As a physician who travels quite a lot, I spend a lot of time on planes listening for that dreaded 'Is there a doctor un board?' announcement. I've been 【C1】______ only once—for a woman who had merely fainted. But the 【C2】______ made me quite curious about how 【C3】______ this kind of thing happens. I wondered what I would do if 【C4】______ with a real midair medical emergency without access 【C5】______ a hospital staff and the usual emergency equipment. So 【C6】______ the New England Journal of Medicine last week 【C7】______ a study about in-flight medical events, I read it 【C8】______ interest. The study estimated that there are a(n) 【C9】______ of 30 in-flight medical emergencies on U.S. flights every day. Most of them are not 【C10】______ fainting and dizziness are the most frequent complaints. 【C11】______ 13% of them— roughly four a day are serious enough to 【C12】______ a pilot to change course. The most common of the serious emergencies 【C13】______ heart trouble, strokes, and difficulty breathing. Let's face it: plane rides are 【C14】______ . For starters, cabin pressures at high altitudes are set at roughly 【C15】______ they would be if you lived at 5,000 to 8,000 feet above sea level. Most people can tolerate these pressures pretty 【C16】______ , but passengers with heart disease 【C17】______ experience chest pains as result of the reduced amount of oxygen flowing through their blood. 【C18】______ common in- flight problem is deep venous thrombosis— the so-called economy class syndrome (综合症). 【C19】______ happens, don't panic. Things are getting better on the in-flight- emergency front. Thanks to more recent legislation, flights with at 【C20】______ one attendant are starting to install emergency medical kits to treat heart attacks. 【C1】