The emergency room doctor who pronounced George Floyd dead has testified that he theorised at the time that Floyd's heart most likely stopped because of a lack of oxygen.
Bradford Langenfeld, who was a senior resident on duty that night at Hennepin County Medical Center and tried to resuscitate Floyd, took the stand at the beginning of week two at former Minneapolis Officer Derek Chauvin's murder trial, as prosecutors sought to establish that it was Chauvin's knee on the black man's neck that killed him.
Dr Langenfeld said Floyd's heart had stopped by the time he arrived at the hospital.
The doctor said that he was not told of any efforts at the scene by bystanders or police to resuscitate Floyd but that paramedics told him they had tried for about 30 minutes.
Dr Langenfeld said Floyd's heart had stopped by the time he arrived at the hospital. Source: Pool Court TV
Under questioning by prosecutor Jerry Blackwell, Dr Langenfeld said that based on the information he had, it was "more likely than the other possibilities" that Floyd's cardiac arrest - the stopping of his heart - was caused by asphyxia, or insufficient oxygen.
Chauvin, 45, is charged with murder and manslaughter in Floyd's death.
The white officer is accused of pinning his knee on the 46-year-old man's neck for nine minutes and 29 seconds as Floyd lay face-down in handcuffs outside a corner market where had been accused of trying to pass a counterfeit $US20 bank note for a pack of cigarettes.
The defence argues that Chauvin did what he was trained to do and that Floyd's use of illegal drugs and his underlying health conditions caused his death.
Chauvin lawyer Eric Nelson questioned Dr Langenfeld about whether some drugs can cause hypoxia, or insufficient oxygen.
The doctor acknowledged that fentanyl and methamphetamine, both of which were found in Floyd's body, can do so.
The county medical examiner's office ultimately classified Floyd's death a homicide - that is, a death at the hands of someone else.
The full report said Floyd died of "cardiopulmonary arrest, complicating law enforcement subdual, restraint and neck compression".
A summary report listed fentanyl intoxication and recent methamphetamine use under "other significant conditions" but not under "cause of death".
Under cross-examination from Nelson, Dr Langenfeld said Floyd's carbon dioxide levels were more than twice as high as levels in a healthy person and he agreed that that could be attributed to a respiratory problem.
But on questioning from the prosecutor, the doctor said the high levels were also consistent with cardiac arrest.
Dr Langenfeld also testified that neither he nor paramedics administered a drug that would reverse the effects of an opioid overdose.
The doctor said giving Narcan once a patient is in cardiac arrest would provide no benefit.
Floyd's treatment by police was captured on widely seen bystander video that sparked protests that rocked Minneapolis and quickly spread to other US cities and beyond and descended into violence in some cases.
Dr Langenfeld said that "any amount of time" a patient spends in cardiac arrest without immediate CPR decreases the chance of a good outcome.
He said there is an approximately 10 to 15 per cent decrease in survival for every minute that CPR is not administered.
Prosecutors in the second week of the trial are also expected to zero in on Chauvin's training in the use of force.
Minneapolis Police Chief Medaria Arradondo took the stand on Monday, testifying about police policy that dictates that whenever it is reasonable to do so, officers must use tactics to de-escalate a situation so as to avoid or minimise the use of force.
Prosecutor Steve Schleicher noted that while some people may become more dangerous under the influence of drugs or alcohol, some may actually be "more vulnerable".
Arradondo agreed and acknowledged that this must also be taken into consideration when officers decide to use force.
Kneeling on Floyd's neck violated training, police chief says
Minneapolis police chief Medaria Arradondo testified that police officer Chauvin kneeling on Floyd's neck violated the department's restraint policy.
Arradondo, testifying at Chauvin's murder trial, said the use of force against Floyd should have ended "once he stopped resisting."
"When Mr Floyd was no longer responsive and even motionless to continue to apply that level of force to a person proned out, handcuffed behind their back, that in no way, shape or form is anything that is by policy, is not part of our training and it is certainly not part of our ethics or our values," Mr Arradondo said.
Mr Arradondo, 54, fired Chauvin and the three other officers involved in the arrest that led to Floyd's death within days of the incident.
