AILSA CHANG, HOST:
In the trial of former Minneapolis police officer Derek Chauvin today, the focus was on the science of breathing. Prosecutors say Chauvin killed George Floyd last May by exerting pressure on his back and neck, and they brought in a highly respected pulmonologist to make that case to the jury. Watching the trial for us in Minneapolis is NPR's Martin Kaste, who joins us now. Hey, Martin.
MARTIN KASTE, BYLINE: Afternoon, Ailsa.
CHANG: All right. So tell us about this pulmonologist. He was the main witness today, I understand - Dr. Martin Tobin.
KASTE: Right. Yeah. He's a doctor who was born in Ireland, and he has decades of experience here in the U.S. And he's focused a lot on the physiology of breathing, the combination of the mechanics of the muscles that expand the lungs, the rates of airflow through the airway, you know, how the oxygen gets to the - where it needs to in the lungs. One of his areas is looking at sleep apnea, when people snore at night and stop breathing sporadically, and how that lowers oxygen levels in the blood. And he said an extreme version of this kind of low oxygen level in the blood is what killed George Floyd.
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MARTIN TOBIN: The cause of the low level of oxygen was shallow breathing - small breaths, small tidal volumes.
CHANG: Wait - shallow breathing. That's not the same thing as no breathing. And there are a lot of people who believe that Floyd was choked to death, right? But that's - is that not what Tobin's saying here?
KASTE: No, he paints a more complex picture than choking. He says the first few minutes or so when Floyd was facedown on the street with the officers on top of him, he could breathe. But at the same time, all these different forces acting on his body - the knee to the neck and the weight on the back, the pressure of his arms being twisted behind him in the cuffs - that increased how much work he had to do, how much work he had to put into each breath. It became exponentially harder, according to Dr. Tobin, to breathe. And he says that gradually just shrank the reserve of oxygen already in the lungs. And then it hit a tipping point, and the brain just finally just wasn't getting the oxygen it needed. And Tobin had this to say about this idea that, because Floyd could talk - I mean, he's heard saying, I can't breathe...
CHANG: Right.
KASTE: ...That that somehow meant he could breathe.
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TOBIN: It tells you how dangerous is the concept of if you can breathe - or if you can speak, you can breathe. Yes, that is true on the surface but highly misleading, a very dangerous mantra to have out there.
CHANG: Interesting. So how much do you think Dr. Tobin's analysis had an effect on the jury?
KASTE: Oh, the reports from the pool reporters in the room in the courtroom indicate that they were very engaged. At one point, Dr. Tobin, you know, very expertly, quickly unbuttoned his collar, loosened his tie to palpate his own neck to demonstrate how you feel certain aspects of the anatomy of the neck. And he told the jurors to do this along with him, to feel their own necks. And they did to the point where the defense raised an objection. And that led the judge here, Peter Cahill, to tell the jury this.
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PETER CAHILL: The witnesses ask you to do certain things. These are not required. You may do them, and he should phrase it more in terms of if you were to do them. And if you wish to do it, that is your choice.
KASTE: And I'll point out that right after the judge said that, the jurors apparently kept doing that. They kept feeling their own necks.
CHANG: So how is the defense responding to this line of argument that Floyd was slowly asphyxiated?
KASTE: Well, they're sowing the kinds of doubts about the calculations that Tobin put into calculating how much force the police actually put on Floyd. They say there are some assumptions there, that actually, the leg moved around more than he takes into account for. But they return to their central theme of drugs, that it was, in their view, the fentanyl or the meth found in Floyd's system that really contributed to his shallowness of breathing or his eventual asphyxia. Meanwhile, the prosecution came back and said through the doctor that that was not the case, that he was breathing normally until he died.
CHANG: That is NPR's Martin Kaste. Thank you, Martin.
KASTE: You're welcome. Transcript provided by NPR, Copyright NPR.
