B type natriuretic peptide (BNP) or N-terminal proBNP (NT-proBNP) are often estimated in patients presenting with breathlessness to look for evidence of heart failure. In obese patients, clinical findings, chest X-ray and even bedside echocardiography has limitations in the diagnosis of heart failure. Abdominal obesity can even cause orthopnea like presentation due to elevated levels of diaphragm. Hence we are more likely to look at BNP/NT-proBNP to establish heart failure in the emergency room. But there is a great limitation for these estimations as BNP levels may be low due to obesity.
An inverse relationship between body mass index and BNP has been demonstrated by Wang TJ et al . It was based on data from 3389 Framingham Study participants without heart failure. Similar pattern was noted for N-terminal proatrial natriuretic peptide. Authors also suggested that the low levels of natriuretic peptides may contribute to their susceptibility to hypertension and related disorders. Association between obesity and low BNP is probably mediated by expression of natriuretic peptide clearance receptor (NPR-C) in adipose tissue.
2707 subjects from the Dallas Heart Study also showed a similar pattern. Body mass index was independently associated with lower BNP and NT-proBNP levels (P<0.001) . But this study did not confirm the proposed enhanced BNP clearance mediated via NPR-C. This was because NT-proBNP which is not believed to bind NPR-C was also low.
The Suita Study of urban Japanese population involved 1,759 subjects without atrial fibrillation or history of ischemic heart disease in the age group 38-95 years . This study showed an inverse relation between BNP and obesity related markers like body fat mass, skinfold thickness and waist circumference.
Results from the Breathing Not Properly Multinational Study suggested a lower cut-point of BNP more than or equal to 54 pg/mL for the severely obese patients for diagnosis of acute heart failure . They suggested a higher cut-point of BNP 170 pg/mL or more in lean patients to increase specificity. Breathing Not Properly Multinational Study involved 7 centres and was a prospective study of 1586 patients who presented to the emergency room with acute breathlessness. BNP was measured on arrival and 1368 participants had height and weight data.
A previous study of 318 patients with heart failure by Mehra MR had shown similar results . They found that BNP levels were significantly lower in obese patients compared to non-obese patients (205 ± 22 and 335 ± 39 pg/ml, respectively; p = 0.0007).