第八版 TINTINALLI EMERGENCY MEDICINE 急診教科書. (目前出到電子版第九版了)
(page 1429)
In Denver at 1610 m (5280 ft), air pressure is 17% less than at sea level. The air of Aspen, Colorado, 2440 m (8000 ft), has 26% less oxygen than sea level. At 5490 m (18,000 ft), there is half the available oxygen, whereas on top of Mount Everest, there is only one third. Oxygen supplementation prevents symptoms of altitude illness during hypobaric exposure, and therefore, hypoxia, not hypobaria per se, is responsible for illness.
In Denver at 1610 m (5280 ft), air pressure is 17% less than at sea level. The air of Aspen, Colorado, 2440 m (8000 ft), has 26% less oxygen than sea level. At 5490 m (18,000 ft), there is half the available oxygen, whereas on top of Mount Everest, there is only one third. Oxygen supplementation prevents symptoms of altitude illness during hypobaric exposure, and therefore, hypoxia, not hypobaria per se, is responsible for illness.
Acute mountain sickness (AMS) occurs at and above 2130 to 2440 m (7000 to 8000 ft) and sometimes at lower altitudes in particularly susceptible individuals.
High altitude, 2440 to 4270 m (8000 to 14,000 ft), is associated with decreased arterial oxygen saturation
(SaO2); marked hypoxemia may occur during exercise and sleep.
(SaO2); marked hypoxemia may occur during exercise and sleep.
Very high altitude, 4270 to 5490 m (14,000 to 18,000 ft), is uncommon in the United States but is encountered by visitors to the mountainous regions of South America and the Himalayas. Abrupt ascent can be dangerous, and a period of acclimatization is required to prevent illness. Extreme altitude, >5490 m (>18,000 ft), is experienced only by mountain climbers and is accompanied by severe hypoxemia and hypocapnia.
At this height, progressive physiologic deterioration eventually outstrips acclimatization, and sustained human habitation is impossible. Because hypoxemia is maximal during sleep, the sleeping altitude is the critical altitude to consider.
Acutely hypoxic individuals become dizzy, faint, and rapidly unconscious if hypoxic stress is sufficient (SaO2 <65%).
(補充 Hypoxemia (low oxygen in your blood) can cause hypoxia (low oxygen in your tissues))
The process of maximizing ventilation, termed ventilatory acclimatization, culminates after 4 to 7 days at a given altitude.
(page 1430)
Exercise capacity, as measured by Vo2max, drops dramatically on ascent to altitude, approximately 10% for each 1000-m (3280-ft) altitude gain above 1500 m (4920 ft).
Exercise capacity, as measured by Vo2max, drops dramatically on ascent to altitude, approximately 10% for each 1000-m (3280-ft) altitude gain above 1500 m (4920 ft).
Miners in South America report that they cannot live at altitudes >5800 m (>19,000 ft) because of weight loss, increasing lethargy, poor-quality sleep, weakness, and headache.
Sleep stages III and IV are reduced at altitude, whereas sleep stage I is increased. More time is spent awake, with a significant increase in arousals, but with only slightly less rapid eye movement time. Frequent nighttime awakenings are a common source of bitter complaints from skiers and others, but they are innocuous and improve with time at altitude. The typical periodic breathing (Cheyne-Stokes respiration) in those sleeping at >2700 m (>8860 ft) consists of 6- to 12-second apneic pauses interspersed with cycles of vigorous ventilation. Intervals of apnea of >20 seconds have been observed at extreme altitudes. Interestingly, the frequent awakenings are not necessarily related to sleep periodic breathing, and they are not related to AMS either. Quality of sleep and arterial oxygenation during sleep improve with acclimatization and with acetazolamide.
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