Sir Ernest Shackleton, knighted by King Edward VII in 1909, was an Antarctic explorer whose exploits involved trekking further south than any other man before him — twice. But perhaps he was best known for leading the 1914 to 1917 Imperial Trans-Antarctic Expedition, in which he aimed to cross Antarctica from sea to sea by traveling directly through the South Pole. When disaster struck the expedition's main ship, the Endurance, Shackleton and his crew were forced to maneuver lifeboats across more than 800 miles of stormy seas to reach the island of South Georgia.

Over the course of these explorations, many of Shackleton's crewmates reported he suffered from bouts of breathlessness and weakness allegedly caused by heart problems. Fast forward nearly 100 years later, and retired anaesthetist Dr. Ian Calder, fresh off his own excursion across South Georgia, has decided to look into these health conditions to determine exactly what went down.

"The evidence rests in diary entries made by Dr. Eric Marshall, the medical officer of Shackleton's second expedition to the Antarctic in 1907 to 1909," Calder said in a press release. The diary, courtesy of the Scott Polar Research Institute in Cambridge, England, provided information detailing several times in which Shackleton felt unwell, or even collapsed, as well as other instances of weakness and breathlessness.

However, it was Marshall's reports of a pulmonary systolic murmur following two separate examinations during the expedition that led Calder and consultant cardiologist Dr. Jan Till to diagnose Shackleton with an atrial septal defect — also known as a "hole in the heart."

According to the Centers for Disease Control and Prevention (CDC), an atrial septal defect is a type of congenital heart defect that develops during pregnancy and remains after birth. If the hole doesn't close during pregnancy or immediately afterward, the hole will increase the amount of blood that flows through the lungs and, over time, may cause damage to the blood vessels in the lungs. This type of damage can lead to problems during adulthood, like heart failure.

The CDC estimates more than 1,900 babies are born each year with this defect, with about 10 percent of them caused by spontaneous genetic mutations.

These days, congenital heart defects can be treated with invasive open-heart surgery, and 3D printing can even be used to plan out the procedure. Although these treatments weren’t available in the early 1900s, it didn’t mean Shackleton was unaware of his condition. In fact, Calder believes he knew he was sick, but avoided visiting the doctor because he wanted to continue exploring. He subsequently died of a heart attack in 1922 at the age of 47.

Source: Calder I, Till J. Shackleton's heart. Journal of the Royal Society of Medicine. 2016.