KANSAS CITY, Mo. (AP) _ A sensor the size of a credit card was implanted in the chest of a 73-year- old man in an experiment that could help treat congestive heart failure more precisely and more easily, doctors said.

An estimated 2.3 million to 3 million Americans suffer from congestive heart failure and it kills about 34,000 Americans a year.

The condition, caused by a damaged heart muscle that slows the flow of blood, can result from high blood pressure, heart attack, atherosclerosis, a congenital heart defect, rheumatic fever or lung disease.

In a two-hour operation on Wednesday, doctors at the Mid America Heart Institute of Saint Luke's Hospital implanted the 2-by-2 1/2 -inch sensor into the chest of Charles R. Wilson, a funeral director from Paola, Kan.

The sensor, implanted below the collarbone, measures the amount of oxygen in the blood and the pressure in the lung's arteries.

A pad placed against the skin over the device relays the information to a computer, where the information can be stored.

The monitoring could be done in a doctor's office.

Doctors hope they can use the information to better adjust the drugs that cause the heart to contract and dilate.

The device could reduce the need for invasive procedures, such as catheterization, now relied on to measure the heart's strength.

''This is entirely experimental right now, but we're very, very excited about it because we think it has a tremendous future in one form or another,'' said Dr. David Steinhaus, one of the doctors who performed Wilson's surgery.

He said he knew of no similar implants.

Wilson was in fair condition Thursday. The device was working as expected.

''He was very ill when he came in and will continue to be very ill. This device does not change his condition, just allows us to monitor it,'' said hospital spokesman Tom Broad.

Although he wasn't hospitalized before the surgery, Wilson receives a continuous intravenous infusion of drugs to increase his heart's ability to contract.

''If it doesn't lengthen his life, perhaps it will for someone else,'' said his wife, Rogenia Wilson.

Wilson was expected to be hospitalized for several days, and was scheduled to be monitored every two weeks for six months after that, Steinhaus said.

Nine other tests are planned over the next year, but widespread use of the monitor isn't expected for at least five years, Steinhaus said.

Dr. Robert Califf of Durham, N.C., chairman of the American Heart Association's Committee on Intensive Cardiac Care, said the implant could add substantial information on a patient's medication needs.

''The obvious questions are what's the cost, the accuracy, the infection rates, and how long will it last. These would all have to be determined through extensive clinical studies,'' he said.