Clarence Dennis, 96, who performed the first open-heart surgery that included the use of a heart-lung machine, died July 11 at a hospice in St. Paul, Minn. He had congestive heart failure.

Dr. Dennis's research into the heart-lung machine began in the late 1930s at the University of Minnesota. Surgeons were trying to figure out how to keep blood flowing to prevent brain damage in a patient while they stopped the heart long enough to make repairs.

In April 1951, Dr. Dennis performed the first open-heart surgery that used a multiple-screen blood oxygenator, or heart-lung machine. The patient died hours later. A staff member's error during surgery led to the death of the second patient on whom the machine was used.

In 1951, Dr. Dennis became chairman of the department of surgery at the State University of New York Downstate Medical Center in Brooklyn. On June 30, 1955, he became the second surgeon in the country to perform a successful open-heart surgery involving a mechanical pump oxygenator, an advance over his earlier model.

Dr. Dennis was born in St. Paul, where his father was a noted surgeon. He was a graduate of Harvard University and of Johns Hopkins University medical school. He later received a master's degree in physiology and, in 1940, a doctorate in surgery, both from the University of Minnesota.

From 1972 to 1974, he was director of the Division of Technological Applications at the National Heart and Lung Institute of the National Institutes of Health. He continued to do research at the State University of New York at Stony Brook from 1974 until he retired in 1988 and moved back to St. Paul.

He came out of retirement in 1991 to serve as director of the Cancer Detection Center at the University of Minnesota until 1996.

He received many awards throughout his career and invented and patented a number of surgical instruments.

His first marriage, to Eleanor Dennis, ended in divorce.

Survivors include his second wife, Mary Dennis; four children from his first marriage; two stepchildren; 14 grandchildren; and five great-grandchildren.