A leading forensic pathologist has told the Derek Chauvin trial that George Floyd was killed by his heart condition and drug use. Dr David Fowler, testifying for the defence, also introduced the idea that vehicle exhaust may have played a part in Floyd’s death by raising the amount of carbon monoxide in his blood and affecting his heart. Fowler, Maryland’s former chief medical examiner who trained in South Africa during the apartheid era, said the combination of cardiac disease, methamphetamine use and carbon monoxide killed the 46-year-old Black man while Chauvin, who is white, was arresting him last May in Minneapolis. “All of those combined to cause Mr Floyd’s death,” he said. Fowler said that the various factors meant the cause of death was medically defined as “undetermined” because it could not be narrowed to one cause alone. Chauvin’s lawyer, Eric Nelson, called Fowler to raise doubt about the testimony of other medical specialists who told the trial that Floyd died because he could not breathe properly as he was pinned to the ground by Chauvin’s knee on his neck for more than nine minutes and by two other police officers. Fowler said he specifically eliminated asphyxia as a cause of death. But he did acknowledge that “restraint” played a part in bringing on the cardiac arrhythmia that killed Floyd. In cross-examination, the prosecution forced a number of important concessions by Fowler, including that Chauvin did have his knee on Floyd’s neck, despite the defence claims, and that the detained man should have received medical care at the scene when he went into cardiac arrest. Chauvin, 45, has denied charges of second- and third- degree murder, and manslaughter, over Floyd’s death which prompted mass protests for racial justice across the US and other parts of the world. Fowler is a controversial witness. He is being sued by the family of a Black teenager, Anton Black, killed by the Maryland police in 2018 after being held face down by three police officers. Fowler certified that Anton Black died from natural causes, with his bipolar disorder a contributing factor. The American Civil Liberties Union (ACLU) has accused Fowler of “creating false narratives about what kills Black people in police encounters”. Last week, medical experts testified for the prosecution that Floyd died because the way that Chauvin and the other police officers pinned him to the ground in the prone position caused brain damage and heart failure. Fowler defended a study, criticised by other medical experts during the trial, that said the prone position – lying face down – is not inherently dangerous. In his questioning, Nelson focused on the impact of carbon monoxide on Floyd’s heart from the running engine of the squad car that he was restrained next to. Fowler said that Floyd did “not exclusively” die of carbon monoxide but that it may have played a part because people with significant heart disease are more adversely affected than healthy people. “This is just another potential insult, another brick in the wall,” he said. However, the prosecution forced Fowler to concede that he did not check on emissions data from the squad car which was a hybrid with lower levels than ordinary vehicles. The doctor also said he could not be sure the car was even running and that he based the claim on seeing water dripping from the exhaust pipe. Fowler also admitted that in calculating the impact of Chauvin’s weight on Floyd’s back, and the amount of air in the detained man’s lung, he did not take into account the significant weight of the equipment the accused former police officer was carrying. Fowler said he was unable to determine the exact time of Floyd’s death in contrast to a pulmonary specialist, Dr Martin Tobin, who testified for the prosecution that the video clearly shows the moment when the detained man suffered brain injury from lack of oxygen and stopped breathing. Nelson has argued that Chauvin’s actions had nothing to do with Floyd’s death, which was entirely the result of his medical problems. But Fowler acknowledged that “the more the individual is stressed, both physically and in other ways, the more the demand on the heart is going to increase”. Nelson needs just one juror to have reasonable doubt to cause a hung jury, although that would not be enough for acquittal and would be likely to force Chauvin to face another trial. Doubt among enough jurors might be enough to see them convict the former police officer of the lesser charge, manslaughter. The defence is expected to finish delivering its evidence by Friday and the case to go to the jury early next week. It is still unknown if Nelson will call Chauvin to testify on his own behalf. Although that would give the accused former officer the opportunity to explain his actions it would also open him to a stringent cross-examination. Three other police officers involved in Floyd’s death are scheduled to be tried together later this year on charges of aiding and abetting murder and manslaughter. The trial continues.