Q: My 40-year-old healthy sister recently had a heart attack. The doctor said it was caused by a tear in a blood vessel in her heart. What causes that? A: Most heart attacks happen when a blood clot blocks an artery feeding the heart. But a small percentage result from a tear in the inner wall of one of the heart’s arteries. The resulting flap or swelling inside the artery wall obstructs normal blood flow. Known as a spontaneous coronary artery dissection, or SCAD, the condition strikes both sexes but is more common in women. In women who are under 50, SCAD is the most common reason for acute coronary syndrome. Expanded awareness of heart disease in women has increased recognition of SCAD, which was considered quite rare just a decade ago. Improved diagnostic tools have also helped. For example, the blood tests to diagnose heart attacks that detect troponin …