overcome the higher degree of pulmonary gas exchange limitations during exercise at both altitude locations, which might have lost a compensatory reserve to defend the hypoxic stress to exercise.  Our results further indicate that the magnitude of decline in arterial oxygen saturation during standard exercise at 2100 m influence the ability to defend exercise VO₂ at higher altitude (4350 m).  Thus it can also be suggested that low altitude women who display greater decline of end exercise SaO₂ during standard exercise at lower hypoxic condition (2100 m) are more prone to decline in exercise O₂ consumption at higher hypoxic situation (4350 m), compared with moderate altitude women.  Evident conclusion can be made from these findings that the moderate altitude women are better fit compared to low altitude women with respect to hypoxic tolerance/HA performance.

Elliot and Atterbom (17) and Patterson et al (18) observed that women tend to adapt to endurance training at altitude in a manner similar to men and also capable of performing hard work at high altitude.  Gore et al (5) and Terrados et al (2) observed that the reduced exercise SaO₂ and pulmonary gas exchange limitations must impose a greater susceptibility to VO_2max decline at mild altitude.  Chapman et al. (14) proposed that elite endurance athletes display varying degrees of pulmonary gas exchange limitations during maximal normoxic exercise and many demonstrate reduced arterial oxygen saturation (SaO₂) at VO_2max. They concluded that athletes who display reduced measures of SaO₂ during maximal exercise in normoxia are more susceptible to declines in VO_2max in mild hypoxia compared with the normoxic athletes.  Their observations support our findings. 

During maximal exercise vigorous hyperventilation occurs which may results in maintaining SaO₂ and VO_2max (19, 20).  Increase in pulmonary ventilation during exercise at high altitude can theoretically improve SaO₂ by raising the alveolar PO₂ and by left shifting in the oxygen hemoglobin dissociation curve.  In our studies, both the groups showed significant increase in ventilation as well as ventilatory equivalent for oxygen (V_E/VO₂ ) to a standard exercise at 4350 m indicates alterations in exercise ventilation in two different hypoxic situations might affect the changing exercise VO₂ . 'This appears to be dependent upon ventilatory influences on SaO₂ maintenance to some extent.  Pulmonary gas exchange limitations to exercise may also display in important role (21).  Powers et al. (22) suggested that pulmonary gas exchange limitations caused a significant reduction of VO_2max of approximately 1% for every 1% reduction in SaO₂ below 92%.  While Chapman et al. (14) suggested approximately a 0.5 % reduction in VO_2max for every 1% drop in SaO₂ below 92 %.  During exercise at high altitude the study groups showed varying degree of change in SaO₂ between two hypoxic conditions.  The low altitude women showed (D 26.8%) fall of SaO₂ and a significant reduction in exercise VO₂. It appears that the low altitude women were not able to prevent exercise VO₂ decrement at high altitude even by significant increase in metabolic and ventilatory demands.  Whereas the moderate altitude women did not achieve maximum oxygen uptake and maintained the almost same exercise oxygen consumption as that of the initial value at 2100 m with less fall in SaO₂  (20.7%), even though both the study groups undergone same protocol of mountaineering training and same intensity of standard exercise test.  Gore et al. (5) demonstrated a significant reduction in VO_2max (5.3 ml/kg/min or 6.9%) in 11 elite level cyclists at a very mild simulated altitude of 580 m.  Again, the VO_2max of a physically active but untrained group did not change with mild hypoxia.  It has been suggested that pulmonary gas exchange limitations and reduced exercise SaO₂ present in many endurance trained athletes must impose a grater susceptibility to VO₂ reduction at mild altitudes.  In this study both the groups undergone same training schedule throughout the mountaineering course, thus it may not be due to the training during the course alone.  It can be suggested that, the maintenance of oxygen consumption to a standard exercise at higher altitude might be due to better ability to withstand more strain which may be dependent on some other factors.  The moderate altitude women are forced to undergo physical exertion while carrying out their normal daily activities due to rugged and steep hilly terrain.  Besides, being exposed (born and raised) to different climate and physical environment these women differ in their social culture and pattern of living.  From very young age, they are habitually climbing with or without load and this being one of their major daily routines, whether it is for education and/or occupation.  The level of habitual physical activity of these women associated with the daily schedule of work along with intense mountaineering training might have exerted in considerable influence on the level of aerobic capacity and thus improved the capability to defend the hypoxic stress at high altitude.  Compared with the study of Chapman et al. (14), our approach for better maintenance of SaO₂ during a standard exercise at both the altitude situations suggests a smaller effect on pulmonary gas exchange limitations on oxygen consumption in moderate altitude women and thus proved to be better fit for hypoxic tolerance/HA performance. 

ACKNOWLEDGEMENTS 

The authors are grateful to Col.  H. S. Chauhan, VSM.  Principal, Major R. Verma, Medical Officer and staff of HMI, Darjeeling for providing all the facilities for field trials.  Warm thanks are due to all the women trainees who volunteered as-subjects.
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REFERENCES  

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