Utility of N-Terminal Pro-Brain Natriuretic Peptide for the Diagnosis of Heart-Failure

Jacob V Jose, Satya N Gupta, Dhayakani Selvakumar

Christian Medical College and Hospital, Vellore

Background: The goal of this study was to evaluate the utility of plasma N-terminal pro-brain natriuretic peptide for the diagnosis of heart failure in patients presenting with shortness of breath.

Methods and Results: We measured plasma levels of N-terminal pro-brain natriuretic peptide in 119 patients presenting with shortness of breath. The patients were divided into two groups based on the Framingham criteria and echocardiographic results—those with heart failure and those not in heart failure. Plasma levels of N-terminal pro-brain natriuretic peptide were compared in the two groups. The mean N-terminal pro-brain natriuretic peptide concentration in patients with heart failure (n=73) was higher than that in those not in heart failure (389±148 fmol/ml v. 142±54 fmol/ml, p<0.001). N-terminal pro-brain natriuretic peptide values increased significantly as the functional severity of heart failure increased (p<0.001). The mean N-terminal pro-brain natriuretic peptide levels were 261±34 fmol/ml for patients in New York Heart Association functional class I, 300±161 fmol/ml for patients in New York Heart Association functional class II, 427±103 fmol/ml for patients in New York Heart Association functional class III and 528±170 fmol/ml for patients in New York Heart Association functional class IV. Using a cut-off value of 200 fmol/ml, the sensitivity of N-terminal pro-brain natriuretic peptide was 97%, specificity was 89% and accuracy for differentiating heart failure from other causes of shortness of breath was 93%.

Conclusions: Our results suggest that N-terminal pro-brain natriuretic peptide can be reliably used for the diagnosis of heart failure in an outpatient setting, and this will improve the ability of clinicians to differentiate patients with shortness of breath due to heart failure from those with other causes of shortness of breath. (Indian Heart J 2003; 55: 35–39)

Key Words: Heart failure, Natriuretic peptide, Echocardiography

Heart failure (HF) is a common disorder, and its prevalence increases with age. In the Framingham Heart Study, the prevalence of HF increased from 0.8% in the 50–59 years age group to 9.1% in the population 70 years of age and older.1,2 The prevalence of HF may continue to rise as the proportion of elderly people within the population increases. Heart failure is not only an important reason for hospitalization but is also second only to hypertension as a cause of outpatient physician visits.3 Most patients in the emergency department present with acute shortness of breath. It is extremely important to differentiate between HF and other causes of shortness of breath. Symptoms and physical examination alone are not sufficient to always make an accurate diagnosis. Currently, echocardiography is considered the gold standard diagnostic test to evaluate patients with suspected ventricular dysfunction but it is expensive and not always easily available.4

Natriuretic peptides (cardioneurohormones)—atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP)—are secreted from the heart in response to increased intracardiac volume or pressure.5,6 Brain natriuretic peptides are specifically secreted from the ventricles in response to volume expansion or pressure overload. Levels of BNP have been shown to be elevated in patients with left ventricular dysfunction and correlate with the New York Heart Association (NYHA) classification, as well as prognosis.7–10

Recently, it has been reported that N-terminal pro-brain natriuretic peptide (NT-proBNP) levels are elevated in patients with cardiac impairment.11,12 NT-proBNP is derived from proBNP when it splits into BNP. In normal subjects, NT-proBNP levels are similar to those of BNP, whereas in those with cardiac impairment, the proportional and absolute increment of NT-proBNP above the normal level exceeds that of BNP, which suggests that it may be a more discerning marker.13,14 Though there are a number of studies regarding the role of BNP in HF, NT-proBNP has not been studied extensively. Thus, this study was undertaken to evaluate the utility of NT-proBNP for the diagnosis of HF.

Methods

Patient characteristics: One hundred nineteen patients presenting with shortness of breath (either acute or chronic) over a 6-month period to our emergency and outpatient departments were enrolled in the study. Associated symptoms were edema, weight gain, cough, or wheezing. Patients with acute coronary syndromes were excluded.

All the subjects underwent physical examination, chest X-ray, supine 12-lead electrocardiogram (ECG) and blood pressure (BP) measurement. Other data recorded included history, risk factors, and drug treatment. Echocardiography was performed in all the patients using a Sonos 5500 machine. Left ventricular end-systolic and diastolic volumes were measured, and ejection fraction determined based on the American Society of Echocardiography recommendations.15

NT-proBNP measurement: Two ml of blood was collected in a chilled clotted tube and centrifuged immediately. The plasma was stored at –80°C until the NT-proBNP assay was carried out. Plasma NT-proBNP concentration was measured using a Biomedica kit. This kit was a competitive enzyme immunoassay (EIA) designed to measure the immunoreactive NT-proBNP in diluted human serum, plasma or urine samples. To achieve high specificity, the kit incorporates an immunoaffinity purified sheep antibody specific for NT-proBNP (8-29) immobilized to the surface of a microtiter plate well.

The assay is based on competitive reaction of the unlabeled peptide in the standards or samples, and horseradish peroxidase-labeled peptide (tracer) for the limiting binding sites of the NT-proBNP (8-29) specific antibody. The assay had a detection limit (±2 SD) of 5 fmol/ml. The intra-assay and interassay coefficients of variation were 6.5% and 4.4%, respectively. The assay was performed within 3 months of sampling. The author, who was involved in NT-proBNP measurement, was blinded to clinical information regarding HF.

Diagnosis of heart failure: We divided our patients into two groups: patients with shortness of breath due to HF and patients with shortness of breath not due to HF. The diagnosis of HF was based on the Framingham criteria and echocardiography.16 For patients with a diagnosis other than HF, confirmation was attempted using the following variables: normal chest X-ray and normal left ventricular function by echocardiogram. In the group of patients with HF, there were patients with systolic and diastolic dysfunction: of the 73 patients with HF, 14 had diastolic dysfunction.

Statistical analysis: To evaluate the utility of NT-proBNP measurement for the diagnosis of HF, we computed sensitivity, specificity, and accuracy. All the values are expressed as mean±2 SD. Comparison among values obtained for the HF group and for those not in HF was made by multivariate analysis. A value of p<0.05 was considered statistically significant. The plasma level of NT-proBNP was also compared in each stage of NYHA functional class in patients with HF. A receiver–operating characteristics curve was obtained to determine various cut-off values of NT-proBNP peptide for the diagnosis of HF.

Baseline characteristics: The baseline characteristics of the overall study group of 119 patients are shown in Table 1. The mean age of the study population was 53.7±12.4 years. There were 78 men (65.5%) and 41 women (34.5%). Patients were grouped according to whether they had HF or not, based on the Framingham criteria. The final assessment revealed that 73 patients (61.3%) had HF as a cause of their shortness of breath, while 46 (38.7%) had a cause other than HF; these included bronchial asthma, chronic obstructive pulmonary disease, pneumonia, carcinoma lung, psychogenic dyspnea. There was no significant difference in the age and sex ratio in both the groups of patients. Patients with HF were more likely to have had a previous history of myocardial infarction and clinical signs of HF.

Results

NT-proBNP and final diagnosis: Patients with a final diagnosis of HF had a mean (±SD) NT-proBNP level of 389±148 fmol/ml compared with 142±54 fmol/ml in patients not in HF. The difference between the groups was statistically significant (p<0.001). Figure 1 is a box plot of NT-proBNP values for patients with HF and those not in HF.

NT-proBNP and clinical severity of heart failure: The mean NT-proBNP level was 261±34 fmol/ml for NYHA functional class I, 300±161 fmol/ml for NYHA functional class II, 427±103 fmol/ml for NYHA functional class III and 528±170 fmol/ml for NYHA functional class IV patients. NT-proBNP values increase significantly as the functional severity of HF increases (p<0.001). Figure 2 shows NT-proBNP values in relation to the NYHA functional class.

NT-proBNP and left ventricular ejection fraction: There was a significant negative linear correlation between plasma NT-proBNP and the left ventricular ejection fraction (r=–0.69; p<0.001) (Fig. 3). The level of NT-proBNP in patients with a left ventricular ejection fraction less than 40% was significantly higher in comparison to its level in patients with left ventricular ejection fraction more than 40% (406±152 fmol/ml v. 171±83 fmol/ml).

Receiver–operating characteristics curve: The capacity of NT-proBNP to differentiate between shortness of breath due to HF and that due to other causes was assessed with a receiver–operating characteristics curve analysis (Fig. 4).  The area under the receiver–operating characteristics curve when NT-proBNP was used to differentiate HF from other causes of shortness of breath was 0.94 (95% confidence interval: 0.90–0.98; p<0.001). An NT-proBNP cut-off value of 200 fmol/ml had a sensitivity of 97%, a specificity of 89% and an accuracy of 93% for differentiating between HF and other causes of shortness of breath.

Multivariate analysis: In multiple logistic regression analysis, we found that NT-proBNP was a strong independent predictor for HF with an odds ratio of 8.94 (95% confidence interval: 3.9–20.48). The other variable, which was better than NT-proBNP, was the presence of S3 gallop (Table 2).

Discussion

Our study was designed to assess the diagnostic value of the plasma level of circulating NT-proBNP as a noninvasive indicator of HF. Our results suggest that NT-proBNP can be used as a reliable marker for identification of left ventricular dysfunction in a group of patients presenting with shortness of breath. We found that a cut-off value of 200 fmol/ml had an accuracy of 93%.

Echocardiography, although currently the gold standard for the diagnosis of left ventricular dysfunction, is costly and has limited availability in an urgent care setting. Dyspneic patients may be unable to hold their breath long enough during an echocardiographic study for a good image to be obtained. In addition, technical difficulty due to obesity and lung disease may also interfere in getting a good echocardiographic image. Therefore, even if echocardiography is available in the emergency department, an accurate, sensitive, and specific blood test for HF would be a useful addition to the clinical armamentarium. Brain natriuretic peptide measurement is a useful tool for evaluating possible left ventricular dysfunction and ventricular failure.10 Insufficient data are available on NT-proBNP, but BNP and NT-proBNP appear to be equivalent markers.17 In our study, the NT-proBNP level was significantly higher in patients with HF in comparison with those not in HF. Our results concur with those reported previously from the West.18–22

Our results suggest that the levels of NT-proBNP increase with increasing severity of HF. NT-proBNP levels were higher in patients in a higher NYHA class. Our results are in accordance with the study by Hunt et al.12 Patients with left ventricular ejection fraction <40% had NT-proBNP levels of 406±152 fmol/ml, whereas patients with left ventricular ejection fraction >40% had values of 171±83 fmol/ml. In the study by Campbell et al.,21 NT-proBNP had a sensitivity of 100% for differentiating patients with left ventricular ejection fraction <45%. In our study, NTproBNP levels showed a negative correlation with the left ventricular ejection fraction (Fig. 3). When we used a cutoff value of 200 fmol/ml for NT-proBNP, the sensitivity was 97% for the diagnosis of HF. Our findings suggest that NTproBNP could be a reliable marker for the diagnosis of HF.

Our results suggest that the determination of cardiac ventricular peptides substantially improves the diagnostic accuracy of the assessment of left ventricular dysfunction in a patient population with suspected HF. The results demonstrate that measurement of NT-proBNP levels in the blood improves the ability of the clinician to differentiate between patients with shortness of breath due to HF and shortness of breath due to other causes in an acute care setting. This should be especially true among patients in whom the diagnosis of HF is not clinically obvious. The BNP/NT-proBNP test is now available in a rapid form, thus making diagnostic information immediately available to the acute care physician. Use of this test in conjunction with other clinical information should lead to a more accurate initial diagnosis of HF.

Limitations of the study: As this study was conducted in a tertiary care center, it needs to be confirmed whether these results can be generalized to the primary care level. A larger study, including the patient’s clinical findings, chest X-ray, and NT-proBNP level may be worthwhile. Also, we did not look at the group of patients who could not be diagnosed to have HF by routine clinical methods. Such a study will give the incremental value of NT-proBNP over routine clinical methods.

Conclusions: Plasma NT-proBNP is a sensitive indicator of cardiac dysfunction. Our results suggest that NT-proBNP can be used reliably for the diagnosis of HF in an outpatient setting. The results demonstrate that measurement of NTproBNP levels will improve the ability of clinicians to differentiate between patients with shortness of breath due to HF and that due to other causes.

Acknowledgment

We would like to express our thanks to Mr Pramod, statistician, for helping us with the statistical work.

Correspondence:

Dr Jacob V Jose,
Department of Cardiology,
CMC Hospital,
Vellore 632004

References

  1. Cheitlin MD, Zipes DP. Cardiovascular disease in special population. In: Braunwald E, Zipes DP, Libby P (eds). Heart disease: a text book of cardiovascular medicine. 6th ed. Philadelphia: WB Saunders Co; 2001. p. 2019

  2. Kannel WB, Belanger AJ. Epidemiology of heart failure. Am Heart J 1991; 121: 951–957

  3. O’Connell JB, Bristow MR. Economic impact of heart failure in the United States: time for a different approach. J Heart Lung Transplant 1994; 13 (Suppl): S107–S112

  4. Dao Q, Krishnaswamy P, Kazanegra R, Harrison A, Amirnovin R, Lenert L, et al. Utility of B-type natriuretic peptide in the diagnosis of congestive heart failure in an urgent-care setting. J Am Coll Cardiol 2001; 37: 379–385

  5. Nagagawa O, Ogawa Y, Itoh H, Suga S, Komatsu Y, Kishimoto I, et al. Rapid transcriptional activation and early mRNA turnover of brain natriuretic peptide in cardiocyte hypertrophy: evidence for brain natriuretic peptide as an "emergency" cardiac hormone against ventricular overload. J Clin Invest 1995; 96: 1280–1287

  6. Maeda K, Tsutamoto T, Wada A, Hisanaga T, Kinoshita M. Plasma brain natriuretic peptide as a biochemical marker of high left ventricular end-diastolic pressure in patients with symptomatic left ventricular dysfunction. Am Heart J 1998; 135: 825–832

  7. Clerico A, Iervasi G, Del Chicca MG, Emdin M, Maffei S, Nannipieri M, et al. Circulating levels of cardiac natriuretic peptides (ANP and BNP) measured by highly sensitive and specific immunoradiometric assays in normal subjects and in patients with different degrees of heart failure. J Endocrinol Invest 1998; 21: 170–179

  8. Omland T, Aakvaag A, Bonarjee VV, Caidahl K, Lie RT, Nilsen DW, et al. Plasma brain natriuretic peptide as an indicator of left ventricular systolic function and long-term survival after acute myocardial infarction. Comparison with plasma atrial natriuretic peptide and N-terminal proatrial natriuretic peptide Circulation 1996; 93: 1963–1969

  9. Krishnaswamy P, Lubien E, Clopton P, Koon J, Kazanegra R, Wanner E, et al. Utility of B-natriuretic peptide levels in identifying patients with left ventricular systolic or diastolic dysfunction. Am J Med 2001;111: 274–279

  10. Maisel AS, Krishnaswamy P, Nowak RM, McCord J, Hollander JE, Duc P, et al. Rapid measurement of B-type natriuretic peptide in the emergency diagnosis of heart failure. N Engl J Med 2002; 347: 161–167

  11. Hunt PJ, Yandle TG, Nicholls MG, Richards AM, Espiner EA. The amino-terminal portion of pro-brain natriuretic peptide (Pro-BNP) circulates in human plasma. Biochem Biophys Res Commun 1995; 214: 1175–1183

  12. Hunt PJ, Richards AM, Nicholls MG, Yandle TG, Doughty RN, Espiner EA. Immunoreactive amino-terminal pro-brain natriuretic peptide (NT-ProBNP): a new marker of cardiac impairment. Clin Endocrinol 1997; 47: 287–296

  13. Richards AM, Nicholls MG, Yandle TG, Frampton C, Espiner EA, Turner JG, et al. Plasma N-terminal pro-brain natriuretic peptide and adrenomedullin: new neurohormonal predictors of left ventricular function and prognosis after myocardial infarction. Circulation 1998; 97: 1921–1929

  14. Hughes D, Talwar S, Squire IB, Davies JE, Ng LL. An immunoluminometric assay for N-terminal pro-brain natriuretic peptide: development of a test for left ventricular dysfunction. Clin Sci (Lond) 1999; 96: 373–380

  15. Schiller NB, Shah PM, Crawford M, DeMaria A, Devereux R, Feigenbaum H, et al. Recommendations for quantification of the left ventricle by two-dimensional echocardiography. American Society of Echocardiography Committee on Standards, Subcommittee on Quantitation of Two-Dimensional Echocardiograms. J Am Soc Echocardiogr 1989; 2: 358–367

  16. Ho KK, Pinsky JL, Kannel WB, Levy D. The epidemiology of heart failure: the Framingham Study. J Am Coll Cardiol 1993; 22: 6A–13A

  17. Mair J, Hammerer-Lercher A, Puschendorf B. The impact of cardiac natriuretic peptide determination on the diagnosis and management of heart failure. Clin Chem Lab Med 2001; 39: 571–588

  18. Schulz H, Langvik TA, Lund Sagen E, Smith J, Ahmadi N, Hall C. Radioimmunoassay for N-terminal probrain natriuretic peptide in human plasma. Scand J Clin Lab Invest 2001; 61: 33–42

  19. Hammerer-Lercher A, Neubauer E, Muller S, Pachinger O, Puschendorf B, Mair J. Head-to-head comparison of N-terminal probrain natriuretic peptide, brain natriuretic peptide and N-terminal pro-atrial natriuretic peptide in diagnosing left ventricular dysfunction. Clin Chim Acta 2001; 310: 193–197

  20. Pemberton CJ, Yandle TG, Rademaker MT, Charles CJ, Aitken GD, Espiner EA. Amino-terminal proBNP in ovine plasma: evidence for enhanced secretion in response to cardiac overload. Am J Physiol 1998; 275: H1200–H1208

  21. Campbell DJ, Mitchelhill KI, Schlicht SM, Booth RJ. Plasma aminoterminal pro-brain natriuretic peptide: a novel approach to the diagnosis of cardiac dysfunction. J Card Fail 2000; 6: 130–139

  22. Talwar S, Squire IB, Downie PF, Mccullough AM, Campton MC, Davies JE, et al. Profile of plasma N-terminal proBNP following acute myocardial infarction; correlation with left ventricular systolic dysfunction. Eur Heart J 2000; 21: 1514–1521