Jenny Simpson Survived a Cardiac Arrest, But Her Diagnosis Means She Cannot Run Again.

Jenny Simpson said Thursday that the sudden cardiac arrest she suffered in June was caused by arrhythmogenic right ventricular cardiomyopathy (ARVC), a rare inherited disease of the heart muscle, and that the condition rules out any return to running. She announced it on Instagram, 65 days after she collapsed at one of Sir Walter Running’s Pop Up Miles in Raleigh, N.C., where she was pacing a group of young girls.

By her own account, scarring in the heart muscle interfered with the signals that regulate her heartbeat, setting off the rhythms that stopped it. “That chapter has ended,” she wrote. “Thankfully, my story hasn’t.”

In ARVC, according to Johns Hopkins Medicine, fatty and fibrous scarring gradually replaces muscle in the right ventricle, weakening the heart and making dangerous arrhythmias more likely. It is usually passed down as a dominant trait, though many who inherit it never develop the disease. A 2025 review in the Journal of Cardiovascular Development and Disease puts its prevalence at between 1 in 1,000 and 1 in 5,000 people. In young athletes it ranks high among the causes of sudden death, according to Johns Hopkins. A cardiac arrest stops the heart through electrical failure, not the blocked blood flow of a heart attack.

Endurance exercise appears to accelerate the disease itself. A 2020 study in Europace of 101 gene-positive relatives found that one of 19 who exercised below 650 MET-hours a year met the diagnostic criteria or had a sustained ventricular arrhythmia, against roughly half of those above it. MET-hours measure total exercise load, and the 2025 review translates that ceiling as about half an hour of brisk walking a day. Athletic carriers also show clinical signs some 10 years earlier than sedentary ones, according to an American College of Cardiology commentary. That research follows known carriers of the gene variant, and none of it speaks to Simpson’s own case: she has published no genetic result or family history.

Bystanders at the track started CPR and used an automated external defibrillator before emergency crews arrived. More than 350,000 cardiac arrests happen outside hospital in the United States each year, and 40.2 percent of those patients get bystander CPR, according to the American Heart Association. Among non-traumatic cases treated by emergency services, 9.1 percent survive to discharge. Speed changes the odds sharply: in a 2000 New England Journal of Medicine study of witnessed casino collapses, where security officers carried defibrillators, 74 percent survived when a shock came within three minutes, against 49 percent later. Cardiac deaths at marathons and half-marathons have fallen by about half since 2010, which researchers credit to bystander CPR and defibrillators on course.

Simpson went into cardiac arrest a second time after reaching the hospital, and her heart kept failing. She was on life support by the time she moved from UNC Rex Hospital in Raleigh to Duke University Hospital, though Fleet Feet reported early signs of recovery within a day of the collapse. By her account, her heart and lungs recovered over the following days until neither needed support. Fleet Feet announced her discharge in June, after nine days at Duke. She is back in Colorado under the care of her Duke team, and has said much remains to learn about her recovery.

The diagnosis did not end her competitive career, which was already over. Simpson retired at the New York City Marathon on Nov. 3, 2024, as the first American woman to win a world 1,500m title, in 4:05.40 in 2011, and the first to medal in the Olympic event, with bronze in Rio in 4:10.53. She won 11 U.S. national titles and eight Fifth Avenue Mile titles, and became Fleet Feet’s first chief running officer in February. “Please do not say that you’re sorry or that you feel bad for me,” she wrote. “If it is true in your heart, please tell me you’re glad that I’m here.”

She said she will speak further on CBS Saturday Morning on Aug. 22, a day before she turns 40, and on The Rich Roll Podcast on Sept. 7.