Although it had been announced on November 29, 1961, that Donald Slayton would be the prime astronaut for the second U.S. manned orbital mission, on March 16, 1962, NASA announced that he would be replaced by M. Scott Carpenter, alternate pilot in the Glenn mission.  This decision, made at NASA Headquarters, was prompted by a minor heart defect, on record since 1959.  The physical disability which caused Astronaut Slayton's disqualification was described as "recurring arterial fibrillation without heart disease."  Little positive information was available concerning either the etiology or prognosis of this condition.  Since the medical profession did not establish a firm prognosis in this case, the decision whether Slayton would fly the mission was one that had to be made by management.

This decision by NASA was to be discussed extensively by the press following a news conference held by NASA at noon that day, with Dr. Hugh Dryden, Deputy Director, NASA; Dr. Roadman, Director, Aerospace Medicine; Astronaut Donald K. Slayton; and John H. (Shorty) Powers, Public Affairs Officer, participating.³  For example, in an interpretive report, William Hines of the Washington Sunday Star wrote:
 

The conference brought out the fact that, based on some medical second-guessing, a hasty change had been made in what supposedly had been well laid plans for the second manned orbital flight.  Lt. Col. William Douglas, the astronauts’ personal physician, did not participate in the medical review of Major Slayton, which is said to have been unanimous.  Dr. Douglas had made no secret of the fact that he believes Major Slayton is fit to fly and should fly.⁴

My own feeling is that Deke is an extremely competent engineering test-pilot and entirely capable of this mission.  In no case has the abnormality interfered, with Deke’s performance.

Following extensive observation and examination by eminent specialists, including Dr. Paul Dudley White (who had attended U.S. former President Eisenhower), it was recommended by the Director of Space Medicine, Office of Manned Space Flight, that Astronaut Slayton be removed from consideration "for any Mercury flights."⁵  On July 11, NASA reported:
 

The principal conclusion of the examinations is that the hazards from the arrhythmia of Slayton's heart, under the particularly stressful circumstances of current manned space flight operations, are too great to recommend that he should make a one-manned solo space Right.  The examinations of Slayton's heart condition included those by members of the Manned Spacecraft Center Medical Staff under a variety of circumstances, two groups of heart specialists convened by the Air Force and a detailed examination by Dr. Paul Dudley White, eminent cardiologist of Boston, Massachusetts.  The conclusions represent the consensus of all the medical specialists involved.⁶

3.  Transcript of News Media Conference, Pilot Change in Mercury-Atlas No. 7, 12:15 p.m., Friday, March 16, 1962.

4.  William Hines, "Slayton's Grounding Raises Questions on Space Program," The Sunday Star (Washington), Mar. 18, 1962. See also NASA Press Release No. 62-67, Mar. 17, 1962.

5.  Brig. Gen. Charles H. Roadman, Director, Aerospace Medicine, Office of Manned Space Flight, Memo for D. Brainerd Holmes, Director of Manned Space Flight, July 10, 1962.

6.  NASA Release No. 62-161, July 11, 1962.  Subsequently, Major Slayton resigned from the Air Force and assumed important duties with MSC in a civilian capacity.