Health insurer Aetna Inc., said its profit skidded 28% in the second quarter due to higher medical expenses in its commercial business, which it expects to continue for the rest of the year. The company said it earned $346.6 million, or 77 cents per share, compared with $480.5 million, or 97 cents per share. Premium revenue grew 12 percent, and medical membership was flat at 19.1 million.