Key Point

In critically ill patients with pulmonary edema, plasma BNP level showed high specificity but low sensitivity for acute lung injury when measured within 24 hours of pulmonary edema onset.

SAN FRANCISCO—When critically ill patients develop pulmonary edema, intensivists want to know if it is because of cardiac dysfunction or acute lung injury. The plasma B type natriuretic peptide (BNP) level may help to make that determination, suggested a recent study by Rana and colleagues.¹

The authors evaluated the diagnostic accuracy of the plasma BNP level in a retrospective cohort of 84 critically ill patients with acute pulmonary edema; the findings were prospectively validated among 120 similar patients with a 43% incidence of acute lung injury. In both cohorts, the plasma BNP level distinguished acute lung injury from cardiogenic pulmonary edema about as well as the pulmonary artery occlusion pressure and ejection fraction did.

"The likelihood ratios for a BNP threshold of 250 pg/mL were comparable to those seen with our traditional hemodynamic measures," observed Ognjen Gajic, MD, one of the authors, at the most recent meeting of the Society of Critical Care Medicine. However, the sensitivity of the plasma BNP level was low, acknowledged Dr. Gajic, a Consultant in Pulmonary and Critical Care Medicine at the Mayo Clinic in Rochester, Minnesota.

The authors hypothesized that the absence of significant plasma BNP elevation would exclude cardiac dysfunction as the principal cause of pulmonary edema, thus confirming a diagnosis of acute lung injury. Plasma BNP was measured within 24 hours of acute pulmonary edema onset.

A plasma BNP level of 250 pg/mL or lower was 48% sensitive and 87% specific for diagnosing acute lung injury in the retrospective cohort. In the prospective cohort, the specificity was slightly higher (90%) and the positive likelihood ratio was 3.9. As many clinicians know, the likelihood ratio combines sensitivity and specificity to estimate the rise or decline in the odds of a disease or condition with a particular diagnostic test.

The area under the curve for the plasma BNP level was 0.71, comparable to the 0.66 and 0.61 found for the pulmonary artery occlusion pressure and the ejection fraction, respectively. The area under the curve for the plasma BNP level improved to 0.82 when patients with renal insufficiency were excluded from the analysis.

The sensitivity of plasma BNP level for diagnosing acute lung injury is low partly because of the high incidence of cardiac dysfunction in patients with acute lung injury, explained Dr. Gajic. He pointed out that the study cohorts had a combined 73 patients with acute lung injury; 23 of these had concomitant acute or chronic cardiac dysfunction.

—Timothy Begany

Reference
1. Rana R, Vlahakis NE, Daniels CE, et al. Utility of early measurement of plasma B type natriuretic peptide (BNP) in the assessment of acute lung injury and cardiogenic pulmonary edema. Presented at: annual meeting of the Society of Critical Care Medicine; January 10, 2006; San Francisco, Calif.

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