A looming threat from a very powerful rival could significantly dampen a key business for the company. Healthcare technology company ResMed's (RMD -5.12%) stock wasn't quite the picture of health on Wednesday. The shares lost more than 5% of their value on news of an analyst's downgrade and his new, bearish outlook. That performance didn't look good against that of the S&P 500 index, which fell a comparatively light 0.3%. A change in recommendation for the worse Well before market open that day, Wolfe Resea ...