Everest photo
Arterial Blood Gases on Mt. Everest

Prepared by Lawrence Martin, MD

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* Values in mm Hg (BP = barometric pressure). Click on location for discussion of blood gas values.

Location Altitude (m/ft) BP* PIO2* pH PaCO2* PaO2* SaO2
Summit 8848 / 29,029 253 43 @7.7 7.5 28 @70%
Balcony 8400 / 27,559 272 47.1 7.53 13.3 24.6 54%
Camp 4 7950 / 26,083 292 51.3 - - - - - - - -
Camp 3 7100 / 23,294 317 56.5 7.53 16.7 35 --
Camp 2 6400 / 20,997 350 63.4 7.51 18.2 40 - -
Camp 1 5900 / 19,358 - - - - - - - - - - - -
Base Camp 5300 / 17,388 403.5 74.7 7.46 20.4 47 - -
Sea level 0 / 0 760 140 7.40 40 >80 98%
NEJM Everest schematic

Figures A & B are from Grocott MPW, et al. Arterial Blood Gases and oxygen content in climbers on Mount Everest. NEJM 2009;360:140-149.

Photo of the "South Col" route is from Mt. Everest South Col Route Maps & Videos.

Mt. Everest was first summited by Sherpa Tenzing Norgay and New Zealander Edmond Hillary with a British expedition on May 29, 1953; they took the South Col route shown. They used supplemental oxygen, as did all subsequent climbers for the next 25 years. On May 8, 1978, Reinhold Messner from Italy, and Peter Habeler from Austria, made the first ascent without supplemental oxygen. Messner summited without supplemental O2 a second time, from a different route, on August 20, 1980. It is now estimated that less than 4% of current climbers go without supplmental oxygen (quoted in NEJM 2009). For detailed geography, climbing routes and history of ascents, see Wikipedia article on Mt. Everest. For first person accounts of climbing Mt. Everest, including the notorious 1996 expedition ("Into Thin Air"), see list of Amazon-linked books at bottom of web page. (See Climbing Mt. Everest for more information on unique feats associated with climbing the mountain.)

The altitudes given in the NEJM article (Figures A & B) are from the Caudwell Xtreme Everest blood gas expedition in May 2007. The altitudes given in the picture of Everest and neighboring mountains are from South Col expeditions detailed in www.alanarnette.com.

(Note: The summit altitude is variously given as either 29,029 feet (8848 M) or 29.035 feet (8850 M), an inconsequential difference.)

Blood gases at summit of Mt. Everest (8848 M, 29.029 ft.), breathing ambient air (FIO2= 21%)
There are no direct blood gas measurements on the summit of Mt. Everest. The
Caudwell Xtreme Everest team that published the 2009 NEJM study was unable to make measurements on the Everest summit as "conditions were too severe, with temperatures at minus 25 degrees centigrade and winds above 20 knots."

There are two simulation studies (ABGs drawn on subjects in a hypobaric chamber to simulate the summit altitude) and one study whereby pH, PaO2 and SaO2 were inferred from exhaled gases obtained in a single climber at the summit (West, et al). In both the simulated studies and the actual climber, survival was possible because the subjects had been able to acclimatize to the low pressure environment for many days. Values for PaCO2, PaO2 and SaO2 in the hypobaric studies are mean of the number of subjects studied.

The pH and SaO2 values in the West study are considerbly higher than the chamber values, because the PaCO2 (assumed equal to the climber's exhaled PCO2) was considerably lower. All authors acknowledge that no chamber can simulate the actual extreme conditions on the summit, which could affect the climber's ventilatory status.

In summary, we know that the PaO2 at the summit, without supplemental oxygen, is at the limit of survivability, and that survival is only possible because of profound, sustained hyperventilation. This point is discussed in greater detail under Sea level.

Setting (#subjects) Reference BP PIO2 pH PaCO2 PaO2 SaO2
Summit Mt. Everest (1) West JB, et al. J Appl Physiol 1983;55:678-87 253 43 @7.7 7.5 28 @70
Hypobaric chamber (6) Sutton JR, et al. J Appl Physiol 1988;64:1309-21 253 43 7.56 11.2 30.3 58
Hypobaric chamber (5) Richalet JP, et al. Adv Exp Med Biol 1999;474:297-317 253 43 7.58 11.9 30.6 - -

Blood Gases at the "Balcony" of Mt. Everest (8400 M, 27559 ft.)
This was the highest point in the
2009 NEJM study where arterial sampling actually took place (see Figures A & B, above). Four climbers had blood drawn from their femoral artery! The climbers had acclimatized at altitude for 60 days, and when sampling took place were without supplemental oxygen for 20 minutes. While the study presents several methodologic problems, the results clearly show survivability in extreme hypoxia. There were no apparent neurologic sequelae. Values in the table are means of 4 subjects at this altitude. Values for O2 content used hemoglobin measurements made at lower altitudes on the mountain.

Measurements at 8400 M (27,559 ft.)
Barometric pressure 272 mm Hg
PaO2 24.6 mm Hg
pH 7.53
PaCO2 13.3 mm Hg
Calculations at 8400 M (27, 559 ft.)
PAO2 (alveolar PO2) 30 mm Hg
A-a O2 difference 5.4 mm Hg
SaO2 54%
HCO3 10.8 mEq/L
O2 content 14.58 ml/dl

For further discussion of PaO2 and the alveolar gas equation at this altitude, please see the section Sea level.

Everest Camps: Base, 1-4

The 2009 NEJM article ( Grocott MPW, et al. Arterial Blood Gases and oxygen content in climbers on Mount Everest. NEJM 2009;360:140-149) provides only some of the data at these elevations. The table at top of the page is filled in from data presented in the text of the article, and also from the article's Figure 2 (shown below).
NEJM Fig. 2

Sea level

Below are the standard values at sea level, compared with those at the summit of Mt. Everest. ("Summit" includes data from both the single climber and the chamber studies; see Summit section.)

Location Altitude (meters / ft) BP PIO2 pH PaCO2 PaO2 SaO2
Summit 8848 / 29,029 253 43 7.56 - @7.7 7.5 - 11.9 28-30.6 58% - @70%
Sea level 0 / 0 760 140 7.40 40 >80 98%

Note that with altitude the barometric pressure (BP) falls, but the FIO2 stays constant at 21% of the atmosphere. The drop in BP reduces the inspired partial pressure of oxygen (PIO2), which in turn reduces the alveolar partial pressure of oxygen (PAO2). In the 2009 NEJM study PAO2 was calculated using the long form of the alveolar gas equation:

PAO2 = PIO2 - PACO2  [ FIO2 + -----------------] 


PIO2 = FIO2 (BP - 47 mm Hg)
	(47 mm Hg is the water vapor pressure in the upper airway, 
	a constant at all altitudes)

PACO2 = Alveolar PCO2, which is assumed to = arterial PCO2 (PaCO2).  It 
	is the value for PaCO2 that is used in the alveolar gas equation.

FIO2 = Fraction of inspired oxygen, which is 21% at all altitudes when 
	breathing ambient air.

R = respiratory quotient, which is ratio of ml CO2 produced/ml O2 taken up by 
	the lungs, normally about 0.80 at rest.

In this study respiratory quotient (R) was measured in 3 of the climbers the day before summiting, at an altitude of 7950 meters. The 3 values were 0.81, 0.72 and 0.70. When using the alveolar gas equation for clinical purposes, R is assumed to be 0.80 and PaCO2 is assumed = to PACO2. These assumptions give the familiar 'abbreviated alveolar gas equation':

PAO2 = PIO2 - 1.2(PaCO2)

At the summit:

PIO2 = .21(253-47) = .21(206) = 43.26 mm Hg

If the climber's PaCO2 stayed anywhere near 40 mm Hg this would not be a survivable situation, as shown by a simple calculation:

PAO2 = PIO2 - 1.2(PaCO2)

PAO2 = 43 mm Hg - (1.2)40 mm Hg = 43 - 48 = -5 mm Hg!!

A negative alveolar PO2 would mean, of course, that oxygen would go the other way - leave the blood and enter the atmosphere Without supplemental oxygen, survival is possible at these altitudes because of prolonged acclimatization, which allows for chronic, sustainable hyperventilation. Thus, using the PCO2 values for the summit (West's value of 7.5 mm Hg and mean of the two hypobaric study PaCO2s = 11.6 mm Hg) and the balcony mean PaCO2 of 13.3 mm Hg:

Summit (29,029 ft.): PAO2 = 43 - 7.5(1.2) = 43 - 9 = 34 mm Hg
Summit (29,029 ft.): PAO2 = 43 - 11.6(1.2) = 43 - 14 = 29 mm Hg

Balcony (27.559 ft): PAO2 = 47 - 13.3(1.2) = 47 - 16 = 31 mm Hg

Note that these values -- 34, 29 and 31 mm Hg -- are for ALVEOLAR PO2. Assuming an alveoar-arterial PO2 difference of @5 mm Hg, we see that the PaO2 breathing ambient air, at either location, is in the mid to high 20s -- surely near the limit of survivability.

As noted above, the vast majority of current climbers use supplementary O2 when attempting the summit of Mt. Everest. An interesting journal letter noted the following data for Everest and K2 summiters between 1978 and 1999. shown in the two tables below. (Supplemental oxygen and mountaineer death rates on Everest and K2. JAMA 284:2000;181)

Mt. Everest # ascents # deaths % deaths
Using supplementary O2 1077 32 3
No supplementary O2 96 8 8.3

The differences were even more striking for K2, the planet's 2nd highest mountain.

K2 # ascents # deaths % deaths
Using supplementary O2 47 0 0
No supplementary O2 117 22 18.9

One can see why the altitude one must reach to summit these mountains is sometimes called the "death zone".

Books on climbing Mt. Everest and K2, with links to Amazon.com

Into Thin Air, by Jon Krakauer

The Climb, by Anatoli Boukreev

Left For Dead, by Beck Weathers

High Exposure: An Enduring Passion for Everest and Unforgiving Places, by David Brashears

No Shortcuts To The Top, by Ed Viesturs

Himalayan Quest, by Ed Viesturs

Everest: Mountain Without Mercy, by Broughton Coburn

K2, The Savage Mountain, by Charles S. Houston

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Initially posted January 15, 2009. Last updated July 26, 2011.