(USA TODAY) -- The first U.S. "economy class syndrome" lawsuits, brought by airline passengers who developed life-threatening blood clots after sitting immobile on long flights, have passed early hurdles and may go to trial this summer.

If airlines lose the suits, passengers may get in-flight exercise tips alongside instruction in inflating their life vests.

Lawyers for plaintiffs in Texas, Alaska and California say the airlines' failure to warn passengers about clot risks may make the syndrome a legal "accident" as defined by the Warsaw Convention.

"Economy class syndrome" is actually a common cardiovascular disorder that strikes about 2 million people a year nationwide. It is deep-vein thrombosis -- blood clots that usually start in leg veins and can break loose to cause life-threatening damage to the lungs and other organs. Although how often it occurs on airplanes is widely disputed, the syndrome is just as likely to strike first-class or business-class passengers.

An English passenger's death after a long Qantas flight two years ago sparked broader awareness of the ailment, and dozens of lawsuits here and abroad followed. Since trips longer than six hours make clots more likely, most cases involve international travel and hinge on the 1929 Warsaw Convention that governs air travel across borders.

Lawsuits against carriers that include Continental, Air France and US Airways focus on a 1985 Supreme Court decision that said airlines are liable under the rules of the convention for "accidents" that cause unexpected or unusual injuries not resulting from "the passenger's own internal reaction to the usual, normal, and expected operation of the aircraft."

Airlines in the past have argued that deep-vein thrombosis occurs because of immobility alone and not because of anything special about flying. Passengers' lawyers say seat belt rules, carts in aisles, cabin air quality and inadequate warnings make airplanes liable.

Uncertainty over whether clots qualify as a Warsaw Convention accident makes the outcome of trials tough to call, says lawyer Truett Akin. He is representing a Jacksonville, Fla., man who suffered a clot-induced stroke on a nine-hour flight to London. In that case, a judge ruled last week that airlines' failure to warn people of economy class syndrome may make it a legal "accident." A trial could start in July. Continental Airlines, the defendant, would not comment on the case. Some airlines, including British Airways, do warn travelers of clot risks on long flights.

In San Francisco, the federal 9th Circuit Court of Appeals recently ruled that a German's case against US Airways could resume, after an earlier decision halted the lawsuit. A London case against United Airlines, American Airlines, Delta and others involving dozens of passengers injured or killed by deep-vein thrombosis held its first hearing in November. And an Australian class-action lawsuit continues against Qantas, British Airways and others.

A U.K.-South African study is underway to determine the true incidence of deep-vein thrombosis among air passengers, but "there is no question the risk of blood clots increases during flights," says expert Samuel Goldhaber of Harvard Medical School.

An American Heart Association meeting report this year found as many as 5% of high-risk passengers, those with past heart disease or strokes, may develop a clot on flights of 10 hours or more. That same report found low odds of clots among healthy individuals.

Copyright 2002 USA TODAY, a division of Gannett Co. Inc.