Until early last week, Richard Nixon's troubles with blood clots seemed confined to his sometimes painfully swollen left leg. The announcement that a clot had evidently passed through his heart and lodged in his right lung suggested that his life may indeed have been imperiled by his condition, if only momentarily. But the presence of such obstructions in the bloodstream is far more common than was generally realized only a few years ago and it is now evident that in only relatively few cases are they truly dangerous.
Nixon has been troubled by phlebitis —inflammation of a vein or veins—in his left...