发信人: dai (逝者如斯·天堂里亦当有青山碧水), 信区: Climbing
标 题: 关于高海拔适应的一篇文章
发信站: BBS 未名空间站 (Thu Jun 26 20:07:30 2008)

W.M. Keck Observatory
Adjustments to Altitude
by Larry Olsen & Andy Perala

The panoramic view from the summit of Mauna Kea offers stunning vistas of
the Big Island, the Pacific Ocean and interstellar space. But the view has a
price. The thin air of the 13,796- foot summit altitude often results in
unusual physiological responses. At this altitude, oxygen levels are
approximately 60% that of sea level. Even minor physical effort in this
rarefied air can leave one gasping for breath. Summit visitors and workers
should be aware of the nature of hypoxia - the deficiency of oxygen in the
air - and its effects on the human body.

Everyone who ascends to the 4,200-meter altitude of the summit of Mauna Kea
will experience hypoxia. Most effects are considered normal physiological
adjustments associated with ascent to high elevations. But there can be a
remarkable range of responses. Typical consequences of hypoxia include
increased respiration and heart rates, fluid release by the blood (edema),
increased blood volume to the brain and lungs at the expense of the
digestive and muscular systems, visual impedance (tunnel vision), and an
increase in alkaline blood pH. These symptoms may or may not lead to Acute
Mountain Sickness (AMS).

Observatory workers are trained to identify the symptoms and possible
complications of altitude. However, visitors and temporary summit workers
should also be aware of these symptoms. Lack of knowledge of some symptoms -
tunnel vision, vertigo - can lead to irrational fears or panic that may
exacerbate the situation. People may faint - which is not usually dangerous
except for the risk of hitting sharp or blunt objects during the collapse.

One of the earliest effects of hypoxia is impairment of judgement - similar
to mild intoxication. When visitors' stays are less than an hour, the
hypoxic symptoms experienced are usually only a mild headache or slight
disorientation. However, visits lasting more than an hour may lead to worse
symptoms that may be indicative of the onset of AMS - and the person
afflicted may not recognize the deterioration. Hence, it is critical to
recognize any worsening in an individual's behavior or condition.

At altitude, a very common reaction is increased urinary output. The body's
kidneys sense the lower level of oxygen immediately and kick into high gear.
The kidneys release a hormone, erythropoetin, that commands the bone marrow
to produce more red blood cells to increase the oxygen-carrying capacity of
the blood. To make room for the increased red cells, the body dumps fluid
from the blood - excess urine and collection of fluid in the body's tissues
are two direct results of these biological actions. On initial ascent, the
body dumps 10-15 percent of the blood supply's plasma. If a summit visitor
were to become fully acclimatized, a process that would take two weeks of
constant presence at altitude, the body's red-blood cell count would
increase 30-50 percent.

The summit of Mauna Kea is very dry - humidity levels can fall to five
percent or less. The mere act of exhaling expels moisture from the body.
Hence, the combination of the kidney's activities and the dry air can lead
rapidly to dehydration. It is imperative that a person drink water, juice or
a Gatorade-type fluid at the summit to avoid dehydration. Some people have
found the electrolytes in sports drinks (i.e., Gatorade, etc.) to be
beneficial in maintaining proper balance in the blood chemistry. The slight
headaches most people experience at the summit of Mauna Kea usually can be
treated effectively with aspirin, Tylenol or Ibuprofen.

Symptoms of the onset of AMS can include worsening headache, nausea, fatigue
, dizziness, acute tunnel vision, slurred speech, memory loss and/or
combinations of these. Headache is the most common symptom experienced - it
should be monitored closely as headaches can worsen rapidly. Children are
much more susceptible to suffering from AMS. In adults susceptibility
appears to be independent of age, sex or physical condition.

AMS results from the body's attempts to adjust to hypoxic stress; the
effects usually occur within 2-6 hours after ascent and then disappear
within 48 hours as the body becomes acclimatized to the altitude. Incidence
and severity of AMS are affected by speed of ascent, previous
acclimatization, altitude achieved and individual susceptibility. Twenty-
five individuals going to the summit at the same time will have 25 different
sets of responses; an individual going to the summit 25 different times
will have 25 different sets of responses. Currently, there is no known
method of predicting how your body will respond to altitude at any given
time.

An additional blood-chemistry alteration that is common at altitude results
from the decreased CO2 in the blood - due to increased respiration - which
causes the blood-pH level to rise. This results in impaired oxygen transport
and Cheyne-Stokes breathing, an irregular cycle of breath cessation and
rapid breathing. The kidneys sense the higher pH levels, and begin the
process of pulling bicarbonate from the blood. Inability to control blood pH
is one of the primary factors in causing the onset of AMS.

Edema is one of the more serious symptoms of AMS. Edemas result from the
body's blood supply dumping fluid (in order for the blood to carry more
oxygen), with most of the fluid concentrating in the lungs and cranial
cavity. This fluid build-up is often re-absorbed without ill effect.

Occasionally, fluid accumulates more rapidly than the body can absorb it.
The result is pulmonary or cerebral edema: two very serious conditions.
Pulmonary or cerebral edema can be fatal. Clinical studies have shown that
at an altitude of 14,000 feet, 0.5 percent of adults and 8 percent of
children under age 16 will suffer from pulmonary edema. Males and females
are equally affected.

In high altitude pulmonary edema (HAPE), the lungs become waterlogged, thus
increasing hypoxic symptoms to potentially critical levels, leading to
respiratory failure. A very rapid resting heartbeat (tachycardia), very
rapid breathing rate (tachypnea), chest pains (dyspnea) and cyanosis (blue
skin) are early indications of HAPE.

High altitude cerebral edema (HACE) is the result of fluid release in the
cranial space. Characteristics include intense headache, loss of
coordination (ataxia), and loss of sensory ability (obtundation) which can
lead to coma. HACE may progress very rapidly and is far more severe than
HAPE. Early symptoms are weakness, disorientation, irrational behavior and
hallucinations.

Although individuals unaccustomed to high altitude may experience more
distress than those who have become acclimatized, this is not always the
case. Non-acclimatized individuals may experience AMS early on, but more
serious problems are unlikely to arise until after several days of exposure.
Individuals who are already acclimatized are at risk for unusual and
potentially lethal fluid accumulation in the lungs if they descend to sea
level and then return rapidly to the summit. In effect, they lose their
acclimatization and may not realize it.

Any abrupt change in symptoms should be taken as a warning sign that AMS may
be accelerating toward one of these more dangerous conditions. Immediate
application of oxygen and descent will usually correct the problem.

The early identification of potentially serious symptoms is essential to
employee and visitor safety at the summit of Mauna Kea. In virtually all
circumstances, serious complications can be averted and are usually
reversible. When detection and prompt care are provided, AMS should not lead
to permanent damage.

CARA employees who regularly work on the summit are required to pass a high-
altitude medical exam. This is not a requirement for visitors or observing
teams. Individuals who are pregnant or under the age of 16 are prohibited
from visiting the observatory. It is highly recommended that individuals
with histories of coronary, cerebral or respiratory disease check with their
doctor before attempting to ascend to altitude.
--
当时明月在，曾照彩云归——


※ 来源:·WWW 未名空间站 海外: mitbbs.com 中国: mitbbs.cn·[FROM: 67.170.]


附件: hyalt.htm (9070 字节) [删除]