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 trial is in its second week and is expected to last for at least one month. The defence is due to begin arguing its case next week.
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.
About three minutes before Mr Chauvin removed his knee, Dr Tobin said, there was “not an ounce of oxygen left in his body”.
“The knee remains on the neck for another three minutes and 27 seconds after he takes his last breath,” Dr Tobin said. “After the officers have found themselves that there’s no pulse, the knee remains on the neck for another two minutes and 44 seconds”.
Dr Tobin used footage captured during Mr Floyd’s arrest to provide his medical opinion on the ninth day of Mr Chauvin’s trial in Minneapolis, Minnesota.
The main medical examiner, who conducted the county-commissioned autopsy, is not expected to give evidence at the trial until Friday.
On Wednesday, the police chief of Minneapolis testified that Mr Chauvin had violated the agency’s policy on the force during Mr Floyd’s arrest.
Medaria Arradondo said the way Mr Chauvin had restrained Mr Floyd was not in line with training and “certainly not part of our ethics and our values”.
Defence attorney Eric Nelson said that an officer’s actions must be viewed from the point of view of an officer at the scene, not in hindsight.
The defence has argued that Mr Floyd’s efforts to resist arrest necessitated the restraint.
It has also contended that in addition to the drug use, an underlying heart condition contributed to Mr Floyd’s death.