Johnson said he became lightheaded and his knees buckled after he took medicine to control his high blood pressure on an empty stomach — against doctors orders — and was unrelated to his chronic kidney disease.
Johnson said he was diagnosed with a kidney disease 32 years ago when he applied to be a Chicago police officer and was required to take a blood test. He said he has managed it since then without medicine.
Mayor Rahm Emanuel said he had "absolute confidence" in Johnson and the leadership of the department.
"He is exactly the person we want at exactly this time," Emanuel said, adding that he had known about Johnson's condition since the day he offered him the job as top cop, and Johnson has kept him up to date on his condition and his search for a new kidney.
"I've always been confident in his ability to do the job," Emanuel said, adding that Johnson "is straightforward, honest and hardcharging."
Johnson said his condition did not affect his ability to lead a normal life or to be the police superintendent. Johnson said he went to the hospital in November after a spike in the level of potassium in his blood.
After the incident Friday morning, Johnson was taken to Christ Hospital, where he was evaluated in the emergency room and saw his nephrologist, Dr. Paul Crawford from Rush Hospital. Johnson returned to work Friday afternoon, and will remain at work.
Crawford said a quarter of all Chicagoans have high blood pressure, and African Americans are three or four times more likely to develop the disease.
"We all need to take better care of ourselves," Crawford said.
Johnson, 56, thanked the staff that treated him, as well as the officers who came to "check on" him and for the dozens of messages of support from Chicagoans.
"The support I received today is a testament why I love this city and why I love this department," Johnson said, calling Emanuel a "great partner."
What happened to Johnson while he and the mayor detailed efforts to prevent gun violence in Chicago's neighborhoods could have happened to anyone who took blood pressure medicine on an empty stomach and was not related to Johnson's condition, known as Glomerulonephritis, Crawford said.
Johnson brushed off a question about whether the incident was brought on by stress.
"If you love something it doesn't stress you, you just do what you have to do," Johnson said.
A donor kidney has not yet been found for the city's top cop. When one is, Johnson said he expects to take three to five weeks off after the surgery set to take place at Rush Medical Center.
Johnson said news reports that he has diabetes and is on dialysis were incorrect.
Johnson said he would not appeal for a kidney donor, but did encourage those who could to register.
"There are a lot of people out there who need a kidney donation," Johnson said. "If you can donate to someone, you change their lives."
Johnson is among the more than 123,000 Americans currently on the organ transplant list. However, only 7,000 people receive a transplant each year — but 12 people die every day waiting for a kidney.
A Chicago Police officer for nearly 30 years, Johnson lived in the now-demolished Cabrini Green public housing complex until the age of 9 when his family moved to the Washington Heights neighborhood.
Emanuel has effusively praised Johnson for leading the department through the aftermath of the video release.
An investigation by the Justice Department released Jan. 13 concluded the unconstitutional use of force by officers and a lack of accountability by the Chicago Police Department created a "deadly cycle" of violence that effected Black and Latino Chicagoans disproportionately.
Emanuel has agreed to negotiate reforms of the police department that will be overseen by a federal judge.
Johnson replaced Garry McCarthy, who had a heart attack in June 2014.
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