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Physicians have long recognized that factors including weight, age, lifestyle and cholesterol levels can affect patients’ risk of heart disease. But as narrator Gillian Anderson repeats several times in the new documentary “The Widowmaker,” about 4 million Americans with no symptoms and none of the common risk factors have died of unanticipated heart attacks in the past three decades.

Written and directed by Patrick Forbes, “Widowmaker” makes the case that many of those lives could have been saved if doctors employed a long-ignored, still-underused procedure: the coronary artery scan, a sort of mammogram of the heart that identifies calcium deposits. “If you find calcium, you know you’ve got [heart] disease,” one doctor says. With such information, a symptom-free patient can be put on a diet/exercise/lifestyle regimen before disaster strikes.

More than 30 years elapsed after the scan’s invention in 1981 before it was accepted as “beneficial” by the American Heart Association. The film blames the delay on the recalcitrance of doctors, hospitals and insurers — many of whom were eager to take a different route: the “highly profitable” use of stents, inserted via catheter, into the blocked arteries of heart patients. But that operation usually takes place only after a heart attack or other traumatic event makes the patient’s disease apparent. That wouldn’t have helped those 4 million asymptomatic heart attack victims.

Interspersed with emotional recollections from people who lost family members to sudden heart attacks and audio clips of terrified 911 calls, the movie is unabashedly on the side of the scan advocates (who call themselves “the calcium club”). The film recently debuted in theaters in New York and Los Angeles, and is available from iTunes, Amazon Instant and other sources.