BNP is often normal in patients with compensated heart failure (without water overloading, even with LV-EF 20-30%), especially in optimally treated persons. During decompensation BNP also  may remain normal in first hours.

Natriuretic peptides have several actions: (1) down-regulating the sympathetic nervous system and the renin-angiotensin-aldosterone system, (2) facilitating natriuresis and diuresis through the afferent and efferent hemodynamic mechanisms of the kidney and distal tubules, (3) decreasing peripheral vascular resistance, and (4) increasing smooth muscle relaxation. Natriuretic peptides also may inhibit cardiac growth and hypertrophy, counteracting the mitogenesis that causes ventricular remodeling.

BNP primarily is secreted by the ventricles in the heart as a response to left ventricular stretching or wall tension. It may be a backup hormone that is activated only after a prolonged period of volume overload. Cardiac myocytes secrete a BNP precursor that is synthesized into proBNP, which consists of 108 amino acids. After it is secreted into the ventricles, proBNP is cleaved into the biologically active C-terminal portion and the biologically inactive N-terminal (NT-proBNP) portion.

Many medications used to treat heart failure (e.g., diuretics such as spironolactone [Aldactone], angiotensin-converting enzyme inhibitors, angiotensin-II receptor blockers) reduce natriuretic peptide concentrations. Therefore, many patients with chronic stable heart failure will have BNP levels in the normal diagnostic range (i.e., BNP level less than 100 pg per mL).

İs  the patient obese?

In obese patients the pro-bnp can sometimes be falsely decreased

Thank you Przemyslaw, Frederico, Serkan, and Gerald for all of the wonderful responses and discussion.  Obesity is a good consideration in this scenario, as well for all of the etiologies of false decreases of BNP.

You can find the description of the physiological basis of this phenomenon in the paper of G. Iervasi et al,  published on the Am J of Physiol and in the following paper of our group: Sorry now I do not remember the year. If you was not able to find it, let me known.

BNP is expressed predominantly in cardiomyocytes of left ventricles, that's why normal levels of the hormone possible with heavy cardiosclerosis.

Congestive heart failure with preserved ejection fraction of the left ventricle suggest of the right ventricle insuffiency or recurrent thromboembolism of the pulmonary artery. In either case, the expression of BNP should not be expected.

Thank you Alexandr and Andrei for the additional insight that you have provided.  Recombinant BNP (nesiritide) is actually a therapy that is being used in the context of acute CHF.