Shares of Thoratec Corp (THOR.O) fell 10 percent in pre-market trade Wednesday, a day after the cardiac device maker cut its full-year sales forecast, citing slower sales growth of its heart pumps in the United States in the second half of 2011.

Thoratec now expects full-year revenue of $418-$423 million.

In August, the company had raised its full-year revenue to $422 million to $430 million from its prior view of $410 million to $425 million.

This inconsistency is a mark of a market in early development and will not raise confidence in future forecasts, Robert W. Baird & Co analyst Lawrence Neibor wrote in a note to clients.

Investors will now question the true growth rate of the market and of Thoratec, Neibor said, lowering his price target on Thoratech's stock to $30 from $33, while maintaining a neutral rating.

Thoratec is a maker of ventricular assist devices, used on heart failure patients, that help a severely weakened heart to pump blood.

Shares of the Pleasanton, California-based company were trading at $31.25 in pre-market trade. They had closed at $34.89 Tuesday on Nasdaq.