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Our continued fascination with the element oxygen (O2), first discovered by Joseph Priestley (1733–1804), is eminently justified, for without it we would simply not survive. Photosynthesis breathes life into what was 1000 million years ago considered to be a reductive atmosphere containing only 1–2% O2. Contemporary estimates now suggest that the green plants on earth combine a total of 150 billion tons of carbon (from CO2) with 25 billion tons of H2 (from H2O) to liberate 400 billion tons of O2 each year, thus accounting for the present day atmospheric content of O2 (20.9%), which has persisted for the last one tenth of the Earth's existence.1
Few are more acutely aware of the importance of the elixir of life than mountaineers when exposed to the innocuous effects of hypobaric hypoxia during ascent to terrestrial high altitude. Since the French mathematician Blaise Pascal (1623–1662) first showed that barometric pressure (and hence the inspired partial pressure of O2 (Pio2)) decreased with altitude, scientists have developed a fascination with the maximal altitude that humans could achieve without the unsporting assistance of supplemental O2. The ensuing battle between scientific prediction and human performance soon blossomed into one of the most colourful sagas in the history of high altitude mountaineering and physiology.2
The aristocratic Italian climber, the Duke of Abruzzi, rocked the scientific world at the turn of the last century by completing an “oxygenless” ascent to 7500 m in the Karakoram despite ominous predictions by the British Alpine Club, “ . . .21 500 ft (6553 m) is near the limit at which man ceases to be capable of slightest further exertion.”3 Naturally, it was not long before attention turned towards Mt Everest, which in 1892 had been declared the highest mountain in the world. Mountaineers were irrevocably drawn to the ultimate challenge, and, in 1924, Edward Norton (1884–1954) ascended to within 300 m of its summit without supplementary O2, a remarkable feat eclipsed by the untimely deaths of George Mallory and Andrew Irvine some two days later.
These courageous attempts motivated a series of pioneering studies by Rodolfo Margaria (1901–1983) and Joseph Barcroft (1872–1947), who subsequently concluded that an oxygenless ascent of Mt Everest would not be possible.4 However, the epochal ascent by Reinhold Messner and Peter Habeler in 1978 put paid to their gloomy predictions; the last 3% of Mt Everest had finally been conquered 54 years after Norton's initial bid, having claimed no fewer than 39 lives in the process. Messner's quote on reaching the top clearly establishes that the summit of Mt Everest was tantalisingly close to the limits of human tolerance, “ . . .Now, after the hours of torment . . . I have nothing more to do than breathe . . .I am nothing more than a single, narrow, gasping lung, floating over the mists and the summits.”5 By 1986, Messner had confirmed his place in mountaineering history having climbed all 14 of the 8000 m giants without supplementary O2.
The physiological significance of these achievements came to light a decade later when two landmark studies6, 7 measured the maximal oxygen uptake (Vo2max) of healthy volunteers on the summit at a mere 1.07–1.17 litres/min or 15 ml/kg/min. These maximal energy expenditures are comparable to those typically observed in exercising patients with congestive heart failure, albeit 5.5 miles lower at sea level! Furthermore, the geographical location of Mt Everest at a latitude of 28°N results in a barometric pressure that is considerably higher than if it were located near one of the poles. The climatic idiosyncrasy of this “equatorial bulge” is fortunate in that, without it, a climber would not be able to summit without supplementary O2.8
It is, however, somewhat discouraging to note that the physical challenge of climbing Mt Everest without supplementary O2 is becoming more difficult as the summit appears to be increasing in height at a rate of 3 cm/year and steadily moving north eastward at 6 cm/year (Geological Society, personal communication). This is a consequence of it being located on the great fault system, which continues to push India under Nepal and China thus creating the Himalayas. These data required the initial measurement of the summit, which in itself has proved one of the most formidable geological challenges since the first attempt using vertical triangulation by James Nicolson during 1847–1849 (8840 m), which failed to take into account the deflection of the vertical produced by the Himalayan chain, the discrepancy between geoid and ellipsoid, and refraction of the atmosphere. The Chinese National Bureau of Surveying and Mapping and the Italian National Research Council have subsequently performed what is arguably the most accurate measurement of the summit using laser distance meters, theodolites, and satellite positioning systems. The value reported in 1992 was 8848.65 (0.35) m above sea level (inclusive of the snow on the summit measured as 2.55 m). These facts raise the irresistible question of how much longer an oxygenless ascent will remain humanly possible.
Figure 1 (based on the data presented in table 1) clearly illustrates that systemic O2 transport is exquisitely sensitive to a decline in Pio2 primarily because of a decrease in alveolar Po2 (Pao2). Extrapolation of this relation predicts the Pio2 at a variety of energy expenditures ranging from 1 metabolic equivalent (MET) to 3.9 METS compared with the measured maximum of about 4.5 METS (assuming 1 MET = 3.5 ml/kg/min). Whole body Vo2max is expressed in relative terms and further reduced to METS to normalise for the unavoidable differences between studies in subject body mass, which is an undoubted confounding variable during attempts to fit pooled data.
The value of 3.5 METS (point B on fig 1) was selected because 3 METS is considered the minimum energy expenditure required for a safe ascent to the summit during the summer months assuming that the climber is working at about 85% of his maximum, which is not unreasonable (J S Milledge, personal communication). Messner's total body mass including equipment was reported to be 74 kg,10 which at an ascent rate of 2 m/min during the last 100 m of the climb would have resulted in a power output close to 150 kg/m/min. According to the data of West et al6 and the aforementioned assumption, this submaximal effort would have required a Vo2max of approximately 3.5 METS.
Seasonal variations in barometric pressure at these extreme altitudes have a major impact on systemic O2 transport and are thus of considerable physiological significance. Previous theoretical calculations have identified that a mere 4 mm Hg decrease in summit barometric pressure would reduce a climber's predicted Vo2max by about 10%.8 Thus, point C on fig 1 refers to the predicted Vo2max of a climber based on the previous assumptions attempting to summit during the winter months, a remarkable feat that was achieved on 22 December 1987 by Sherpa Ang Rita. Retrospective analyses indicated that the summit pressure during this ascent was 247 mm Hg,11 only 3–4 mm Hg lower than that observed during the American Medical Research Expedition to Everest (AMREE)6 and Operation Everest II7 (point A).
Incorporating this information into the linear function presented in fig 2 (based on the data summarised in table 1), we can identify the corresponding altitude at each of the respective Pio2 values which equates to 9972.7 m (3.5 METS, point B), 9662.1 m (3.9 METS, point C), and 11 913.9 m (1 MET, point D).
Assuming a constant growth rate of 3 cm/year and present (2001) height of 8848.92 m, a climber with a Vo2max of 3.5 METS would therefore have to make his assault on the summit during the summer months before about 39 460 ad. A winter ascent would need to be launched before 29 107 ad. By about 104 167 ad, a climber's Vo2max on the summit would equate to his basal metabolic rate measured at sea level, emphasising the futility of any summit bid. Incidentally, these dates could be extended by another 85 years if the snow on the summit melted.
A climber with a sea level Vo2max that is comparatively higher than the values incorporated in fig 1 could further extend the theoretical timescale of an oxygenless ascent. Data obtained during AMREE6 clearly indicate a slight leftward displacement of the Vo2max α Pio2 curve, effected in part by a 24% higher sea level Vo2max when compared with the data obtained by Pugh et al.9 Hypoxic chemosensitivity may also affect these predictions, with previous studies indicating that climbers with a “brisk” hypoxic ventilatory response are capable of reaching higher altitudes on the mountain.12 However, the hyperventilatory responses observed at high altitude are energetically more demanding, accounting for 26% of an individual's Vo2max at 5050 m compared with 5.5% at sea level (assuming a mechanical efficiency of 5%).13 Thus it would appear that the mechanical power of breathing may impair a climber's ability to perform external work on the mountain. The limits for reaching the theoretical summit are also affected by subtle changes in barometric pressure as the comparative example of Messner's summer ascent versus Sherpa Ang Rita's winter ascent clearly indicates; a successful winter bid would have to occur about 10 millenia before a summer bid despite a mere 3–4 mm Hg difference in summit pressure. More pronounced seasonal changes in summit pressure, which can differ by up to as much as 11.5 mm Hg at Everest's present altitude, could further reduce the theoretical timescale of an oxygenless ascent.14 Changes in the geological (summit growth rate) and climatic (Fio2 and barometric pressure α altitude relation possibly as the result of global rewarming) assumptions on which these calculations are based have obvious implications for summit dates; whether the predictions are correct, only time will tell!
Despite its appeal, the risks inherent in an ascent to such extreme altitudes by “fair means” are quite considerable and can increase a climber's susceptibility to hypoxic brain damage, as studies showing clear evidence for residual impairment of central nervous system function after return to sea level15 would seem to suggest. Furthermore, recent epidemiological data clearly indicate that an oxygenless ascent/descent of Mt Everest and K2 is associated with more fatalities16 probably because of the proposed therapeutic benefits of supplemental O2 as a means of reducing the incidence of “medical” (related to altitude illness) and “traumatic” (caused by accident as the result of misjudgement) deaths.17 Thus, although high altitude mountaineers may find some reassurance that, at least for the next 40 millennia, an oxygenless ascent will have some chance of succeeding, the ethical implications of such a remarkable feat warrant due consideration. The physical and intellectual challenge posed by Everest lives on!
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