The prosecution in the Derek Chauvin murder trial had a comeback moment Thursday in what’s been an occasionally foundering week. For its surge, the state can thank formidable medical experts who convincingly testified that the police actions last Memorial Day were substantially responsible for George Floyd’s death. Ironically, that is the inverse of what the state suggested in prosecutor Jerry Blackwell’s opening statement (which I wrote about here). Besides the powerful audio and video evidence presented through the testimony of eyewitnesses, Blackwell asserted that the state’s case would be cinched by testimony on the use of force: The jury would learn that Chauvin’s methods of physical restraint were totally beyond the pale. The prosecutor was not entirely dismissive of the medical experts, but he did downplay their anticipated contributions. That is another irony in a case that teems with them. Blackwell himself has been principally responsible for the medical proof in the prosecutors’ presentation, and he has done a masterful job of it. In the opening, though, he cautioned jurors not to worry so much about the abstruse legal mumbo-jumbo; instead, trust your eyes, and trust what you see and hear on the recordings. Some of this is understandable. The state realized that defense counsel Eric Nelson had plenty to work with on the medical front: Floyd’s drug use and underlying cardio-pulmonary conditions. But prosecutors clearly overestimated the strength of their use-of-force testimony. In fact, there were times during the testimony of the Minneapolis Police Department (MPD) witnesses, particularly that of Lieutenant Johnny Mercil, the MPD’s top defensive-tactics trainer, that an observer could be forgiven for forgetting that prosecutors, rather than Nelson, had called them to the stand. Nelson not only established that Chauvin had not choked Floyd and had not used an improper neck hold, he also emphasized video images from police body-cam recorders, which showed that police did not constantly press all their weight on Floyd, and that Chauvin was not perched on Floyd’s neck for the nine crucial minutes — he put some pressure on the neck, but he was coming up from between the shoulder blades. Ironically (there’s that word again), this is not as exculpatory as the defense may have hoped — indeed, not exculpatory at all. That is because Floyd did not really die in the way we’ve been led to believe that he died during most of the last year. It is not that his airways were closed near his throat; it is that his chest was pressed to the point that his lungs could no longer function. Of the nine minutes and 29 seconds of recorded footage that the state has hyped as the plinth of its case, it has concentrated most on the final four minutes. They are indeed ghastly, and they represent the part of the case that, to my mind, is hardest for Chauvin (and the other three cops, who will be tried in August) to explain away: Floyd’s breathing stopped, he had no pulse, and yet the police did nothing to help him other than wait for the ambulance they’d called. In particular, they never even started chest compressions, which they are trained to administer when a person in their care is pulseless. In reality, though, it turns out that these last four minutes may be the least important. For all intents and purposes, Floyd was already dead due to the first five minutes. This was made convincingly clear by Dr. Martin J. Tobin, a renowned pulmonologist from Ireland, who was a scintillating witness — amiable, authoritative, and accessible. Dr. Tobin explained that Floyd died because of intolerably low oxygen intake, principally due to his being kept in a prone position on the asphalt street while Chauvin and two other since-fired MPD cops exerted physical pressure on his back, as well as the base of his neck. The neck pressure is what looks most awful, but it was the detention aspect of least consequence. Over the first five minutes of detention on the street, Floyd’s lungs were gradually failing, and his oxygen level dropped dangerously. He showed classic signs of it — robust, coherent chatter at the start, then low, slurred speech and patently labored breathing at the end. His squirming and the way he pressed his own face into the ground were not signs of being strangled; they were desperate efforts to move his chest into a position where he could draw breath. Chillingly, Dr. Tobin identified the precise moment when he could detect “the moment the life goes out of [Floyd’s] body.” It happened just after one of Floyd’s legs jutted out wildly, a telltale sign of anoxic seizure. “There’s a flickering and then it disappears,” Tobin remarked as, on the video screen, jurors saw death’s gaze fall over Floyd’s face. The four minutes that followed were horrifying, but even if the former policemen had tried, there was no saving Floyd at that point. For Chauvin, the most damaging part of Dr. Tobin’s account may be his conclusion that even a perfectly healthy person would have had pulmonary failure under the physical restraint Floyd endured. What wasn’t as immediately obvious Thursday, but will come clear this summer at the next trial arising out of Floyd’s death, is that this was a devastating day for the three other former police involved in the detention. This is especially so for Thomas Lane and Alexander Keung, who were helping Chauvin hold Floyd down. The medical testimony moves the center of attention from Chauvin’s infamous neck hold down to Floyd’s shoulders and back, and the lower body restraint that made it impossible for him to move into a better breathing position. That is, it makes Lane and Keung every bit as responsible as Chauvin for the physical restraint; and it further highlights that Tou Thao, the fourth officer standing nearby and focused on crowd control, took no action as Floyd slowly faded. Nelson, who has been very effective using the state’s police witnesses to advance Chauvin’s case, never laid a glove on Dr. Tobin. Yesterday’s medical testimony also included Louisville police surgeon Bill Smock and forensic toxicologist David Isenschmid. Like Dr. Tobin, they discounted defense theories that drug abuse, arterial blockage, or other cardiopulmonary conditions could have caused Floyd’s death. Keep in mind that the state’s presentation on this score is not dispositive. The defense will also have an opportunity to elicit testimony from medical experts — and to examine Hennepin County medical examiner Andrew Baker, who is expected to testify Friday and has been less condemnatory of the police actions. Significantly, the cause of death testimony does not settle the matters of Chauvin’s knowledge and intent. As a police officer, he is not expected to be a medical expert. Moreover, proof that he caused Floyd’s death is not proof that Chauvin intended to cause Floyd’s death, intended to cause him real injury, or acted in a manner that was criminally irresponsible. That will have to be established to the jury’s satisfaction. But suffice it to say, Thursday was a very bad day for the defense. The prosecution has gotten its bearings back as it nears the end.