Altitude Acclimatization

Contents

1. The guideline and its arithmetic
2. What changes in the first days
3. What changes over the following weeks
4. Effort, fitness and performance
5. Limits, losses and lifelong residents
6. The next step in research

Between 2011 and 2014 Candace Cooley practiced in the Colorado ski town of Breckenridge, a resort that the CDC Yellow Book places among the common high altitude destinations, with guests sleeping at about 9,600 feet.¹ The guideline that governs how a lowlander should reach such a height fits in a sentence. As the CDC chapter relays the Wilderness Medical Society advice, travelers should avoid a first night at 9,000 feet or more when coming straight from low ground, gain no more than about 1,650 feet of sleeping altitude a night once above 9,800 feet, and add an extra night for every 3,300 feet gained.¹ The chapter then adds a caveat that is easy to miss: these reasonable rules will still be too fast for some people and too slow for others.¹ Behind the numbers sits a physiological process, acclimatization, that unfolds in stages over days and then weeks.

The guideline and its arithmetic

The rule counts where a person sleeps, not how high she climbs during the day. The Merck Manual Professional Edition frames it for climbers above about 10,000 feet: no more than roughly 1,600 feet of new sleeping altitude a day, a rest night at the same height every three or four days, and extra rest days whenever terrain forces a larger jump, so the average stays under the limit. On rest days climbers can go higher and return to sleep lower, and a group should move at the pace of its slowest member.²

Figure

The ascent guideline, step by step

1. 01
   Limit the nightly gainAbove about 10,000 feet, raise sleeping altitude by no more than about 1,600 feet a day.
2. 02
   Build in rest daysSleep at the same altitude every third or fourth day.
3. 03
   Average out big daysWhen terrain forces a larger single gain, add rest days so the average over the ascent stays below the limit.
4. 04
   Climb high, sleep lowOn rest days climbers may go higher but return to the lower level to sleep.
5. 05
   Pace the groupA climbing party moves at the pace of its slowest member.

Night matters because oxygen is lowest then. The CDC notes that blood oxygen falls furthest during sleep, so day trips that end back at a lower elevation by nightfall are far less stressful, and that two or three nights at about 8,000 to 9,000 feet before going higher is markedly protective against acute mountain sickness.¹ A 1997 Spokesman Review article about skiers in Colorado described the same rhythm at the resorts, where symptoms in new arrivals usually faded within three to five days, once the body had adjusted.⁵

What changes in the first days

The CDC describes an acute phase covering roughly the initial three to five days following ascent, marked by a steady rise in breathing, better oxygenation and shifts in blood flow to the brain. New red cells play no part in this early stage; instead a fall in plasma volume concentrates the hemoglobin already present.¹

An NIH hosted review of how the body meets thin air lays out the chain of events. Sensors in the carotid body detect low oxygen and drive deeper, faster breathing, known as the hypoxic ventilatory response. That extra breathing blows off carbon dioxide and leaves the blood alkaline, which in turn tends to damp the urge to breathe. Within minutes the kidneys begin to excrete bicarbonate to correct the imbalance, a response that can continue for hours or days, and a hypoxia driven loss of fluid shrinks plasma volume and raises the hematocrit.³

The heart joins in. A cardiologist interviewed for a Steamboat Pilot column on heart health at altitude said the air in that town holds about 25 percent fewer oxygen molecules than at sea level, so the heart beats faster to compensate, with the greatest limits in the first 48 hours.⁹ A science column in the Aspen Times adds that blood pressure rises on arrival and that increased urination, part of the bicarbonate correction, helps explain why short term visitors so often become dehydrated.⁶

What changes over the following weeks

The Merck Manual lists the features of a fuller acclimatization: sustained overbreathing, blood pH that normalizes within days as bicarbonate leaves in the urine, ventilation that can then rise further, cardiac output that climbs at first and later returns to baseline, and red cell mass and aerobic capacity that build over several weeks.² The NIH review places the red cell response later still: the kidneys release erythropoietin, but a measurable rise in red cell production takes several days and is not complete for weeks or months, so it matters little on a short trip.³

Figure

Acclimatization in two stages

	Within the first days	Over the following weeks
Breathing	Sustained deeper, faster breathing raises oxygen in the lungs and blood	Breathing can rise further once blood chemistry settles
Blood acidity	Overbreathing makes the blood alkaline	The kidneys excrete bicarbonate and pH returns toward normal within days
Heart output	Rises at first	Returns toward baseline over time
Red cells and stamina	Little change yet	Red cell mass and tolerance for aerobic work increase over several weeks

Coaches see the same timetable. A physical therapist interviewed for an Aspen Times guide to running at altitude put the growth of the red cell count at two to three weeks, a period in which runners struggle to hold their usual pace.⁷ A researcher quoted by Outside separated the two systems: breathing adjusts within a day or two at moderate heights, while the cardiovascular system needs about three weeks, and longer the higher one goes.¹⁰

Effort, fitness and performance

Fitness does not substitute for time. The Merck Manual states that physical fitness offers no protection against altitude illness and in some settings is linked to higher risk.² The NIH review adds that strenuous effort straight after a rapid ascent raises the risk of illness, and it quantifies the cost of thin air: peak oxygen uptake falls to roughly 85 percent of the lowland figure at 3,000 meters and to 60 percent at 5,000 meters.³ The CDC notes that acclimatization improves sleep, comfort and submaximal endurance, but maximal performance at altitude always stays below what the same person manages at low elevation.¹ How this plays out for women who train and compete is taken up in a separate entry on women in sport.

Figure

What thin air takes away

Inspired oxygen pressure at about 3,000 meters, relative to sea level

70%

Maximum oxygen consumption at 3,000 meters, relative to sea level

85%

Maximum oxygen consumption at 5,000 meters, relative to sea level

60%

Limits, losses and lifelong residents

Acclimatization has a ceiling. The Merck Manual says most people cope reasonably well with about 10,000 feet after several days, that each further gain in height demands more time, and that no one can fully acclimatize to permanent residence above about 17,000 feet. It distinguishes acclimatization, the individual’s response, from adaptation, the inherited traits of long settled highland peoples on the Tibetan plateau and in the Ethiopian highlands. And the gains fade: after a few days at low altitude acclimatization is gradually lost, so a returning climber should again follow a graded ascent.² The NIH review notes that Andean highlanders carry more hemoglobin while Himalayan populations breathe harder, differences likely to have a genetic basis.³

For residents, some changes never fully settle. The Aspen Times column notes that the blood’s chemistry and its supply of red cells settle within weeks to months, yet breathing and sleep never revert to the lowland pattern; that persistent disruption is the subject of sleep at altitude.⁶ Physicians interviewed for a Steamboat Pilot series on living at altitude described the main long term change as a higher hemoglobin level and described residents of every age as generally adjusting well and without long delay, while cautioning that the evidence on why high country residents tend to be healthier remains unsettled.⁸

The next step in research

The practical goal of current work is to compress the timetable. The CDC notes that acetazolamide hastens acclimatization so that the ventilatory adjustment that normally takes three to five days takes about one, and that a stay above 9,000 feet for two or more nights within two weeks before a trip is useful.¹ Military researchers have pursued the same aim. A 2008 U.S. Army report described volunteers who slept for a week in low oxygen chambers before five days at a laboratory atop the 14,110 foot summit of Pikes Peak, to test whether the chambers improved their adjustment; one physiologist noted that some people simply adapt faster, for unknown reasons.⁴ The NIH review cautions that no set of resting hypoxic training settings has yet been shown to reliably prevent illness.³ Whether the gains of earlier trips make later ones easier is also open; the Outside researcher found that returning visitors seemed to adjust faster, but suspected the reason was partly psychological.¹⁰ How any of these findings bear on one woman’s plans is something her physician should weigh with her.