On Saturday night, Senator Lindsey Graham, Republican of South Carolina, died of an aortic dissection — a rip in the aorta, the main artery carrying blood from the heart to the rest of the body.

Such a tear is a medical emergency. Fifty percent of patients make it to the hospital, and about half survive, said Dr. Michael Mack, a heart surgeon who is chair of the research institute Baylor Scott & White Health in Dallas.

Dr. Mack added that an aortic dissection is the first thing that comes to mind when a patient who is feeling well dies suddenly.

The wall of the aorta has three layers, said Dr. Kendra J. Grubb, a heart surgeon formerly at Emory University and now at Medtronic, the medical device company. A dissection occurs in the inner layer, allowing blood to pour in between the layers of the tubelike vessel.

It’s most likely caused by an underlying aortic aneurysm — a bubble in the vessel, similar to what can occur in old bicycle tubes. Aortic aneurysms can be caused by high blood pressure, atherosclerosis or some rare underlying genetic conditions, like bicuspid aortic valves.

Patients usually describe an excruciating pain, from the chest to the back, “like a knife to the heart,” Dr. Grubb said.

Treatment depends on where in the artery the tear is. If it occurs after the section that goes to the brain, doctors can try to repair it, threading a wire from the groin to the aorta that carries material on the vessel to seal the rip.

If the tear occurs in the part of the aorta that carries blood to the brain, treatment involves emergency open-heart surgery. The patient is put on a heart-lung machine, the body is cooled, and a surgeon cuts out the torn section of the aorta. The surgeon then inserts a graft made of fabric.