Medical expert on breathing, lungs testifies during Derek Chauvin trial

Dr. Martin Tobin told the jury that George Floyd died because of the actions of former officer Derek Chauvin, not because of drugs or a history of heart disease.
2:35 | 04/08/21

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Transcript for Medical expert on breathing, lungs testifies during Derek Chauvin trial
The news continues here, and now to the trial of former police officer Derek chauvin. The medical expert on the stand saying what Floyd was subjected to would have killed a healthy person. Alex Perez on the scene in Minneapolis again tonight. Reporter: The medical expert was direct and unshakable, telling the jury George Floyd died because of the actions of former police officer Derek chauvin -- not because of drugs or a history of heart disease. The cause of death is a low level of oxygen that caused the brain damage and caused the heart to stop. Reporter: Dr. Martin Tobin, an expert on breathing and lung function, testifying he watched videos of Floyd's final moments hundreds of times, explaining what he saw when the officers pinned Floyd down on the asphalt, handcuffed. It's like the left side is in a vice. It was almost to an effect of a surgeon going on and removed a lung. Reporter: Tobin testifying the officer's actions interfered with central features of how we breathe. To understand the knee on the neck, you need to examine your own necks, all of you here in the jury, like I am doing now. Reporter: The jury captivated and most did touch their necks as the doctor spoke. He walked them through the video, frame by frame. Pointing to chauvin's body position in this image. What you're seeing is that the toe of his boot is no longer touching the ground. This means that all of his body weight is being directed down at Mr. Floyd's neck. Reporter: He showed how Floyd struggled to stay alive. He's using his fingers and his knuckles against the street to try to crank up the right side of his chest. This is his only way to try and get air to get into the right lung. Reporter: The doctor describing what happened when Floyd became nonresponsive. The knee remains on the neck for another 3 minutes and 27 seconds after he takes his last breath. After there's no pulse, the knee remains on the neck for another 2 minutes and 44 seconds. Reporter: And he was emphatic it was the officer's actions that killed Floyd, nothing else. A healthy person subjected to what Mr. Floyd was subjected to would have died as a result of what he was subjected to. Reporter: And David, the defense pushed back, but the doctor did not budge, telling the jury in detail how all of his research led him to the same conclusion that Derek chauvin's actions killed George Floyd. David? Alex Perez in Minneapolis tonight. Alex, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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