The insurer-hospital relationship isn't a simple zero-sum game, but Goldman Sachs' Wednesday downgrade of HMO shares such as Aetna and Humana because of rising costs should be a positive for hospitals' admissions and — if reimbursement rates don't fall too much — their bottom lines. Judging by the stock prices of HCA, CHS, LifePoint and Vanguard, investors didn't feel that way Wednesday, but it's worth noting that the market was in a rather foul mood for much of the day.