A study published in The New England Journal of Medicine estimated that there are an average of 30 in-flight medical emergencies

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问题     A study published in The New England Journal of Medicine estimated that there are an average of 30 in-flight medical emergencies on U.S. flights every day. Most of them are not grave; fainting, dizziness and hyperventilation are the most frequent complaints. But 13% of them—roughly four a day—are serious enough to require a pilot to change course. The most common of the serious emergencies include heart trouble (46%), strokes and other neurological problems (18%), and difficult breathing (6%).
    Let’s face it: plane riders are stressful. For starters, cabin pressures at high altitudes are set at roughly what they would be if you lived at 5,000 to 8,000 feet above sea level. Most people can tolerate these pressures pretty easily, but passengers with heart disease may experience chest pains as a result of the reduced amount of oxygen flowing through their blood. Low pressure can also cause the air in body cavities to expand—as much as 30%. Again, most people won’t notice anything beyond mild stomach cramping. But if you’ve recently had an operation, your wound could open. And if a medical device has been implanted in your body—a splint, a tracheotomy tube or a catheter—it could expand and cause injury.
    Another common in-flight problem is deep venous thrombosis—the so-called economy-class syndrome. When you sit too long in a cramped position, the blood in our legs tends to clot. Most people just get sore calves. But blood clots, left untreated, could travel to the lungs, causing breathing difficulties and even death. Such clots are readily prevented by keeping blood flowing; walk and stretch your legs when possible.
    Whatever you do, don’t panic. Things are looking up on the in-flight-emergency front. Doctors who come to passengers’ aid used to worry about getting sued; their fears have lifted somewhat since the 1998 Aviation Medical Assistance Act gave them "good Samaritan" protection. And thanks to more recent legislation, flights with at least one attendant are starting to install emergency medical kits with automated defibrillators to treat heart attacks.
    Are you still wondering if you are healthy enough to fly? If you can walk 150 feet, or climb a flight of stairs without getting winded, you’ll probably do just fine. Having a doctor close by doesn’t hurt, either.
According to the passage, why does deep venous thrombosis usually happen?

选项 A、Because the economy class is not spacious enough.
B、Because there are too many economy-class passengers.
C、Because passengers are not allowed to walk during the flight.
D、Because the low pressure in the cabin prevents blood flowing smoothly.

答案A

解析 本题的关键词是deep venous thrombosis,定位到原文第三段。原文第三段第二句讲到深静脉血栓形成的原因,即腿部的血液容易结块是因为人坐在狭窄的位置上(a cramped position)时间过长,而这与选项A经济舱不够宽敞(not spacious enough)属于相同含义,故正确。选项B属于主观推导,原文并未提及经济舱内乘客人数是多是少。选项C、D属于主观推导,由第三段第五句可知,走动或保持血流通畅是预防深静脉血栓形成的措施。这两个选项就是根据这句话设置的干扰项。第三段:简要介绍深静脉血栓的形成原因、危害及预防措施。
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