The medical-devices business continues moving erratically in an uncertain environment. Take the case of Boston Scientific (NYSE:BSX).

Its stent business is beating the stuffing out of Johnson & Johnson's (NYSE:JNJ) stents. Its cardiac rhythm management business appears more resilient than that of St. Jude Medical (NYSE:STJ).

Yet Boston Scientific's stock was being pounded -- down more than 16% Tuesday. The main reason is that the company cut its full-year earnings forecast, excluding one-time items, to a range of $0.75 to $0.79 from its prediction of $0.82 to $0.86 in July.

The forecast follows by two weeks a warning from St. Jude Medical that third-quarter earnings would be lower than expected because hospitals weren't buying as many devices. Boston Scientific said it didn't have this problem, but investors weren't mollified.

Boston Scientific's third-quarter report, released Monday night, featured adjusted earnings per share of $0.12 versus the Wall Street consensus of $0.14. Revenue of $2.025 billion was just under the average analyst forecast of $2.04 billion and 2.4% above the year-ago quarter. On a GAAP basis, the company earned $0.13 versus a year-ago loss of $0.04.

Lower-than-expected growth
Sales of cardiac rhythm management (CRM) devices -- pacemakers and implantable cardioverter defibrillators -- didn't grow as expected, but they still brought in 8% more revenue over the year-ago quarter.

Two weeks ago, St. Jude Medical warned that projected third-quarter revenue for CRM devices would be below or at the low end of previous estimates. It also warned that it would miss its earlier earnings guidance.

Stents stay the course
Boston Scientific's drug-coated stent business stayed strong amid competition from longtime foe Johnson & Johnson and newer entrants Medtronic (NYSE:MDT) and Abbott Laboratories (NYSE:ABT). Drug-coated stents are mesh-like devices inserted into arteries to facilitate blood flow after doctors reduce artery-clogging plaque. The stent slowly releases a drug to lower the risk of an artery becoming blocked again.

For the quarter, Boston Scientific said it had a 41% worldwide market share and a 49% U.S. market share. Worldwide sales of drug-coated and non-coated stents rose to $452 million from $446 million, including a small gain in the U.S. and a tiny decline in foreign markets.

In contrast, sales of Johnson & Johnson's Cypher stents fell 28% worldwide in the third quarter versus the year-ago period, and the U.S. market share fell 11 points to 12% year over year.

The warning from Boston Scientific and St. Jude Medical prompted a swift reaction in the stock market. Until Congress makes up its mind on health-care legislation, investors in medical-device companies will continue to experience the financial equivalent of irregular heartbeats.

See why fellow Fool Dr. Brian Orelli believes that just because you're a big pharmaceutical company doesn't mean you'll do well.

Fool contributor Robert Steyer doesn't own shares of any companies cited in this story. Johnson & Johnson is an Income Investor pick, and the Fool owns shares of Medtronic. The Fool has a disclosure policy.