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.

Interested in more info on HCA Holdings? Add it to your watchlist.

Fool contributor Anders Bylund holds no position in any of the companies discussed here. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool is investors writing for investors.