(CNN) -- The wife of former Utah doctor Martin MacNeill drowned and drugs may have played a role in her death, according to a retired medical examiner who took the stand Friday in MacNeill's murder trial.
"It was clear she was inhaling a significant amount of water," said Dr. Joshua Perper.
Perper was the chief medical examiner for Broward County, Florida, before he retired two years ago.
Prosecutors' basic theory is that Martin MacNeill drugged and drowned his wife. But they're having to overcome the findings of the original medical examiner who performed Michele MacNeill's autopsy and determined she died of natural causes from heart disease.
Prosecutors say Martin MacNeill killed his wife on April 11, 2007, so he could continue an affair with Gypsy Willis. Michele MacNeill was found with several drugs in her system -- including Valium, Percocet, Phenergan and Ambien -- the day she died.
Martin MacNeill has pleaded not guilty to first-degree murder and obstruction of justice in her death and is currently on trial in Provo, Utah.
Perper took up most of Friday, the10th day of trial, telling jurors why he believed "within the reasonable degree of medical certainty" that Michele MacNeill drowned.
He based his conclusions, in part, on the fact that -- according to a first responder who treated Michele MacNeill the day of her death -- she regurgitated three to four cups of fluid as they tried to resuscitate her and then an additional three cups of fluid later.
Key chemicals in Michele MacNeill's blood were also "remarkably diluted," according to Perper. He said the level of dilution could only be explained by water entering her lungs and crossing through tiny blood vessels and into her bloodstream.
Perper also said he did a thorough review of Michele MacNeill's heart tissue samples and did not find any evidence of myocarditis, or inflammation of the heart. The original medical examiner attributed Michele MacNeill's heart disease to this inflammation and also to high blood pressure. The fact that she had high blood pressure is not a disputed issue in this case.
On cross-examination by Martin MacNeill's defense, however, Perper admitted he still couldn't rule the manner of Michele MacNeill's death a homicide. He listed it as "undetermined" instead.
Defense attorney Randy Spencer also suggested the first responder may have been mistaken when estimating how much water Michele MacNeill expelled as CPR was performed. He took a bottle of water and poured some into a pitcher. "Fair to say that it's pretty hard to estimate exactly how much water is coming out?" Spencer asked.
"I have to rely my determination on what he said. He didn't say to us half a cup of water. He said to us it was a significant amount, which he estimated," said Perper.
Perper also left open the possibility that an irregular heartbeat may have contributed to Michele MacNeill's death. A cardiologist who testified earlier in the day, however, said he didn't believe that Michele MacNeill's heart disease was significant enough to cause an irregular heartbeat or sudden death.
"In my experience, patients with mild myocarditis have mild problems related to it. They don't have serious arrhythmias or life threatening conditions," said Dr. David Cragun. He also pointed out that Michele MacNeill didn't show any symptoms like chest pain or shortness of breath before her death.
The prosecution has said it plans to call four prison inmates to the stand Tuesday to testify about how Martin MacNeill allegedly told them that he killed his wife and was going to get away with it.
Martin MacNeill faces life in prison if convicted.
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