In a large study from Canada, high levels of circulating surfactant protein-D (SP-D) in the blood were found to be strongly predictive of death from heart and vascular causes, independent of established risk factors for heart attack and stroke.
SP-D levels rise in response to lung inflammation, and the finding suggests that elevated levels in the blood in response to lung damage from smoking, air pollution, and other causes may have a direct role in cardiovascular disease, lung specialist and study researcher Don D. Sin, MD, MPH, of Vancouver's St. Paul Hospital tells WebMD.
"We know that things like smoking and inhaling a lot of air pollution are associated with an increased risk for heart attack and stroke," he says, adding that the study suggests that lung inflammation is directly linked to heart disease.
Checking for Heart Risk
To test the theory, the researchers analyzed data from a study that included 806 people with suspected coronary artery disease undergoing angiography testing.
Just under a third (30%) of the participants died during an average of 12 years of follow-up, and the participants with the highest blood levels of SP-D had more than four times the risk of death from cardiovascular causes than participants with the lowest levels of the protein.
A second analysis of a study involving current and ex-smokers revealed that circulating SP-D levels were significantly higher in people who died or were hospitalized for heart or vascular problems.
While the associations were strong, Sin says the study was not designed to prove that elevated circulating SP-D causes heart attacks and strokes.
"Obviously these findings need to be replicated," he says, adding that different types of studies will be needed to confirm that circulating SP-D directly contributes to heart and vascular disease.
It also remains to be seen if measuring SP-D in the blood will prove useful for evaluating cardiovascular risk.
Test for Heart Disease?
Sin is hopeful, telling WebMD that measuring SP-D levels in the blood of patients with established artery disease may help doctors better understand patients' risk.
In the study, published online June 8 in the European Heart Journal, SP-D proved to be a stronger predictor of heart death than the established cardiovascular blood test C-reactive protein (CRP).
But New York City cardiologist Tara Narula, MD, says it remains to be seen if SP-D will ever be a useful test for heart and vascular disease.
"This study suggests that this surfactant protein is associated with cardiovascular disease and mortality, but it doesn't tell us if this association is causal," she says.