Physiology of extreme altitude




Introduction


It is a remarkable coincidence that when humans are well acclimatized to high altitude, they can just reach the highest point on earth (8848 m) without breathing supplemental oxygen. This feat was first realized in 1978 after many physiologists and physicians interested in high altitude medicine and physiology had previously predicted that it would not be possible (West 1998). It was truly the end of an era when Messner and Habeler reached the summit of Mount Everest (8848 m) on May 8, 1978, without supplemental oxygen. Likewise, the remarkable first and only ascent of Everest in winter without oxygen by Ang Rita Sherpa in December 1987 is testament to the likely limits of performance at high altitude. Nowadays, technical climbing without supplemental oxygen is conducted successfully on peaks >8000 m both in summer and winter, although it is only a minority of elite mountaineers that attempt and succeed in this endeavor. This chapter examines the profound physiological changes that are necessary for humans to survive and do small amounts of work at extreme altitudes, such as the summit of Mount Everest. It includes an analysis of the factors that limit performance at these great altitudes and shows that such ascents are possible only if both the physiological makeup of the climber and physical factors, such as barometric pressure, are right.


History of Climbing at Very High Altitudes


Sixteenth to nineteenth centuries


It has been known for many centuries that very high altitude has a deleterious effect on the human body and that the amount of work that a person can do becomes more and more limited as the altitude increases. Until the end of the 19th century, however, most of the climbing and exploration was limited to what would today be considered more moderate altitudes (<6000 m).


One of the first descriptions of the disabling effects of high altitude was given by the Jesuit missionary Joseph de Acosta, who accompanied the early Spanish conquistadores to Peru in the 16th century. He described how, as he traveled over a high mountain, he “was suddenly surprised with so mortal and strange a pang, that I was ready to fall from the top to the ground.” His dramatic description was first published in 1590 (Acosta 1590).


In the 18th century, climbers in the European Alps reported a variety of disagreeable sensations which now seem to us greatly exaggerated. For example, the physicist de Saussure, who was the third person to reach the summit of Mont Blanc, reported during the climb:


When he was near the summit, he complained of extreme exhaustion:


On the summit itself, he reported:


These dramatic complaints at an altitude of only 4807 m or less reflect a combination of almost no acclimatization and the fear of the unknown.


In the 19th century, numerous ascents were made of higher mountains, including those in the Andes, and there were abundant accounts of the disabling effects of extreme altitude. In 1879, Whymper made the first ascent of Chimborazo and described how, at an altitude of 5079 m (16,664 ft), he was incapacitated by the thin air:


However, Whymper and his two guides gradually recovered their strength and in fact his lively account shows that he was aware of the beneficial effects of high altitude acclimatization.


In the latter part of the 19th century, there was considerable interest in the highest altitude that could be tolerated by climbers. Thomas W. Hinchliff, President of the (British) Alpine Club (1875–77), wrote an account of his travels around the world and described his feelings as he looked at the view from Santiago in Chile.


Twentieth century


With the onset of the 20th century, mountaineers began exploring the extremes of elevation. In 1909, the Duke of Abruzzi attempted an ascent of K2 (8611 m) in the Karakoram Range on the China-Pakistan border, and although his party was unsuccessful in reaching the summit, they attained the remarkable altitude of 7500 m without supplemental oxygen. According to the Duke’s biographer, one of the reasons given for this expedition was “to see how high man can go” (de Filippi 1912), and certainly the climb had a dramatic effect on both the mountaineering and the medical communities interested in high altitude tolerance. In contrast to the florid accounts of paralyzing fatigue and breathlessness given by de Saussure, Whymper and others at much lower altitudes, the Duke made light of the physiological problems associated with this great altitude. However, as discussed in Chapter 8, his feat prompted heated arguments among physiologists about whether the lungs actively secreted oxygen at this previously unheard-of altitude.


Ten years later, a milestone in the history of the physiology of extreme altitude was provided by the British physiologist, Alexander M. Kellas, whose contributions were largely overlooked for many years. Kellas was a lecturer in chemistry at the Middlesex Hospital Medical School in London during the first two decades of the 20th century, but, despite this full-time faculty position, managed to make eight expeditions to the Himalayas, and probably spent more time above 6100 m than anyone else. In 1919, he wrote an extensive paper entitled, “A consideration of the possibility of ascending Mount Everest,” which was not published until 2001 (Kellas 2001). In this, he analyzed the physiology of a climber near the Everest summit, including a discussion of the summit altitude, barometric pressure, alveolar PO2, arterial oxygen saturation, maximal oxygen consumption, and maximal ascent rate. On the basis of his study, he concluded that:


The importance of this study was not that he reached the correct conclusion. Kellas had so little data that many of his calculations were erroneous. However, Kellas asked all the right questions and can claim the distinction of being the first physiologist to seriously analyze the limiting factors at the highest point on earth (West 1987). It was not until almost 60 years later that all of his predictions were fulfilled. For a biography of Kellas, see Mitchell and Rodway (2011).


Kellas was a member of the first official reconnaissance expedition to Everest in 1921, but tragically died during the approach march just as the expedition had its first view of the mountain they came to climb. Three years later, EF Norton, who was a member of the third Everest expedition, reached a height of about 8589 m on the north side of Everest without supplemental oxygen. He was accompanied to just below that altitude by Dr. TH Somervell, who collected alveolar gas samples at an altitude of 7010 m, though unfortunately these were stored in rubber bladders through which the carbon dioxide rapidly diffused (Somervell 1925). Somervell also referred to the extreme breathlessness at that altitude, stating that “for every step forward and upward, 7 to 10 complete respirations were required.”


The summit of Everest was finally attained in 1953 by Hillary and Tenzing (Hunt 1953). Naturally, this was a landmark event in the physiology of extreme altitude, but the fact that the two climbers used supplemental oxygen still did not answer the question of whether it was possible to reach the summit breathing air. Hillary did remove his oxygen mask on the summit for about 10 minutes and at the end of the time reported:


Nevertheless, the fact that he could survive for a few minutes without additional oxygen came as a surprise to some physicians who had predicted that he would lose consciousness during that brief time. However, there was a precedent for surviving for this period of time on the summit in the experiment Operation Everest I, carried out by Houston and Riley in 1945. As briefly described in Chapter 1, four volunteers spent 34 days in a low-pressure chamber of whom two were able to tolerate 20 minutes without supplemental oxygen at the equivalent of the summit altitude. In fact, the equivalent altitude was even higher than the true summit of Mount Everest because the standard atmosphere pressure was inadvertently used.


Additional information on whether there was enough oxygen in the air to allow a climber to reach the Everest summit while breathing air was obtained by Pugh and his colleagues during the 1960–61 Silver Hut Expedition (Pugh et al. 1964). Measurements of maximal oxygen consumption were made using a bicycle ergometer on a group of physiologists who wintered at an altitude of 5800 m and who were therefore extremely well acclimatized to this altitude. Figure 18.3 (lower curve) shows the results of measurements made up to an altitude of 7440 m. Note that extrapolation of the line to a barometric pressure of 250 mmHg on the Everest summit suggested that almost all the oxygen available would be required for the basal oxygen uptake. (For details of the extrapolation procedure, refer to West and Wagner 1980.) Thus, these results strongly suggested that a climber who could reach the Everest summit without supplemental oxygen would be very near the limit of human tolerance.


This ultimate climbing achievement occurred when Reinhold Messner and Peter Habeler reached the summit of Everest without supplemental oxygen in May 1978. Messner’s account (Messner 1979) makes it clear that he had very little in reserve:


And when he eventually reaches the summit:


The long period of 25 years between the first ascent of Everest in 1953 and this first “oxygen-less” ascent also suggests that humans are near the limit of tolerance at this extreme elevation. Again, as indicated earlier, Norton ascended to within 300 m of the Everest summit as early as 1924 without oxygen, but it was not until 1978 that climbers reached the top without supplemental oxygen. Thus, the last 300 m took 54 years!


Since that historic climb, Messner has further confirmed his outstanding tolerance to the extreme hypoxia of great altitudes. In 1980, he became the first person to ascend Everest alone without supplemental oxygen (Messner 1981), and in 1986, he became the first man to climb all 14 of the 8000 m peaks without supplemental oxygen. These accomplishments assure him a place not only in the history of mountaineering but also in the history of the physiology of extreme altitude. Nowadays, technical climbing to altitudes >8000 m without breathing supplemental oxygen is conducted successfully by elite mountaineers in both summer and winter.


Characteristics and History of Ascents on Mount Everest


While most would agree that there are many technically more demanding climbing objectives, the majority of the detailed history of climbing at extreme altitude is based on the documented ascents of Mount Everest. As illustrated in Figures 19.1 and 19.2, some important observations can be made from analyses of the open-access Himalayan database (https:/​/​www.himalayandatabase.com). First, while ascents of Everest have obviously increased since the 1950s, including the increased number of female climbers, the proportion of those not using supplemental oxygen has declined. Overall, it is estimated that ∼1.2% of successful summits of Everest have been conducted without supplemental oxygen. This observation is important, as much of our understanding of the physiology at extreme altitude is based on either chamber experiments or upon field studies where supplemental oxygen was used and then discontinued for a period of time before physiological measures were obtained. Second, it is also interesting to note that there is a marked variation of age—15 to 81 years—of successful ascents of Everest while on supplemental oxygen. In contrast, the variation in age range—25 to 55 years—is much less in the minority of those who have ascended Everest without supplemental oxygen. Therefore, it is obvious that supplemental oxygen can offset many of the adverse influences of hypoxemia on the human body and facilitate an improvement in performance. By the end of 2019, only one climber (Sherpa Ang Rita) had summitted Everest in winter without supplemental oxygen.

Figure 19.1

Figure 19.1Panel A: Percentage of successful summits of Mount Everest (1953 to 2018) that have (gray bars) and have not (dashed lines) used supplemental O2. The relative contribution of males to females is also reported. Panel B illustrates the relative and absolute numbers of ascent with and without supplemental O2 from 1953 to 2018. Data retrieved from the open-access Himalayan database (https:/​/​www.himalayandatabase.com; August 2019, courtesy of Richard Salisbury).

Figure 19.2

Figure 19.2Illustration of ascents of Mount Everest (1953 to 2018) by age including the use (gray bars) or not (blue bars) of supplemental O2. Data retrieved from the open-access Himalayan database (https:/​/​www.himalayandatabase.com; August 2019, courtesy of Richard Salisbury).


Physiology of Extreme Altitude


This section is devoted to human performance at altitudes over 8000 m and the analysis that follows is based primarily on data from five studies: 1) The 1960–61 Silver Hut Expedition, during which data on maximal oxygen consumptions were obtained as high as 7440 m (PB 300 mmHg) and alveolar gas samples were taken as high as 7830 m (PB 288 mmHg); 2) the 1981 American Medical Research Expedition to Everest (AMREE), which extended the work of the Silver Hut Expedition and took measurements on the summit including barometric pressure, alveolar gas samples, and electrocardiograms, as well as measurements between the summit and the highest camp situated at 8050 m (PB 284 mmHg); 3) Operation Everest II in 1985; 4) Operation Everest III (COMEX ’97) in 1997. In both Operation Everest II and III several volunteers were gradually decompressed over a period of 31 to 40 days in hypobaric chambers to extreme altitudes approaching that of Mount Everest); and 5) the 1997 Caudwell Xtreme Everest Expedition where arterial blood gases, following removal of supplemental oxygen, were sampled at an altitude of 8400 m following descent from the summit (Grocott et al. 2009). Although the degree of acclimatization in the chamber studies was not as great as in the earlier field studies, they were still able to provide a considerable amount of valuable data.


Barometric pressure


Barometric pressure is a critical variable in physiological performance at extreme altitude because it determines the inspired PO2, the first step in the oxygen cascade from the atmosphere to the mitochondria. As pointed out in Chapter 2, there has been considerable confusion in the past about the relationships between barometric pressure and altitude on high mountains such as the Himalayan chain. The resulting errors are particularly important at extreme altitude where maximal oxygen consumption is exquisitely sensitive to barometric pressure. It is remarkable that Paul Bert gave nearly the correct value of barometric pressure for the Everest summit in Appendix I of his classic book La Pression Barométrique (Bert 1878). His figure of 248 mmHg was based on an extrapolation of measurements made by Jourdanet and others at various locations including the Andes (Jourdanet 1875).


However, when the standard atmosphere was introduced and used extensively by aviation physiologists in the 1930s and 1940s, it was erroneously applied to Mount Everest, giving a value of 236 mmHg, which is much too low. Nevertheless, this figure was used by several high altitude physiologists. For example, during Operation Everest I, they were exposed to a pressure of 236 mmHg and their alveolar PO2 fell to as low as 21 mmHg (Riley and Houston 1951)! As the next section shows, this is about 14 mmHg less than that of a well-acclimatized climber on the summit of Mount Everest.


As described in Chapter 2, Dr. Christopher Pizzo measured a barometric pressure of 253 mmHg on the Everest summit on October 24, 1981. This was about 2 mmHg higher than that expected from the mean barometric pressure for that month based on extensive weather balloon data (see Chapter 2). The discrepancy can be accounted for by normal variation and the high pressure system that made the weather ideal for climbing that day. The reading of 253 mmHg was within 1 mmHg of the pressure predicted for an altitude of 8848 m from radiosonde balloons released in New Delhi, India, on the same day (West et al. 1983a). Several direct measurements on the summit since 1981 have given similar values (see Chapter 2).


Measurements of V.O2max on AMREE (West et al. 1983b) and Operation Everest II (Sutton et al. 1988), as well as the analysis described below show that exercise performance at these extreme altitudes is exquisitely sensitive to barometric pressure. For example, on AMREE, a decrease in inspired PO2 of only 1 mmHg resulted in a fall of V.O2max by about 63 mL min−1 (West 1999). This is partly because the lung is working very low on the hemoglobin-oxygen dissociation curve where the slope is steep. As a consequence, a fall of barometric pressure of as little as 3 mmHg (less than twice the daily standard deviation) will apparently cause a reduction of maximal oxygen uptake of about 4%. This means that even the daily variations of barometric pressure caused by weather may affect physical performance.


Seasonal variations of barometric pressure can be expected to have a marked effect on maximal oxygen uptake. As already noted, mean barometric pressure falls from nearly 255 mmHg in the summer months to only 243 mmHg in midwinter, a decrease predicted to reduce maximal oxygen uptake by about 15%. This might explain why it took until 1987 before Mount Everest was climbed in winter without supplemental oxygen and until 2016 until a similar style ascent was made of Nanga Parbat (8126 m). Although the very cold temperatures and high winds significantly limit climbing efforts at this time of year, the reduced barometric pressure must certainly contribute. As pointed out in Chapter 2, the location of Mount Everest at 28°N latitude is fortunate because the barometric pressure at its summit is considerably higher than would be the case if it were at a higher latitude. As an example, if Denali (6190 m, 63°N latitude) were actually 8848 m, its barometric pressure for May and October (preferred climbing months for Everest) would be only 223 mmHg, which would make it impossible to reach the summit without supplemental oxygen.


A similar argument would apply if the barometric pressure on the Everest summit were only 236 mmHg, as predicted from the standard atmosphere model. The reduction of pressure by 17 mmHg below that measured by Pizzo would reduce the maximal oxygen consumption by over 20%, according to the analysis presented in this chapter. It seems very probable that climbing Everest without supplemental oxygen under these conditions would be impossible. Thus, the higher barometric pressure that Everest enjoys because of its near equatorial latitude makes it just possible for humans to reach the highest point on earth. Of the 14 mountains >8000 m, K2 remains the only unclimbed peak in the winter, with the most recent failed attempt taking place in the winter of 2019.


Alveolar gas composition


On ascent to high altitude, the alveolar PO2 falls because of the reduction in the inspired PO2. At the same time, alveolar PCO2 falls because of increasing hyperventilation. As described in Chapter 8, Rahn and Otis (1949) clarified the differences between unacclimatized and fully acclimatized subjects at high altitude by plotting their alveolar PO2 and PCO2 values on an oxygen-carbon dioxide diagram (Chapter 9). There are apparently differences between the results obtained in the field, that is, on expeditions to high altitude, and those obtained from simulated ascents in a hypobaric chamber. The field studies will be discussed first followed by those using simulated ascents.


Figure 19.3 shows alveolar PCO2 plotted against barometric pressure at extreme altitude from field studies. The closed circles show data reported by Greene (1934), Warren (1939), Pugh (1957), and Gill et al. (1962). The triangles show data obtained on the AMREE (West et al. 1983c). It can be seen that alveolar PCO2 declines approximately linearly as barometric pressure falls and that the partial pressure on the summit of Mount Everest is about 7–8 mmHg. The measurements made on the summit itself had high respiratory exchange ratio (R) values, for reasons that are not clear. However, the data obtained at the slightly lower altitude of 8400 m (PB 267 mmHg) had a mean R value of 0.82 with a PCO2 of 8.0 mmHg, which means we can be confident of the very low values at this great altitude.

Figure 19.3

Figure 19.3Alveolar PCO2 against barometric pressure at extreme altitudes. Triangles show the means of measurements on the American Medical Research Expedition to Everest (AMREE). Circles are results from previous investigators at barometric pressures below 350 mmHg (see Table 19.1. (Source: West et al. 1983c.)


Figure 19.4 shows the line drawn by Rahn and Otis (1949) for fully acclimatized subjects together with additional data obtained at barometric pressures below 350 mmHg (Table 19.1). Note that the AMREE data (triangles) fit well with the extrapolation of the line. This method of plotting the data shows that as well-acclimatized humans go to higher and higher altitudes, the PO2 falls because of the decreasing inspired PO2, and the PCO2 falls because of the increasing hyperventilation. However, above an altitude of about 7000 m (PB 325 mmHg), the alveolar PO2 becomes essentially constant at a value of about 35–37 mmHg. Measurements of alveolar PO2 up to an altitude of 8000 m by Peacock and Jones (1997) are in good agreement with these data. This means that successful climbers are able to defend their alveolar PO2 by the process of extreme hyperventilation. In other words, they insulate the PO2 of their alveolar gas from the falling value in the atmosphere around them. This appears to be the most important feature of acclimatization at extreme altitude.

Figure 19.4

Figure 19.4Oxygen-carbon dioxide diagram showing alveolar gas values collated by Rahn and Otis (1949) (circles) together with values obtained at extreme altitudes by the American Medical Research Expedition to Everest (AMREE) (triangles). (Source: West et al. 1983c.)













































































Table 19.1 Alveolar PO2 and PCO2 in acclimatized subjects at barometric pressures below 350 mmHg

Source


Barometric pressure (mmHg)


PO2 (mmHg)


PCO2 (mmHg)


Respiratory exchange ratio (R)


(Greene 1934)


337


40.7


17.7


0.87



305


43.0


  9.2


0.79


(Warren 1939)


337a


37.0


15.6


0.60


(Pugh 1957)


347


39.3


21.0


0.87



337


35.5


21.3


0.87



308


34.1


16.9


0.77


(Gill et al. 1962)


344


38.1


20.7


0.82



300


33.7


15.8


0.78



288


32.8


14.3


0.77


(West et al. 1983c)


284


36.1


11.0


0.78



267


36.7


  8.0


0.82



253


37.6


  7.5


1.49


All pressure values are given in mmHg.


a Barometric pressure estimated from the curve of Zuntz et al. (1906).


Not everyone can generate the enormous increase in ventilation required for the very low PCO2 values shown in Figures 19.3 and 19.4. The pattern of alveolar gas values shown in Figure 19.4 is only obtained if sufficient time is allowed for full respiratory acclimatization. Figure 19.5 compares the results found in unacclimatized and fully acclimatized subjects at high altitude with alveolar gas data reported from two hypobaric chamber experiments in which the simulated rate of ascent was much faster. It can be seen that in Operation Everest I (Riley and Houston 1951), the subjects reached the simulated summit after only 31 days, and at the extreme altitudes, the data fell close to the region predicted by the line for unacclimatized humans. In Operation Everest II (Malconian et al. 1993), the ascent was a little slower, with the first simulated summit excursion occurring after 36 days. However, the alveolar gas values at extreme altitudes still deviated considerably from those found in fully acclimatized subjects. Little information is available about the time required for full respiratory acclimatization at extreme altitudes (>8000 m), but Figure 19.5 suggests that 36 days is inadequate, whereas 77 days clearly is much better. However, it may be that other factors, such as the level of physical activity and extent of periodic breathing during sleep, are also important.

Figure 19.5

Figure 19.5Oxygen-carbon dioxide diagram showing the two lines described by Rahn and Otis (1949) for unacclimatized and acclimatized subjects at high altitude. In addition, data from the American Medical Expedition to Everest (AMREE), Operation Everest I (OEI), and Operation Everest II (OEII) are included. Note that the OEI subjects were poorly acclimatized at extreme altitudes, whereas the OEII subjects had intermediate values. (Source: West 1999.)


Arterial blood composition


Until recently, no one had attempted to sample arterial blood at extreme altitude because of the obvious technical difficulties, although a possible procedure had been suggested (Catron et al. 2006). A landmark advance was made during the Caudwell Xtreme Everest Expedition when four arterial samples were collected at an altitude of 8400 m and barometric pressure of 272 mmHg (Grocott et al. 2009). The original intent was to sample blood on the summit, but this was infeasible due to a variety of logistical factors. A small tent was erected on the “balcony” about 450 m below the Everest summit during the descent, and samples were taken from the right femoral artery of four climbers. They used supplemental oxygen for climbing, but breathed ambient air for 20 minutes before the samples were taken. The blood samples were placed in an ice-water slurry and rapidly transported down to 6400 m, where the analyses were made using a Siemens blood gas analyzer that was modified for use at high altitude.


Table 19.2 shows the results for the PO2, PCO2, and pH and some other variables. Figure 19.6 shows the PO2 and PCO2 values plotted on an O2–CO2 diagram together with the mean results from Operation Everest II and Operation Everest III. More information on these is given later in the chapter. The chamber data were obtained at a barometric pressure of 253 mmHg corresponding to the Everest summit so that slightly lower values for PO2 and PCO2 might be expected. In addition to the absolute PO2 values, it is also useful to note the measured alveolar-arterial oxygen difference in the subjects from the Xtreme Everest expedition. At first glance, the mean difference of 5.4 mmHg would seem normal. However, the normal range for this parameter decreases as the PIO2 falls with ascent and, in fact, at the altitude at which these samples were drawn, the alveolar-arterial oxygen difference should be <2 mmHg. As a result, the observed mean difference of 5.4 mmHg indicates the presence of abnormal gas exchange, which, as discussed further below, likely reflects the effects of diffusion limitation as well as increased ventilation-­perfusion mismatch.


Table 19.2 Arterial blood values at an altitude 8400 m, PB 272 mmHg from the Caudwell Xtreme Everest Expedition

Table 19.2
Figure 19.6

Figure 19.6Arterial PO2 (x-axis) and PCO2 (y-axis) for four subjects from the Caudwell Xtreme Everest Expedition taken at an altitude of 8400 m, PB 272 mmHg (Grocott et al. 2009). Also shown are the means of the summit measurements from Operation Everest II and III.


Figure 19.6 shows that two of the subjects on the Xtreme Everest expedition had PO2 and PCO2 values that fit well with those from Operation Everest II and Operation Everest III. However, the results from the other two subjects were far different for reasons that remain unclear (West 2009). Possibly these two subjects had low hypoxic ventilatory responses. Not shown in the figure are the calculated values from one subject from AMREE on the summit. His PO2 was 28 mmHg, which fits well with the other data, but his PCO2 was 7–8 mmHg, which is substantially lower. This might be related to his exceptionally high ventilatory response to hypoxia.


Incidentally, the altitude of 8400 m and barometric pressure of 272 mmHg do not match previous measurements on Everest discussed in Chapter 2. They give an altitude of 8309 m for that pressure. Perhaps the altitude was incorrectly estimated or the barometer was reading low. A barometer reading on the summit would have clarified this, but unfortunately, that was not recorded.


Acid–base status


Base Excess


Relatively little is known about acid–base changes at extreme altitude, despite the importance of this topic. Some data are available from two well-acclimatized subjects who participated in AMREE, based on blood samples taken the morning after they had reached the summit. Venous blood samples taken at the highest camp (8050 m; PB 267 mmHg) showed a mean base excess of –7.2 mmol L−1. This was a considerably higher base excess than expected (in other words, the base deficit was less than predicted), and the result was an extremely high arterial pH of over 7.7 calculated for the Everest summit (West et al. 1983c). This calculation is based on the measured alveolar PCO2 and base excess. It assumes that there was no change in base excess in the previous 24 hours and that a climber resting on the summit had a negligible blood lactate concentration (see below). In addition, the measured alveolar PCO2 of 7.5 mmHg is assumed to apply to the arterial blood.


A remarkable feature of these base excess values is that they were essentially unchanged from those measured in 14 subjects living for several weeks at camp 2 (6300 m, PB 351 mmHg) where the mean value was –8.7 ± 1.7 mmol L−1 (Winslow et al. 1984). This suggests that base excess was changing extremely slowly above an altitude of 6300 m. The reason for this is not known, but may be related to variability in the use of supplemental O2 and the chronic volume depletion that was observed in climbers living at 6300 m. At this altitude, the serum osmolality was 302 ± 4 mmol kg−1, which was significantly higher (p <0.01) than in the same subjects at sea level, where the value was 290 ± 1 mmol kg−1 (Blume et al. 1984). It is known that the kidney gives a higher priority to correcting dehydration than acid–base disturbances, and in order to excrete more bicarbonate to reduce the base excess, it would be necessary to lose corresponding cations, which would aggravate the volume depletion. This may be the basis for the slow renal bicarbonate excretion.


These acid–base changes may be part of the explanation of why climbers can spend only a relatively short time at extreme altitudes, say above 8000 m. It was pointed out in Chapter 8 that the marked respiratory alkalosis, which increases the oxygen affinity of the hemoglobin at extreme altitude is beneficial because it accelerates the loading of oxygen by the pulmonary capillaries. If a climber remains at extreme altitude for several days, presumably there is some renal excretion of bicarbonate (though this appears to be slow) and the resulting metabolic compensation would move the pH back toward 7.4. Thus, the advantage of a left-shifted dissociation curve would tend to be lost.


One way to counter this disadvantage during a climb of Mount Everest would be to put in the high camps and then return to base camp at a lower altitude for several days. This period at medium altitude would then allow the body to adjust again to this more moderate oxygen deprivation and enable the blood pH to stabilize nearer its normal value. The final summit assault would then be as rapid as possible to take advantage of the nearly uncompensated respiratory alkalosis. In fact, this was the pattern adopted by Messner and Habeler in their first ascent of Mount Everest without supplemental oxygen in 1978 and is now used by most climbers.


The Lactate Paradox


In humans acutely exposed to high altitude during the International High Altitude Expedition to Chile in 1935, it was reported that blood lactate remained very low in acclimatized subjects at high altitude even during maximal work (Edwards 1936). Figure 19.7 shows data on resting and maximal blood lactate obtained by Cerretelli (1980). Also shown are measurements made at 6300 m after maximal exercise at the rate of 900 kg min−1, corresponding to an oxygen uptake of 1.75 L min−1 (West 1986). The mean value after exercise at 6300 m was only 3.0 mmol L−1 despite an arterial PO2 of less than 35 mmHg and therefore extreme tissue hypoxia. Note that extrapolation of the line relating maximal blood lactate concentration to altitude suggests that after maximal exercise at altitudes exceeding 7500 m, there is no increase in blood lactate concentration despite the extreme oxygen deprivation. This is indeed a paradox since it has often been observed that lactate release occurs during tissue hypoxia and the tissue PO2 must be extremely low under these conditions.

Figure 19.7

Figure 19.7Maximal blood lactate as a function of altitude. Most of the data are redrawn from Cerretelli (1980). The filled circles and triangles show data for acclimatized Caucasians (C); the open circles and triangles are for high altitude natives (N). The data for 6300 m are from the American Medical Research Expedition to Everest (AMREE) for acclimatized lowlanders (West 1986). The points marked Sutton et al. are from Operation Everest II (Sutton et al. 1988).


The blood lactate concentrations after maximal exercise were appreciably higher on Operation Everest II (Sutton et al. 1988). For example, at an inspired PO2 of 63 mmHg, the mean lactate concentration following maximal exercise was 4.7 mmol L−1, which is about 56% higher than on the AMREE for the same inspired PO2. Moreover, the “summit” measurements on Operation Everest II gave a blood lactate concentration of 3.4 mmol L−1, a higher value than that found at only 6300 m on the AMREE (Figure 19.7). It was initially believed that the low lactate concentrations following maximal exercise at high altitude come about only as a result of high altitude acclimatization because acute hypoxia causes very high lactate levels. Presumably, therefore, the higher values seen on Operation Everest II compared with the AMREE and other field studies can be explained by the lesser degree of acclimatization.


The topic of the lactate paradox was extensively discussed in a Point–Counterpoint feature in Journal of Applied Physiology following initial statements on whether the paradox exists (van Hall 2007; West 2007). The discussion includes a large number of references, which the interested reader should consult. One hypothesis is that on acute exposure to hypoxia, sympathetic stimulation leads to augmented muscle lactate production and blood lactate concentration through beta-adrenergic-mediated increases in glycolytic flux. By contrast, chronic hypoxia causes beta-adrenergic adaptation, and it was originally believed that there was a reduced lactate response to exercise after acclimatization (Kayser 1996). However, studies on unacclimatized and acclimatized subjects at 4300 m altitude have not supported this hypothesis (Brooks et al. 1998). Consistent with this interpretation, based on the observations by Danish researchers (Lundby et al. 2000), it was revealed that the net lactate release from the active leg was higher after six weeks at 5260 m compared with acute hypoxia at sea level (van Hall et al. 2009). Therefore, it seems that sea-level residents in the course of acclimatization to high altitude do not exhibit a reduced capacity for the active muscle to produce lactate. These findings imply an enhanced lactate utilization with prolonged acclimatization to altitude, and supports the absence of a lactate paradox in lowlanders sufficiently acclimatized to altitude. However, although the underlying mechanisms that regulate blood lactate at rest and during exercise at high altitude remain unresolved, differences in magnitude of altitude exposure, degree of acclimatization, and experimental methodologies (absolute versus relative exercise testing) (Cerretelli 1980; Swenson 2016) likely explain some of these discrepancies. The latter point is important as testing at the same absolute workload (e.g., 200 watts) at both sea level and high altitude will result in a much greater relative workload because of the altitude-induced reductions in V.O2max; as such, the relative comparisons of given blood concentrations will be confounded. See Chapter 18 for further details.


Cardiac output


Intuitively, it would be reasonable to expect increased cardiac output for a given work rate at extreme altitude compared with sea level, as it is known that cardiac output increases as a result of acute hypoxia (Chapter 11). Furthermore, the oxygen concentration of the arterial blood is extremely low at very high altitude, and an increase in cardiac output would be expected to help compensate for the reduced oxygen delivery. Paradoxically, however, the relationship between cardiac output and oxygen uptake in acclimatized subjects at an altitude of 5800 m is essentially the same as at sea level (Chapters 11 and 18), and this apparently holds true even at extreme altitudes, although data are sparse. However, because of the altitude-induced reductions in V.O2max, maximal cardiac output is also reduced. Theoretical calculations indicate, however, that that increasing cardiac output for the conditions on the Everest summit did not improve calculated V.O2max because of diffusion limitation (Wagner 1996).


Pulmonary diffusing capacity


As discussed in Chapter 8, oxygen transfer during exercise at high altitude is, in part, diffusion limited, and all calculations suggest that this limitation will be exaggerated at the extreme altitudes near the summit of Mount Everest. However, very few data on diffusing capacity at extreme altitude are available. Available measurements at an altitude of 5800 m (PB 380 mmHg) indicate that the diffusing capacity for carbon monoxide during exercise is essentially unchanged from the sea-level value, except for the expected increase caused by the faster rate of combination of carbon monoxide with hemoglobin under the prevailing hypoxic conditions (West 1962). These data indicate that the diffusing capacity of the pulmonary membrane itself is unaltered by acclimatization. A recent study of lowlanders who trekked into the Everest base camp (5400 m) and spent two weeks there reported a substantial increase in diffusing capacity for carbon monoxide (Agostoni et al. 2011), but this seems to be at variance with most previous studies.


Measurements of the diffusing capacity for carbon monoxide at different alveolar PO2 values allow calculation of the pulmonary capillary blood volume. Again, in measurements made at 5800 m, there appeared to be little change in capillary blood volume, although there was a suggestion that it was slightly lower, possibly as a result of hypoxic pulmonary vasoconstriction (West 1962). If we accept the conclusion that capillary blood volume is unchanged, and that the cardiac output/oxygen consumption relationship is the same as at sea level, this implies that capillary transit time in the lung is normal since this is given by capillary blood volume divided by cardiac output (Roughton 1945).


Using these data, it is possible to calculate the changes in PO2 along the pulmonary capillary for a climber at rest on the summit of Mount Everest. This shows that the rate of oxygenation is extremely slow and that the end-capillary PO2 is much lower than the alveolar value, indicating severe diffusion limitation of oxygen transfer. This topic is discussed further in Chapter 8.


PO2 of venous blood


During maximal exercise at extreme altitude, peripheral tissue oxygen extraction results in a very low venous PO2 in the exercising muscles, which in turn, reduces the PO2 of mixed venous blood. In order to analyze the relationships between many variables and determine what limits exercise performance at extreme altitude, one possible assumption is that the body does not tolerate a mixed venous PO2 below a certain threshold, for example 15 mmHg (West 1983; West and Wagner 1980). This assumption received strong support from Operation Everest II, where direct measurements of mixed venous PO2 gave similar values (Sutton et al. 1988). For example, during 60 W of exercise on the “summit,” the mixed venous PO2 had a mean value of 14.8 ± 1 mmHg, and at 120 W, which was the highest work level, the mean mixed venous PO2 was 13.8 ± 0.6 mmHg.


Heat loss by hyperventilation


Matthews (1932) argued that tolerance to extreme altitude might be limited by the high rate of heat loss from the lungs as a result of the extreme hyperventilation. However, subsequent experience has not supported this early notion. Calculations of net heat loss are complex because the upper respiratory tract acts as a heat exchanger. During expiration, expired gas warms the respiratory tract, and this heat is then available to warm the cold inspired gas. Climbers who have reached the summit of Mount Everest without supplemental oxygen have not been affected by cold beyond the extent expected from the very low temperatures of the environment. When Pizzo reached the summit to take his alveolar gas samples during the course of the AMREE, he became overheated during the climb and photographs taken on the summit when he was breathing air show that he was not even wearing his down jacket, which he carried with him in his backpack (West 1985).


Oxygen cost of ventilation


A climber at extreme altitude has considerable hyperventilation at rest, and even more during moderate exercise, as evidenced by the measured alveolar PCO2 of 7–8 mmHg at the Everest summit and an arterial value of 10.3 mmHg at 8400 m (Grocott et al. 2009). Since it is known that the carbon dioxide production both at rest and for a given work level is independent of altitude, we can conclude that the alveolar ventilation at these elevations was four to five times the resting value. Even small amounts of physical activity will greatly increase this. If we take the normal resting ventilation to be 7–8 L min−1, this means that the resting ventilation on the summit is at least 40 L min−1.


Assuming a mechanical efficiency of 5% (Cibella et al. 1999), the oxygen cost of breathing at high altitude and sea level amounts to 26 and 5.5% of V.O2max, respectively. The authors concluded that, at high altitude, the mechanical power of breathing may substantially limit the ability to do external work. They also calculated what they called the “critical ventilation,” that is, the ventilation at which the mechanical power of breathing was so high that increasing ventilation above this level did not provide additional oxygen for external work. At the altitude of 5050 m, the maximal exercise ventilation exceeded the critical ventilation even when the efficiency was assumed to be as high as 20% (Cibella et al. 1999; Figure 19.8). It should be noted, of course, that the vast majority (>98%) of climbers at extreme altitude use supplemental oxygen and, as such, will negate much of the influence of the high altitude on gas exchange impairment and potential oxygen cost of breathing.

Figure 19.8

Figure 19.8Increase in oxygen consumption divided by the increase in ventilation for four subjects at an altitude of 5050 m. The solid line shows the relationship for total oxygen consumption; the dashed lines show the relationship for the oxygen consumption of the respiratory muscles, assuming mechanical efficiencies of 5%, 10%, and 20%. Arrows show the maximal exercise ventilation. The intersection of the solid and dashed lines shows the critical ventilation above which no increase in external work was possible because the oxygen consumption of the respiratory muscles was so high. In three of the subjects, the maximum ventilation exceeded the critical ventilation for all assumed mechanical efficiencies, though in one of the subjects, this was only the case for an efficiency of 5%. (Source: Cibella et al. 1999.)


Metabolic changes


Substantial changes in the muscle may represent important metabolic responses to exposure to environmental hypoxia (O’Brien et al. 2019). At the cellular level, the transcriptional response to hypoxia is mediated by the hypoxia-inducible factor (HIF) pathway and results in promotion of glycolytic capacity and suppression of oxidative metabolism. In the Sherpa population, lower muscle peroxisome proliferator activated receptor alpha expression is associated with a decreased capacity for fatty acid oxidation, potentially improving the efficiency of oxygen utilization. In lowlanders on a gradual ascent to base camp on Mount Everest (5300 m), a similar suppression of fatty acid oxidation occurs, although the underlying molecular mechanism appears to differ along with the consequences. Moreover, although subacute exposure to high altitude (19 days after initiating ascent to Everest base camp) was not associated with mitochondrial loss, after 66 days at altitude and ascent beyond 6400 m, mitochondrial densities fell by 21%, with loss of 73% of subsarcolemmal mitochondria. Correspondingly, levels of the transcriptional coactivator PGC-1α fell by 35%, suggesting down regulation of mitochondrial biogenesis (Levett et al. 2012). Unlike lowlanders, however, Sherpas appear to be protected against oxidative stress and the accumulation of intramuscular lipid intermediates at high altitude (Horscroft et al. 2017). Moreover, Sherpas are able to defend muscle ATP and phosphocreatine levels in the face of decreased oxygen delivery, possibly due to suppression of ATP demand pathways (Murray et al. 2018). The molecular mechanisms allowing Sherpas to successfully live, work, and reproduce at altitude may hold key information about the cellular and genetic factors that limit performance at extreme altitude. Despite new insight into these molecular mechanisms, physiological testing (anthropometry, electrocardiography, pulmonary function, strength, echocardiography, and exercise capacity) at 1325 m and 2063 m in two world-record-holding Sherpa revealed they were in the normal ranges for the general population (McIntosh et al. 2011). It might be that the reported alterations in the molecular phenotype in Sherpa become of more functional importance for performance at extreme altitudes.


Studies Using Hypobaric Chambers


A question that is frequently asked is why perform field studies, for example on Mount Everest, when the low pressure conditions can be simulated in a hypobaric chamber. Three extensive studies have been carried out in this way and they have certainly produced important information on how humans respond to low pressure. However, for some unclear reason(s), potentially due to the confounding influence of physical inactivity (in the chamber) or differences in the duration of exposure to extremely low barometric pressure, the results from hypobaric chambers are different from field studies.


Operation Everest I


This was carried out in 1944 under the leadership of Charles Houston and Richard Riley at the US Naval School of Aviation Medicine in Pensacola, Florida (Riley and Houston 1951). Four Navy volunteers were placed in a small chamber for 35 days and the pressure was gradually reduced to that believed to occur on the summit of Mount Everest. However, as indicated earlier, the standard atmosphere was used and the chamber pressure was actually reduced to as low as 234 mmHg, which corresponds to an altitude of about 9400 m, some 550 m above the Everest summit. The small chamber measured only about 3.0 × 3.0 × 2.1 m and contained four bunks. After 29 days, the pressure was reduced to that at the “summit” and two of the subjects were able to tolerate the severe hypoxia at rest during air breathing, while the other two needed oxygen. During these studies, alveolar PO2 values in the low 20s were measured (Riley and Houston 1951). These are presumably the lowest values of alveolar PO2 ever recorded for periods of several minutes. As indicated earlier (Figure 19.3), the subjects of Operation Everest I showed almost no acclimatization at the extreme altitudes as judged from their alveolar gas values. The reason for this is unclear, but it is noted that only one out of the four subjects exercised regularly (albeit briefly) on a bicycle ergometer and this physical inactivity may have been a contributing factor. Nevertheless, the collective findings from Operation Everest I clearly demonstrated for the first time how acclimatization to high altitude consists of a series of integrated adaptations or responses, designed to restore tissue oxygenation in the face of hypoxic hypoxia. These collective changes are illustrated in Figure 19.9. This was the first comprehensive laboratory description that acclimatization requires changes in all body systems involved in the uptake of oxygen into the body, the transport of that oxygen to the tissues, and the unloading of that oxygen at the tissues.

Figure 19.9

Figure 19.9Adaptive physiological changes that occur on an ascent to high altitude. This annotated graph from Houston and Riley’s classic paper shows a composite chart of selected data from all four subjects (Riley and Houston 1951). After an initial three-day observation period at sea level (SL), volunteers were subjected to barometric pressures so as to simulate an ascent of 2000 ft day-1 up to 8000 ft, followed by 1000 ft day-1 up to 15,000 ft, and then 500 ft day-1 thereafter up to 22,000 ft (6706 m) for a total of 35 days. The end barometric pressure was ∼320 Torr. The final points on the graph indicate values observed on the return to SL. BTPS, body temperature, and ambient pressure (saturated with H2O); HbO2, oxyhemoglobin. (Source: Riley and Houston 1951.)


Operation Everest II


This took place in 1985 and again was spearheaded by Charles Houston (Houston et al. 1987). A sophisticated hypobaric chamber at the US Army Research Institute of Environmental Medicine in Natick, Massachusetts, was used, and eight subjects aged 21–29 years spent 40 days and nights in the chamber. The subjects were gradually decompressed over a period of about 35 days followed by excursions to the “summit” where the inspired PO2 was 43 mmHg, which this time was the correct value. Not all the subjects made it to the “summit,” but nevertheless very valuable physiological information was obtained, because the investigators were able to perform more invasive procedures, such as right heart catheterization, than can normally be performed in a field environment. Individuals who become ill during such invasive procedures can rapidly be removed from the chamber and taken to a hospital.


Some of the most important studies were on the pulmonary circulation, and these were referred to in Chapter 11. Right heart catheterization was carried out at barometric pressures of 760 mmHg, 347 mmHg, 282 mmHg, and 240 mmHg. For the last measurement, the oxygen concentration in the chamber was 22% giving an inspired PO2 of 43 mmHg. The mean pulmonary arterial pressure at rest at sea level was 15 ± 0.9 mmHg, but this increased to 34 ± 3 mmHg at a barometric pressure of 282 mmHg (Groves et al. 1987). At the same time, the pulmonary vascular resistance increased from 1.2 to 4.3 mmHg L−1 min−1. When the subjects performed maximal exercise, the increases in mean pulmonary artery pressure were even more remarkable, rising from 33 ± 1 mmHg at sea level to 54 ± 2 mmHg at a barometric pressure of 282 mmHg. However, the pulmonary artery wedge pressure was unchanged with the increase in simulated altitude, indicating that the increase in pulmonary artery pressure was related to increased pulmonary vascular resistance rather than left heart dysfunction.


Cardiac output was measured and shown to have the same relationship with oxygen consumption as at sea level (Chapter 11), confirming earlier measurements made on the Silver Hut Expedition (Pugh 1964). However, heart rate as a function of work level was higher as altitude increased. A particularly interesting finding was that when the subjects breathed 100% oxygen at the high altitudes, pulmonary vascular resistance did not return to the sea level values. This finding, which is supported by more recent field data (Luks et al. 2017), indicated a substantial degree of irreversibility in pulmonary vascular resistance after two or three weeks of hypoxia that is highly suggestive of vascular remodeling.


Additional information was found regarding pulmonary gas exchange, as the investigators used the multiple inert gas elimination technique to separate the effect of ventilation-perfusion inequality from that of diffusion limitation. These studies were referred to in Chapter 8. Diffusion limitation of oxygen transfer across the blood-gas barrier occurred at oxygen uptakes greater than 3 L min−1 at sea level, and at less than 1 L min−1 on the “summit.” This is a graphic demonstration of diffusion limitation at extreme altitude. A new finding was the increasing ventilation-perfusion inequality from rest to exercise at all altitudes, which indirect evidence suggested may have been caused by interstitial pulmonary edema. Table 19.3 summarizes the arterial blood gases during rest and maximal exercise on Operation Everest II.


Table 19.3 Barometric pressures, equivalent altitudes, and arterial blood gases during rest and maximal exercise on Operation Everest II

Table 19.3

During Operation Everest II, skeletal muscle volume was inferred from computed tomography scans of the arms and legs. Muscle area decreased by about 14% during the “ascent.” The biopsies showed that this could be accounted for by a significant decrease in the cross-sectional area of both type I and type II fibers. As a result, there was an apparent increase in capillary volume density although this was not significant. Muscle enzymes were also measured and showed that at the highest altitude of 282 mmHg where biopsies were taken, there were significant reductions in succinic dehydrogenase, citrate synthase and hexokinase compared with measurements made after returning to sea level. Finally, the biopsies showed significant reductions in muscle lactate concentrations at the higher altitudes consistent with the low blood lactate concentrations referred to earlier in this chapter. As noted earlier, an important consideration in the interpretation of prolonged chamber studies is the profound influence of physical inactivity. As outlined in Chapter 16, many of the endocrine changes (that can influence fuel metabolism, among other things) are markedly influenced by both exercise and hypoxia. Although the participants in Operation Everest II were, on the whole, more active than those in Operation Everest I, they were still not as well acclimatized as those on AMREE. The physical activity levels are clearly much less in the chamber than those in the field.


Operation Everest III


This hypobaric chamber experiment in 1997 was carried out at the COMEX facility in Toulouse, France, and had a number of similarities with Operation Everest II (Richalet 2010). However, an innovative feature was that the eight volunteers preacclimatized in the Vallot Observatory (4350 m) for several days before spending a total of 31 days in the hypobaric chamber, ultimately reaching the summit barometric pressure of 253 mmHg. The arterial blood-gas values were similar to those found on Operation Everest II (Table 19.3) with a “summit” arterial PO2 of 31 mmHg, PCO2 of 12 mmHg, and pH of 7.58. The fact that the PCO2 was higher than on AMREE in both of the chamber studies is consistent with a lesser degree of acclimatization (compare Figure 19.5). Body weight fell by an average of 5.4 kg, again in line with findings from Operation Everest II and AMREE. Cardiovascular measurements largely confirmed those made on Operation Everest II (Boussuges et al. 2000). A noteworthy finding was transient neurological disorders, which were attributed to gas emboli, and the marked changes in mood of some of the subjects (Nicolas et al. 2000).


What Limits Exercise Performance at Extreme Altitude?


Concept of limitation


The oxygen cascade from the atmosphere to the mitochondria includes the processes of convective and diffusive ventilation of oxygen to the alveoli, diffusion of oxygen across the blood-gas barrier, uptake of oxygen by the hemoglobin in the pulmonary capillaries, convective flow of the blood to the peripheral capillaries, unloading of the oxygen from the hemoglobin, diffusion to the mitochondria, and utilization of oxygen by the electron transport system. How can we determine to what extent each of these factors is limiting exercise at extreme altitude?


One approach is to use the analogy of a turbine that is fed by water flowing through a pipe that has a series of constrictions in it. Clearly, all sections of the pipe limit the flow of water to some extent. However, a useful description of the extent to which flow is limited by any particular section of the pipe can be found by calculating the percentage change (say 5%) in total flow for a given change in diameter at that point. In carrying out this calculation, we assume that all other factors remain unchanged. Such an analysis can only be carried out if the whole system is modeled using a computer.


Limitations to oxygen uptake on the summit of Mount Everest


The model analysis described above has been carried out for a hypothetical subject exercising on the summit of Mount Everest (West 1983). Some assumptions and extrapolations are necessary because so few data have yet been obtained at these great altitudes. In general, the physiological variables have been noted above and are summarized in Table 19.4.








































Table 19.4 Key variables for the analysis of factors limiting oxygen uptake on the summit of Mount Everest

Key variables measured


Barometric pressure


253 mmHg


Alveolar PCO2


7.5 mmHg


Hemoglobin concentration


18.4 g dL−1


P50 at pH 7.4


29.6 mmHg


Base excess


27.2 mmol L−1


Assumed variables


Respiratory exchange ratio


1.0


Cardiac output/oxygen uptake


Same as sea level


Maximal DMO2a


100 mL min−1 mmHg−1


Capillary transit time


0.75 s


Minimum PO2 in mixed venous blood


15 mmHg


a DMO2, diffusing capacity of the membrane for oxygen.

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Jul 25, 2021 | Posted by in RESPIRATORY | Comments Off on Physiology of extreme altitude
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