DYER, Ind. (AP) _ A federal grand jury indicted a medical device salesman on charges that he sold heart pacemakers that were falsely labeled, improperly sterilized and in some cases already used in other patients.

Michael Walton, 43, of Hammond allegedly operated under 34 alias and 11 corporate names in a number of states, federal officials said.

A 13-count indictment against Walton and six employees was unsealed Monday after he was arrested Saturday in Las Vegas.

Walton's attorney, Brent Stratton of Chicago, denied all of the charges and blamed them on a disgruntled employee of Walton's now defunct Cardiotronics business.

''I am not aware of any harm that ever came to a patient as a result of Mike's sales or involvement,'' said Stratton. He said he had not seen a copy of the 75-page indictment or consulted with Walton since his client appeared late Monday before a federal magistrate in Las Vegas for an initial hearing on the charges.

The magistrate ordered Walton held overnight until another hearing today on a government request to have him returned to Indiana and detain until a trial.

The indictment alleges six instances since 1983 in which Walton sold pacemakers that already had been surgically implanted and then removed from a patient. There is one known case in which a used pacemaker was implanted in a second person, U.S. Attorney John F. Hoehner said.

Walton also was charged with offering individual doctors unlawful gratuities for buying pacemakers for Medicare patients. The inducements included cash, tickets to sporting and entertainment events, payment for travel and prostitutes, Hoehner said.

He was charged with conspiracy to tamper with consumer products, five counts of tampering with pacemakers, conspiracy to tamper with heart pacemakers and connecting wires, one count of unlawfully offering a gratuity to a physician, three counts of mail fraud and two counts of possession of an implement used to make false identification documents.

The indictment did not say how Walton may have obtained used pacemakers. Hoehner said there are no allegations that hospitals were aware of Walton's alleged tampering.

Among other things, Walton and six of his employees are accused of changing identifying information, such as the make, model or serial number of some pacemakers so that they could not be traced back to the manufacturer, the prosecutor said.

In addition, some pacemakers were falsely labeled as being more technically advanced than they were, and expiration dates of some pacemakers allegedly were altered by as much as two years.

''This meant that the pacemaker battery could stop sooner than expected by the patient, which would mean the patient could prematurely require further surgery to have the expired pacemaker replaced,'' Hoehner said in a statement.

Prosecutors consider some patients to have been injured because they required a second operation to put in a new pacemaker. But no deaths can be linked positively to the pacemakers because the patients were either elderly or in poor health before the implants, Hoehner said today.

Warning letters have been sent to physicians who received pacemakers suspected of being altered in Indiana, Missouri, Illinois, Wisconsin, Michigan, Ohio, Southern California, Florida and Delaware, Hoehner said.

Physicians were notified that the true expiration dates of the pacemakers had passed before they were implanted and that there also was a possibility of improper sterilization and mislabeling.

Walton is accused of conspiracy to defraud the U.S. Food and Drug Administration since 1978 by causing false registration information to be submitted to the Federal Cardiac Pacemaker Registry, which tracks the performance of such devices.

The mail fraud charges involved false bills sent to hospitals in Indiana, Illinois, Michigan and Wisconsin since 1984.

If convicted on all counts, Walton could face a maximum sentence of 95 years in prison and a $3.25 million fine.