George Floyd died from a lack of oxygen as he was pinned "in a vice" against the street during his arrest, a doctor at the trial of ex-police officer Derek Chauvin in Minneapolis has said. Intensive care doctor Martin Tobin said Mr Floyd had died of asphyxia from the way he was restrained. Mr Chauvin, 45, was filmed kneeling on Mr Floyd's neck for more than nine minutes during his arrest last May. The ex-officer is on trial for murder and has denied the charges against him. Dr Tobin also contradicted the defence's argument that the painkiller fentanyl was the cause of Mr Floyd's inability to breathe. A toxicology report released last June said that Mr Floyd had fentanyl in his system. However, Dr Tobin said that Mr Floyd's breathing did not slow down enough for the drug to have been a factor in his total loss of oxygen. The prosecution in the case of Mr Chauvin has now begun hearing from medical experts. Asked about the positioning of the handcuffs behind Mr Floyd's back, coupled with the downward pressure of Mr Chauvin's weight against him, Dr Tobin - an expert in pulmonary medicine - said that Mr Floyd's ability to expand his chest was severely impaired. He said it was his medical opinion that Mr Floyd died after a low level of oxygen stopped his heart. "[George Floyd] is jammed down against the street, so the street is playing a major role in preventing him from expanding his chest. "He's against a hard asphalt street, so the way they're pushing down on his handcuffs combined with the street... it's like the left side is in a vice."