The astronauts' physician, William Douglas, recognized that Slayton had a condition medically termed as idiopathic atrial fibrillation - occasional irregularity of a muscle at the top of the heart, caused by unknown factors - when the astronauts first rode the centrifuge in August 1959 at Johnsville. Douglas noted Slayton was performing his tasks in magnificent fashion, but he still thought it best to consult with the chief of cardiology service at the Philadelphia Navy Hospital. The consultant assured Slayton and Douglas that the condition was of no consequence and should not influence Slayton's eventual choice as a flight astronaut. The astronaut's physician did not accept this appraisal as a final diagnostic decision. He and Slayton visited the Air Force's School of Aviation Medicine in San Antonio, Texas, where a member of the internal medicine staff voiced the same opinion. Sometime later Douglas learned that this individual wrote to Administrator James E. Webb, making a recommendation that Slayton should not be assigned a flight.
After sojourns at various medical centers, Douglas informed Mercury Director Gilruth of Slayton's condition during the fall of 1959. Gilruth, in turn, briefed NASA Headquarters in Washington. Douglas also relayed the information to the Air Force Surgeon General's office and was advised to take no action. For some time thereafter the "Slayton file" lay dormant. The astronaut was selected as a pilot in November 1961 and began training for his flight.
Shortly after the beginning of the new year, NASA Administrator Webb, remembering the dissenting vote he had received from an Air Force physician, and, mindful of the fact that Slayton was an Air Force officer on loan, directed a complete reevaluation of the case. In response Douglas called together Stanley White, William S. Augerson, and James P. Henry, physicians assigned to the Mercury program, to study the matter in detail. Their considered recommendation was that Slayton should continue as the pilot for MA-7. From MSC, Douglas journeyed to Washington to brief Brigadier General Charles H. Roadman and Colonel George M. Knauf, Chief and Deputy Chief of the Office of Space Medicine in NASA Headquarters. These doctors also recommended that Slayton remain on space flight status. The reopening of the case was brought to the attention of the Air Force Surgeon General, who convened a board of eight flight surgeons to review the matter. The MSC physician appeared before that body, presenting it with every facet of the medical file. Slayton also appeared. The board judged Slayton to be "fully qualified as an Air Force pilot and as an astronaut."
Administrator Webb referred the case to a group of three nationally eminent cardiologists - Proctor Harvey, professor of cardiology, Georgetown University; Thomas Mattingly, heart specialist, Washington Hospital Center; and Eugene Braunwall, National Institutes of Health. Their consensus was that they were  unable to state conclusively whether Slayton's physiological performance would be jeopardized by his heart condition. Because of this unknown, they felt that if NASA had an available astronaut who did not "fibrillate," then he should be used rather than Slayton. Braunwall added that if there was sufficient time he would like to subject Slayton to some physiological tests.62
Asked several years later if he had known about his heart condition when he was chosen for Project Mercury, Slayton replied:63
No, I didn't, but in the examinations prior to the August 1959 centrifuge program at AMAL the medics discovered that my heart skipped a beat now and then. I went ahead with the centrifuge runs and began to watch myself very closely, noticing that quite often after supper my pulse would be irregular. I would get out and run a mile and everything was normal again. I was terribly concerned over what in my diet might be causing it, but every hypothesis turned up wrong. Concern in STG and even NASA Headquarters got so great in 1960 that I was sent to all kinds of exhaustive examinations under the best heart specialists in the country - in Philadelphia, San Antonio, and New York City. I was examined by different groups of heart specialists who could find nothing wrong. Even Paul Dudley White, Ike's personal physician, gave me a clean bill of health but rendered an operational rather than a diagnostic decision, recommending that the unknown factor in my heart murmur not be added to all the other unknowns for manned space flight.The Slayton decision was irrevocable, even though Gilruth and William Douglas disagreed with the high-level medical verdict. Slayton, they felt, had withstood greater stresses in the training program than he would have experienced had he been rocketed into orbit. On the other hand, Administrator Webb, because of the unknown elements, concurred with the cardiologists that it was neither safe nor politic to subject an individual who had a heart condition, however slight, to the stresses of orbital flight when there were other flight- trained astronauts available.
Shortly after the replacement, Douglas, having completed a three-year tour of detached duty with NASA, returned to his career service, the Air Force. Some newsmen were quick to conclude that this action suggested bitterness. They had not known that Douglas had been invited to the medical hearings but had known that Douglas had been outspoken in his opposition to Slayton's removal from flight status. Stanley White denied the charge in a news conference, maintaining that Douglas' return to the Air Force had been arranged for "better than six months."64 Of the original team of astronauts, Slayton had been considered the professional test pilot par excellence, largely because of his overwhelming experience and flight time. He soon became the coordinator of astronaut activities. He never abandoned hope that he still might make a space flight. As late as December 1964, more than a year and a half after Project Mercury had completed its last flight and when Project Gemini was nearing its first manned flight, the unlucky astronaut remarked, "I've never been grounded and I'm not now. I still hope to get my chance to go beyond the atmosphere."
62 Astronautical and Aeronautical Events of 1962, 36; Donald K. Slayton, interview, Houston, Dec. 16, 1964; letter, William Douglas to L. S. S., Jr., Aug. 17, 1965.
63 During the December 1964 interview Slayton demurred at naming the civilian panel, but newsmen had been less reticent. See Washington Post, March 16, 1962; New York Times, March 16, 1962. For other material on the Slayton case, see Mae M. Link, Space Medicine in Project Mercury, NASA SP-4003 (Washington, 1965). Slayton was not examined personally by Paul Dudley White until June 15, 1962. At that time speculation was revived about Slayton's possible selection for a space flight. Washington Evening Star, June 15, 1962.
64 Washington Evening Star, March 21, 1962; Slayton interview; Paul E. Purser, interview, Houston, Jan. 4, 1965.