Brain Natriuretic Peptide (BNP): A Cardiac Biomarker
BNP is a hormone secreted primarily by cardiac myocytes in response to mechanical stretch, serving as a biomarker for cardiac stress and heart failure that helps diagnose, risk-stratify, and monitor treatment response in patients with cardiac dysfunction. 1
What is BNP?
BNP (B-type natriuretic peptide) is part of the natriuretic peptide family that includes:
- BNP: Released primarily from ventricular myocytes
- ANP (Atrial natriuretic peptide): Released primarily from atrial tissue
- CNP (C-type natriuretic peptide): Released primarily from vascular endothelium
Production and Metabolism
- BNP is formed as a pre-prohormone, which is split to form proBNP
- ProBNP is further broken down to:
- The active hormone BNP 1-32
- The inactive N-terminal fragment NT-proBNP 1
The primary stimulus for BNP production is myocyte stretch caused by:
- Pressure overload
- Volume overload
- Hypoxia (which may reflect myocyte stretching in hypoxic regions) 1
Physiological Actions
BNP functions as a counter-regulatory system to the renin-angiotensin system, with effects including:
- Natriuresis (increased sodium excretion)
- Diuresis (increased urine output)
- Vasodilation
- Blood pressure reduction
- Inhibition of renin, aldosterone, and vasopressin secretion 1, 2
Normal Values and Influencing Factors
Normal BNP levels vary by age and sex:
| Patient age | BNP (males) | BNP (females) |
|---|---|---|
| Young adults | < 25 pg/ml | < 25 pg/ml |
| 45-59 years | < 100 pg/ml | < 100 pg/ml |
| > 60 years | < 98 pg/ml | Higher than males |
Factors affecting BNP levels:
- Sex: Males have consistently lower levels than females (likely due to androgen suppression)
- Body mass index: Higher BMI results in lower normal values
- Renal function: Impaired renal function can increase levels
- Age: Levels tend to increase with age 1
Clinical Utility
Diagnosis of Heart Failure
BNP is particularly useful in diagnosing heart failure in patients with acute dyspnea:
- BNP < 100 pg/ml: Excludes heart failure (90% sensitivity)
- BNP > 400 pg/ml: Strongly suggests heart failure (76% specificity) 1
When used in combination with clinical judgment, BNP significantly improves diagnostic accuracy compared to either alone 1.
Prognostic Value
BNP provides important prognostic information:
- Higher levels correlate with increased mortality risk
- For each 100 pg/ml increase in BNP, relative risk of death increases by 35% 1
- Predicts hospitalization and disease progression 1, 3
Treatment Monitoring
BNP levels typically decrease with effective heart failure treatment:
- Reduction seen with ACE inhibitors, angiotensin receptor blockers, and spironolactone
- Response to beta-blockers may be mixed (potential transient elevation initially) 1
BNP vs. NT-proBNP
Both markers are clinically useful but have important differences:
- Half-life: BNP ~20 minutes vs. NT-proBNP 1-2 hours
- Stability: NT-proBNP has greater stability in plasma
- Detection range: NT-proBNP has wider detection range (30-35,000 pg/ml)
- Sensitivity to hemodynamic changes: NT-proBNP less sensitive to rapid fluctuations 1
Clinical Pitfalls
Several non-heart failure conditions can elevate BNP:
- Acute coronary syndromes
- Pulmonary embolism
- Renal failure
- Sepsis
- Advanced age
- Atrial fibrillation
- Pulmonary hypertension 1, 4
Always interpret BNP values in the clinical context rather than in isolation, as even mildly elevated levels are associated with increased risk of adverse outcomes 1, 3.
Therapeutic Applications
Recombinant BNP (nesiritide) has been developed as a treatment for acute heart failure, providing vasodilation and potentially improving symptoms, though its role in routine management remains limited 5.