Whitney Houston drowned, but how?

Whitney Houston died because of a combination of cocaine, heart disease and, ultimately, drowning.

"It's possible she fell asleep," says cardiologist Gina Lundberg, medical director of the Heart Center for Women at St. Joseph's Hospital of Atlanta. "Based on the fact that the coroner lists drowning first would suggest she was alive when she went under the water, so she didn't have a heart attack first."

The Los Angeles County coroner's preliminary report Thursday found heart disease had caused blockages in Houston's arteries, common in drug users, but could not determine whether she had had a heart attack.

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Healthy arteries can withstand oxygen deprivation longer than arteries blocked by plaque, which is caused by "high cholesterol, high blood pressure, elevated blood glucose and genetic factors," Lundberg says. "But we don't know if she had any of those risk factors."

Final report due in 2 weeks

Houston died Feb. 11 at the Beverly Hilton, where she was discovered unresponsive and submerged in a bathtub.

The coroner's report points to chronic cocaine use, says chief investigator Craig Harvey.

"We feel that the cocaine coupled with the …heart disease complicated her condition," Harvey says. "Chances are if she did not have pre-existing heart disease and cocaine use, she might not have drowned."

The amount of chemicals found in Houston's body will be revealed in the final report expected in about two weeks.

Long-term cocaine use can cause inflammation and chronic damage to the blood vessels around the heart, which can lead to hardening of the arteries, says Cam Patterson, chief of cardiology at the University of North Carolina-Chapel Hill.

In the short term, a hit of cocaine can cause abnormal heart rhythms that can lead to sudden cardiac death.

"That could explain why someone seen just a few minutes earlier could suddenly drown," Patterson says.

The report presents a confusing picture that further tissue tests may answer, says Mark Boguski, a doctor at Harvard Medical School and editor of CelebrityDiagnosis.com. He's not surprised that Houston, 48, had some hardening of the arteries, "a routine finding" at her age, but he's perplexed that she apparently fell asleep or lost consciousness under the influence of cocaine, a stimulant. "It doesn't put you to sleep," he says. "It wakes you up."

Long struggle with drugs

Drowning likely wasn't the true cause of death, says pathologist Michael Fishbein of the Ronald Reagan UCLA Medical Center: "Cocaine is known to cause sudden cardiac death."

It can trigger heart disease as well, he adds, making heart rhythm failures more likely. "The fact that she was in a tub when she died may have been coincidental. She likely would have died just walking around."

The coroner's report suggests Houston had some water in her lungs, Fishbein says, leading to the verdict of drowning. But that may have been the result of gasping breaths she took while under water, long after her heart had stopped and organ death had started.

Houston's struggles with substance abuse were long suspected and, in recent years, confirmed in reports.

In a 2002 interview with Diane Sawyer, Houston did not deny using cocaine or pills but said sternly: "I make too much money to ever smoke crack. … Crack is whack."

Houston staged a comeback with 2009 album I Look to You and a world tour. Several shows were panned for the singer's shambolic delivery and inability to hit high notes, stirring speculation of a relapse.

Last year, she had outpatient treatment for drug and alcohol abuse.

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