George Floyd’s Cause of Death Is Crucial in Trial. Forensic Pathologists Explain.

Dr. Andrew Baker may be one of the most important witnesses to be called in the murder trial of Derek Chauvin. It was his duty to determine why George Floyd died.

Dr. Baker, the Hennepin County medical examiner, conducted Mr. Floyd’s initial autopsy and determined ultimately that his heart and lungs stopped functioning while he was being subdued, restrained and compressed by police officers.

The manner of Mr. Floyd’s death, Dr. Baker concluded, was homicide. And in the months since, almost everything he said in the autopsy report has been parsed and pored over by experts and laypeople alike.

In opening statements last week, the prosecution indicated that they would pursue an unusual strategy: challenging some aspects of Dr. Baker’s findings and introducing a reason that he did not cite — insufficient oxygen — as Mr. Floyd’s cause of death.

For their part, Mr. Chauvin’s defense team told jurors that the pre-existing heart disease, high blood pressure and recent drug use documented by Dr. Baker led to Mr. Floyd’s death from what they said was a heart arrhythmia.

As the trial nears a phase where Mr. Floyd’s cause of death will take center stage, we talked with several forensic pathologists uninvolved in the case to explain some of the terms used in the proceedings, how they determine the cause and manner of death and how this relates to the case. Here is what we learned.

What is a medical examiner?

In the United States, some jurisdictions use coroners and others use medical examiners to determine the cause of certain deaths, including those that occur in police custody. Unlike coroners, medical examiners are required to be physicians. Dr. Baker is certified in the field of forensic pathology and is the chief medical examiner of Hennepin County. The medical examiner’s office is an independent agency. It is not part of law enforcement.

Is there always one cause of death?

When someone dies, a death certificate is filled out for both public health and legal reasons. The form includes a cause of death in the first section and contributing factors in the second section. “We usually have to come up with one cause,” said Dr. Judy Melinek, a board-certified forensic pathologist. Everything else significant “that might be wrong with a person is ‘contributing.’”

Pathologists describe the cause of death as the immediate injury or disease that leads to death. It is the “disease or injury which starts the lethal sequence of events without an intervening cause,” Dr. Melinek said.

What is the manner of death?

The manner of death refers to the circumstances surrounding the death. There are usually five choices (a few jurisdictions include more): natural, accident, suicide, homicide or undetermined.

Homicide is often described as “death at the hands of another or others.” A homicide is not necessarily criminal — homicides can be a matter of self-defense, for example. The courts, not medical examiners, determine criminal culpability.

Is more than an autopsy considered?

In addition to examining the body, which usually occurs quickly, pathologists consider other materials such as the police report, videos, medical records and toxicology reports, said Dr. Priya Banerjee, a board-certified forensic pathologist. “We don’t practice in a black box,” she said. Until all the results of the investigation are in, she said, she typically says the case is pending further study.

Are the cause and manner of death always clear?

In a vast majority of cases, the cause and manner of death are evident, forensic pathologists say. But in some situations, the professional opinions of well-trained and experienced experts can differ.

“Some cases are much more complicated than others,” Dr. Banerjee said. When an “autopsy wasn’t a slam dunk,” Dr. Banerjee said, the written cause of death can be “more verbose because it’s taking into consideration many things.”

That can occur when an autopsy does not reveal a lethal injury like a gunshot wound to the brain. “Autopsies are good at showing demonstrable changes in the body’s tissues,” said Dr. Christopher Happy, a board-certified forensic pathologist, “but they’re not good at showing things that were functional, like a seizure or respiratory depression or an arrhythmia unless there’s some lesion associated with that.”

What did Dr. Baker ultimately determine?

Dr. Baker described Mr. Floyd’s cause of death as “cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression.” The manner of death, he wrote, was homicide.

The use of the term cardiopulmonary arrest led to public confusion because some people wrongly assumed it meant that Mr. Floyd had a heart attack. Cardiopulmonary arrest means “the heart stops beating and the lungs stop moving,” Dr. Cyril Wecht said.

Some pathologists say they do not include it as a cause of death because it describes all deaths.

Dr. Baker also detailed “other significant conditions” including pre-existing ones such as severe disease of the vessels of Mr. Floyd’s heart. He also described laboratory findings of the opiate drug fentanyl and methamphetamines in Mr. Floyd’s blood. Not including these under cause of death means he concluded that “those were there before but didn’t start the lethal sequence of events,” Dr. Melinek said. Listing them “is meant to clarify” what made Mr. Floyd more vulnerable to the cause of death, she said, “not excuse it.”

Here, context matters. Dr. Baker told prosecutors that if Mr. Floyd had been “found dead at home alone” with “no other apparent causes,” they wrote, it could have been acceptable to determine that Mr. Floyd died of an overdose because of the relatively high levels of fentanyl found in his blood collected at the hospital.

Instead, recordings revealed both the prolonged restraint of Mr. Floyd just before his death and also that he appeared agitated rather than lethargic, which could suggest tolerance to higher doses of fentanyl. The drug typically “causes you to become relaxed,” Dr. Wecht said.

By contrast, Dr. Melinek said, Mr. Chauvin’s defense attorneys appear to be trying to use the medical findings to convince the jury that Mr. Floyd “was essentially a ticking time bomb, already had pre-existing conditions that made this endpoint happen, not because excessive force was used.”

Only asserting that he was at higher risk of death from police restraint because of underlying health conditions or drug use would likely be legally insufficient, some experts said.

What did his preliminary findings say?

The medical examiner’s office had not finished its investigation when prosecutors filed a charging document saying that preliminary findings had shown no physical evidence to support a diagnosis of “traumatic asphyxia or strangulation.” It said that the combined effects of police restraint, underlying health conditions including heart disease and “any potential intoxicants in his system likely contributed to his death.”

Forensic pathologists said that in high-profile cases, it can be problematic to release findings early because they may be misinterpreted or incomplete. “I basically never release preliminary cause of death,” Dr. Banerjee said.

What did a second autopsy of Mr. Floyd find?

After the initial autopsy, forensic pathologists no longer have access to an intact body, and sometimes organs or tissues are unavailable, having been removed for further study. Even so, doctors sometimes document findings that an examiner missed in the first autopsy or that were not apparent.

“There are multiple reasons you might see something at second autopsy that’s not seen at the first and vice versa,” Dr. Melinek said.

Mr. Floyd’s family hired Dr. Michael Baden and Dr. Allecia Wilson to perform a second autopsy. Both experts said that pressure on Mr. Floyd’s neck and back during his restraint by the police led him to die of asphyxia, a term Dr. Baker did not use in his official report.

The word asphyxia derives from an ancient Greek term that means “without pulse.” Doctors now use it to mean the deprivation of oxygen, which can occur for many reasons. At the center of the case is whether Mr. Chauvin’s actions led to any of them, such as inhibiting the movement of the diaphragm that allows the lungs to expand, or reducing the flow of blood carrying oxygen to vital tissues. Oxygen deprivation may occur not only from neck compression, experts said, but also from pressure on the back when someone is lying face down, and it may not leave major physical traces.

The cause of death described by both Dr. Baker and the pathologists who performed the second autopsy amounted to substantially the same thing, some experts said, which was “that external pressure in a prone position to Mr. Floyd caused his death,” Dr. Banerjee said.

Shaila Dewan contributed reporting.