Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of health-care service provider HCA Holdings (NYSE: HCA) could use some TLC today after falling as much as 20% on nearly ten times the average trading volume.

So what: The company released second-quarter results this morning, and investors didn't like 'em one bit. Earnings fell 36% year-over-year on 4% higher revenue.

Now what: Management pinned the dramatically thinner margins on an unfavorable shift from high-value surgical procedures to "less acute medical cases." Hooray for healthy humans, of course, but it's bad news for hospital operators. Industry peers Health Management Associates (NYSE: HMA), Universal Health Services (NYSE: UHS), and Tenet Healthcare (NYSE: THC) all dropped as much as 5% on HCA's news. All three will report earnings over the next week, letting us know how broad this sudden no-surgery trend really is. Add them all to your Foolish watchlist and keep your eyes open.

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