Bernard Lown, an eminent cardiologist who forged a dual legacy in medicine and world affairs by pioneering lifesaving treatments for heart attacks and co-founding an international group of physicians that won a Nobel Peace Prize for its efforts to end the nuclear arms race, died Feb. 16 at his home in Chestnut Hill, Mass. He was 99.

He had congestive heart failure, said a granddaughter, Ariel Lown Lewiton, adding that her grandfather oversaw his own medical treatment.

A Jewish immigrant from Lithuania, Dr. Lown came to the United States before the outbreak of World War II and taught himself English by memorizing the pages of a dictionary.

He ascended the ranks of medicine and academia to become a professor of cardiology at the Harvard School of Public Health, a senior physician at Brigham and Women’s Hospital in Boston and founder of a cardiovascular clinic in Chestnut Hill that bears his name.

He is credited with helping lead major advances in care for cardiac patients. Among them, in the early 1960s, was the development of a defibrillator that used direct current to reestablish a normal heartbeat when an arrhythmia occurs.

The device — the paddles often wielded in television medical dramas — quickly entered widespread use and led to more advanced implantable cardioverter-defibrillators placed inside the chest.

In addition, Dr. Lown helped demonstrate the utility of the drug lidocaine for cardiac patients and showed that nitrous oxide — popularly known as laughing gas — could be used to relieve pain caused by heart attacks and improve outcomes by lessening a patient’s anxiety and therefore his or her heart rate and blood pressure.

Dr. Lown had observed the promise of laughing gas at a hospital in Moscow during a United States-Soviet health exchange in 1968.

“When I walked into the hospital’s coronary care unit, I couldn’t get over the fact that there were little tanks of nitrous oxide gas near every patient’s bed,” Dr. Lown told the New York Times.

The exchange was neither the beginning nor the end of Dr. Lown’s long commitment to international cooperation between physicians during the Cold War. Profoundly disturbed by the threat of nuclear war, he saw an opportunity — and responsibility — for doctors on both sides of the Iron Curtain to oppose nuclear armament.

“Look, here’s the problem,” Dr. Lown recalled writing to Yevgeniy Chazov, a Soviet cardiologist he knew, in the late 1970s. “You and I have been concerned with the issue of sudden death. Sudden cardiac death is not what’s threatening us, but sudden nuclear death. You and I have got to get together.”

In 1980, Dr. Lown and Chazov — along with Jim Muller, Eric Chivian and Herbert Abrams of the United States and Mikhail Kuzin and Leonid Ilyin of the Soviet Union — founded the International Physicians for the Prevention of Nuclear War ­(IPPNW).

The doctors quickly attracted attention. They implored the United States and the Soviet Union to cease nuclear testing and to reject the first use of nuclear weapons in the broader goal of ending the arms race.

By 1985, the group counted 135,000 members in 40 countries, among them 28,000 in the United States and 60,000 in the Soviet Union. That year, the group won the Nobel Peace Prize for having “performed a considerable service to mankind by spreading authoritative information and by creating an awareness of the catastrophic consequences of atomic warfare.”

Dr. Lown and Chazov traveled to Norway to collect the honor.

“We are both cardiologists and usually speak about the heart,” Dr. Lown said in his Nobel speech. “Today, we speak from the heart.”

Chazov’s involvement sparked a minor furor: He was a member of his country’s Communist Party Central Committee, had served as personal physician to high-ranking Soviet leaders and had publicly criticized Andrei D. Sakharov, the Soviet physicist and dissident who won the Nobel Peace Prize in 1975.

Some American observers regarded the physicians’ group as politically and geopolitically naive, unwittingly offering the Soviets a moral victory.

But Dr. Lown and his colleagues were insistent.

“We physicians who shepherd human life from birth to death have a moral imperative to resist with all our being the drift toward the brink,” he said in the Nobel speech. “The threatened inhabitants on this fragile planet must speak out for those generations yet unborn, for posterity has no lobby with politicians.”

Dr. Lown was born Boruch Latz on June 7, 1921, in Utena, Lithuania, where a grandfather was a rabbi. The eldest of four children, he grew up in a middle-class family that operated a mill, among other business interests that they relinquished in 1935 when they fled to the United States amid encroaching anti-Semitism.

The family ultimately joined an uncle of Dr. Lown’s who had come to the United States at the turn of the century. His daughter, Louise Lown, would become Dr. Lown’s wife.

Bernard Lown — as he became known in his new American home — spent his adolescence in Maine, where his father and uncle worked in the shoe manufacturing trade. He received a bachelor’s degree in zoology from the University of Maine in 1942 and a medical degree from Johns Hopkins University in 1945.

Dr. Lown was briefly kicked out of medical school when, in an early display of his commitment to social justice, he intentionally mislabeled blood bank samples from Black and White donors. His purpose was to demonstrate the absurdity of the blood bank’s segregationist policies.

Dr. Lown became interested in the dangers of nuclear armament in the late 1950s and early 1960s. In an interview with the Miami Herald, he recalled telling himself, “Lown, you have copped out for far too long.”

He helped establish the organization Physicians for Social Responsibility and spurred medical research on the effects of nuclear weapons in an effort that culminating in the founding of the IPPNW.

In addition to that organization, Dr. Lown helped found SatelLife, a nonprofit organization that seeks to use technology and communications to improve health care in poor countries.

In his medical practice, Dr. Lown advocated a holistic, humanistic approach that he described in a book, “The Lost Art of Healing” (1996). He was also the author of a memoir, “Prescription for Survival: A Doctor’s Journey to End Nuclear Madness” (2008).

Dr. Lown’s wife died in 2019. Survivors include three children, Fredric Lown of Brookline, Mass., Anne Lown of Manhattan and Naomi Lown of Arlington, Mass.; five grandchildren; and a great-grandson.

Dr. Lown labored for years over purposeful, sustained collaboration between the Cold War superpowers. But the day before the Nobel ceremony in 1985, he found himself thrust by coincidence into what a Washington Post correspondent described as an “impromptu display of East-West cooperation.”

He and Chazov were speaking at a news conference in Oslo when a Soviet television journalist collapsed from cardiac arrest. The two doctors tore off their coats and rushed to his side, performing chest compressions. When an ambulance arrived, the crew used a defibrillator to shock the patient’s heart, and then delivered an injection of lidocaine.

“Suddenly I looked up to the heavens and said, ‘Somebody up there is intervening to make a point, a parable,’ ” Dr. Lown told the Post-Standard of Syracuse, N.Y. in 1988. It was a parable about “what is relevant is life — not worrying about ideology, about politics, about whether he is a commie or a capitalist.”