Dr. Martin J. Tobin, a pulmonologist, says George Floyd ‘died from a low level of oxygen.’

Dr. Martin J. Tobin, a pulmonologist and critical care physician from the Chicago area, was the first witness prosecutors called to the stand in the Derek Chauvin trial on Thursday.

When prosecutors asked if he had formed a medical opinion on what had caused George Floyd’s death, Dr. Tobin said, “Mr. Floyd died from a low level of oxygen, and this caused damage to his brain that we see, and it also caused a P.E.A. arrhythmia because his heart stopped,” referring to pulseless electrical activity, or cardiac arrest.

The low level of oxygen was caused by “shallow breathing,” he said. Mr. Floyd’s prone position and his being handcuffed, and Mr. Chauvin’s knee on his neck and back, he said, contributed to the shallow breathing.

Dr. Tobin added that the position in which Mr. Floyd was handcuffed, with the force of the officers compounding with the asphalt street, ultimately prevented him from being able to breathe fully.

“It’s how the handcuffs are being held, how they’re being pushed, where they’re being pushed that totally interfere with central features of how we breathe,” he said.

Mr. Chauvin’s knee placed on the left side of his chest would have limited the amount of air being able to enter the left lung, Dr. Tobin said, as if “a surgeon had gone in and removed the lung,” he said.

Dr. Tobin attended medical school at University College Dublin and is an expert in acute respiratory failure, mechanical ventilation and neuromuscular control of breathing.

He is certified by the American Board of Internal Medicine and the American Board of Internal Medicine Subspecialty Pulmonary Disease.