B-type natriuretic peptide secretion without change in intra-cardiac pressure

Objective: In clinical cardiology, B-type natriuretic peptide (BNP) is used as a non-invasive surrogate marker for intra-cardiac filling pressures, particularly in patients with heart failure. It is unknown whether and to what extent increase in intravascular volume and/or sympathetic tone while maintaining constant intra-cardiac pressures leads to an increase in levels of BNP in vivo.

Design and methods: We aimed to test this hypothesis in an experimental in vivo model of patients directly after off-pump coronary artery bypass grafting admitted to the intensive care unit. These patients require high volumes of intravenous fluids titrated to keep intra-cardiac filling pressures and arterial blood pressure in the normal range while awakening from deep general anesthesia. In 27 consecutive patients, intra-cardiac filling pressures (using a pulmonary artery catheter) and levels of BNP were measured simultaneously every 6h.

Results: At 0, 6, 12, and 18h, the pulmonary capillary wedge pressure remained constant (12±4, 13±3, 12±3, and 13±3mmHg, respectively; p=0.351). Similarly, right heart filling pressures did not change during the study period. In contrast, BNP levels increased significantly during the study period: Median levels were 82 [IQR 37-162] pg/ml at 0h, 153 [92-246] pg/ml at 6h, 274 [156-392] pg/ml at 12h, and 320 [200-528] pg/ml at 18h (p<0.001). No significant correlation between BNP levels and pulmonary capillary wedge pressures was found (r=0.052; p=0.604).

Conclusions: After cardiac surgery, BNP cannot be considered a reliable non-invasive surrogate for PCWP. In vivo, substantial BNP secretion occurs independently of PCWP in a setting of increasing intravascular volume and consciousness/sympathetic tone.