ABSTRACT: The Studies of Left Ventricular Dysfunction (SOLVD) trials demonstrated that early intervention in congestive heart failure (CHF) improves survival. However, early CHF is mainly a clinical diagnosis based on New York Heart Association criteria and, until recently, no easy and inexpensive screening test existed. There are now several such tests that employ radioimmunoassays (RIAs) to measure cardiac peptides in a single plasma sample; results help determine the likelihood that CHF is present but do not definitively establish the diagnosis. The vessel dilator RIA is the most specific and sensitive for differentiating persons with mild CHF from healthy ones; intravenous administration of this cardiac peptide hormone has beneficial hemodynamic, diuretic, and natriuretic properties in persons who have CHF. Brain natriuretic peptide (BNP) measured by fluorescence immunoassay is useful in the emergency department, because a result may be obtained in as little as 15 minutes. This assay may indicate CHF; further tests are recommended to define the diagnosis. BNP increases with other causes of dyspnea, including pulmonary hypertension, pulmonary emboli, and renal failure, so it is not specific for CHF. BNP also increases with age, and measured values are higher in women than in men.
The diagnosis of mild congestive heart failure (CHF) is difficult. It is mainly a clinical diagnosis based on assessment of a patient's symptoms according to the New York Heart Association (NYHA) classification of CHF, which grades the disease from class I (mild) to class IV (severe).
The NYHA criteria for a class I diagnosis are: "No limitation of physical activity. Ordinary physical activity does not cause undue fatigue, palpitations, dyspnea, or anginal pain." Using these criteria, it is often difficult to distinguish persons with class I CHF from healthy ones. Remes and colleagues1 showed that only 25% to 50% of patients thought to have heart failure by their primary care physicians actually did when examined with invasive cardiac tests. Early, accurate diagnosis of heart failure has become more important than ever, given the results of the Studies of Left Ventricular Dysfunction (SOLVD) trials, which indicated that early intervention improves survival.2,3
Until recently, there have been no easy, inexpensive screening tests for early, mild CHF. Now, however, there are several such tests that employ radioimmunoassays (RIAs) to measure cardiac peptides in a single plasma sample from a patient being screened for this disease. The results of these RIAs are used to determine the likelihood that CHF is present. Some of these tests are employed by primary care physicians in Europe, but their use has not yet become routine in clinical laboratories in this country. One of these peptides, brain natriuretic peptide (BNP)-measured by the Biosite fluorescence immunoassay rather than by RIA-is now processed in a number of clinical laboratories in this country.
In this article, I review the efficacy of each of these tests in the screening of patients who may have CHF-and the therapeutic potential of new peptides made in the heart.
THE HEART AS AN ENDOCRINE ORGAN
The heart is now known to synthesize a family of peptide hormones.4,5 These hormones, synthesized by cardiac myocytes in 3 separate genes, are referred to collectively as atrial natriuretic peptides, because their main site of synthesis in healthy adults is the atria. Most of these peptides have natriuretic properties.5,6 Each of the peptides also has blood pressure-lowering, diuretic, and/or kaliuretic properties.7,8 The physiologic effects of these peptides are illustrated in Figure 1.
These peptide hormones are stored as 3 prohormones for immediate release in response to stimuli. The prohormones are 126 amino acid (aa) atrial natriuretic peptide (ANP), 108 aa BNP, and 126 aa C-natriuretic peptide (CNP) prohormones. The atrial natriuretic peptides that originate from the ANP prohormone are the best characterized: only the C-terminal ends of the BNP and CNP prohormones have been investigated for biologic activity.9,10
Four peptide hormones originate from the ANP prohormone (Figure 2). These hormones, numbered by their aa sequences beginning at the N-terminal end of the ANP prohormone, consist of the first 30 aa's of the prohormone (proANP aa 1-30; long-acting natriuretic peptide); aa 31 through 67 (proANP 31-67; vessel dilator); aa 79 through 98 (proANP 79-98; kaliuretic peptide); and aa 99 through 126 (ANP).
The natriuretic effects of long-acting natriuretic peptide and vessel dilator have a mechanism of action different from that of ANP in that they enhance the synthesis of prostaglandin E2 and inhibit renal Na1-K1-ATPase.11,12 The diuretic and natriuretic effects of ANP, on the other hand, are thought to be mediated by cyclic 3′,5′guanosine monophosphate (cyclic GMP).4
BNP, so named because it was first isolated from porcine brain,9 is 10 times more abundant in the heart than in the brain.13 The aa sequence of BNP is remarkably similar to that of ANP: only 4 aa's are different in the 17-aa ring structure common to both peptides.9 BNP and an N-terminal proBNP peptide (aa 1-77) both circulate in humans.14
CNP, the most recently discovered member of the family of atrial natriuretic peptides, was also originally found in the brain.10 There is evidence to suggest that it is also present in the heart.15 CNP has a ring structure and aa sequences nearly identical to those of ANP and BNP, but it differs because it has no carboxy terminal tail.10 Because of its ring structure, CNP can bind to the same receptors as ANP and BNP, although each has different affinities.
CIRCULATING CONCENTRATIONS OF ATRIAL PEPTIDES IN SICKNESS AND IN HEALTH
Vessel dilator and long-acting natriuretic peptides circulate at concentrations 10 to 24 times higher than ANP in healthy humans (Table).4,16 Kaliuretic peptide circulates at a level 3 times higher than ANP.7 In addition, the half-lives of the peptide hormones originating from the N-terminus of the ANP prohormone are at least 15-fold longer than ANP.16 BNP, the N-terminus of the BNP prohormone, and CNP circulate at very low levels, constituting less than 3% of the circulating atrial natriuretic peptides in healthy persons.14,15,17 Together, ANP, BNP, N-terminus of the BNP prohormone, and CNP constitute less than 5% of the circulating peptides.4,16 Long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide constitute more than 95% of the known circulating atrial natriuretic peptides.4,16,17
In persons with CHF, the release of cardiac peptide hormones is stimulated by a number of physiologic and hormonal factors, including stretching of the atria and ventricular hypertrophy. The clue to early CHF resides in an increase of these peptides above recognized normal levels.
SCREENING FOR CHF
The first RIA found to have the ability to detect early asymptomatic or minimally symptomatic CHF by measurement of a plasma marker was the vessel dilator RIA.16 In the investigation reported by Winters and colleagues,16 the RIA differentiated 30 patients with CHF from 54 healthy controls. Ten different peptide hormones have been evaluated simultaneously from the same plasma samples from persons with NYHA classes I through IV CHF and compared with those from healthy persons to determine which ones might yield the best assay for early heart failure.15
Three of the N-terminal proANP assays best detected early heart failure, although with varying degrees of success: ProANPs 31-67 (vessel dilator), 1-30 (long-acting natriuretic peptide), and 79-98 (kaliuretic peptide) RIAs exhibited 100% (P = .01), 83% (P = .09), and 50% (P = .74) sensitivity, respectively, in differentiating patients with class I CHF from healthy controls.17 Moreover, vessel dilator RIA detects vessel dilator itself in plasma and not the whole N-terminus of the prohormone, as do the other assays devised to detect other portions of the N-terminus of the ANP prohormone. Assays of ANP, BNP, N-terminal proBNP, CNP, adrenal medullin, neuropeptide Y, and endothelin could not differentiate patients with mild CHF from healthy persons. The levels of all of these peptides (except adrenomedullin), however, were increased in the circulation of persons with severe heart failure.17
Logistic regression analysis revealed that only vessel dilator RIA significantly (P = .0001) discriminated between patients with early CHF (5226 ± 377 pg/mL) and healthy persons (1595 ± 157 pg/mL).17 Thus, when the peptide markers are compared head-to-head by RIA, data indicate that vessel dilator is the most sensitive marker in detecting NYHA class I CHF. The peptides measured in these assays were also found to be independent markers of CHF with respect to left ventricular ejection fraction.
Vessel dilator correlates with increasing severity of CHF better than ejection fraction does, and thus the vessel dilator RIA provides valuable diagnostic information to augment traditional measurements.16,17 In the Survival and Ventricular Enlargement (SAVE) trial, similar data showed that N-terminal proANP-but not ANP or other neurohormones-was an independent predictor of CHF and cardiovascular mortality when ejection fraction was included in the statistical model.18
Measurement of BNP by the Biosite fluorescence immunoassay is being evaluated by a number of clinical laboratories.19-26 The advantage of a fluorescence assay is that the results are available in a shorter time (approximately 15 minutes) than the results of an RIA, which allows for clinical decisions to be made more quickly in the emergency department (ED). The fluorescence assay was described in 1 study as a tool to determine whether dyspnea is of pulmonary or cardiac origin.19 However, the false-positive rate for a diagnosis of CHF in this study was nearly 40%.20 Moreover, BNP is elevated not only in CHF but also in pulmonary conditions such as primary pulmonary hypertension21 and pulmonary emboli.22 The Biosite assay is not specific to any disease state. The levels of BNP are actually higher in persons with renal failure than in persons with CHF.
The authors of the BNP fluorescence immunoassay study suggest that the cutoff for a diagnosis of CHF should be no greater than 100 pg/mL.19 In their laboratory, however, patients with lung cancer averaged 120 pg/mL and those with acute pulmonary embolism, 207 pg/mL.22 Unlike some of the other natriuretic peptides, BNP appears associated mainly with right ventricular dysfunction in both pulmonary disease (other than cancer) and CHF.21,23
BNP levels increase with age,24-26 so that differentiating a person with class I CHF from a healthy person by using the fluorescence immunoassay may be difficult. Furthermore, there is considerable overlap in BNP levels in persons with CHF who have preserved systolic function and persons without CHF.27 Because BNP levels are also affected by comorbidity and drug therapy and are higher in women, the plasma BNP measurement should not be used in isolation from the clinical context.24-26
BNP is the only natriuretic peptide marker now measured by fluorescence immunoassay. In spite of its limitations, it appears helpful as a "rule-out" test in the ED in patients with dyspnea and suspected new CHF; however, it cannot replace echocardiography or a full cardiology assessment.26 Because vessel dilator is much more effective than BNP by RIA at differentiating early CHF,17 one would expect that when vessel dilator can be measured by a fluorescence immunoassay, it will be more helpful than BNP in the diagnosis of CHF in the emergent setting.
USING CARDIAC PEPTIDES AS MARKERS
Plasma markers useful for screening should be collected and immediately placed on ice, centrifuged, and stored at −80°C before assay. Blood samples of most of the natriuretic peptides intended for measurement remain stable at room temperature for several hours. Samples have been stored for 7 years or more at −80°C, with nearly identical results when examined by the vessel dilator assay 7 years apart.28 Thus, blood samples can be drawn by primary care physicians in even the most rural communities and sent to hospital clinical laboratories to help discern which patients may need further invasive cardiac evaluation.
Although these cardiac markers are very good for screening for CHF, they do not definitively establish the diagnosis. Cardiac peptides are elevated in the blood of patients with both diastolic and systolic dysfunction. An echocardiogram may therefore be appropriate after an elevated level of a plasma marker has been observed. At this point, it is best to refer the patient to a cardiologist for further evaluation.
In reading the literature on CHF and cardiac markers, you will often see the term "N-terminal proANP." Since there are 3 N-terminal proANPassays (vessel dilator, long-acting natriuretic peptide, and kaliuretic peptide), and since the 3 are unequal in their value as screening tools for CHF, you must know which specific assay the authors mean when they simply refer to "N-terminal proANP" assay.
On the basis of the evidence, I recommend using the vessel dilator RIA alone for screening persons with symptoms of CHF. I would reserve the long-acting natriuretic peptide RIA (second best at differentiating patients with early CHF from healthy persons when all of the natriuretic peptides were evaluated17) to help resolve ambiguous results obtained with the vessel dilator assay. If, on the other hand, the measured result of the vessel dilator RIA is markedly increased, it is not necessary to perform a second RIA to confirm the result; most laboratories perform RIAs in duplicate or triplicate to make sure they are correct.
TREATMENT WITH CARDIAC PEPTIDES
High-dose administration of ANP produces little or no diuresis or natriuresis in persons with CHF.29 Some investigators have found that BNP infused for 90 minutes into patients with CHF leads to a very small increase in urinary volume (90 ± 38 vs 67 ± 27 mL/h) and a small increase in urinary sodium concentration (2.6 ± 2.4 vs 1.4 ± 1.2 mEq/h).30 Others have found that BNP causes no diuresis in these patients.31
The results of these studies have provided evidence for the use of Nesteride (synthetic BNP) for the treatment of acute CHF. Nesteride is given for 8 hours intravenously with the patient in bed. This agent has a very short half-life (several minutes) and is not useful for chronic CHF, because it would have to be given up to 10 times per hour subcutaneously or intravenously on an outpatient basis.
In contrast, when vessel dilator peptide was given to patients with class III CHF intravenously for 60 minutes in one study, it increased urinary flow 2- to 13-fold and sustained that increase for 3 hours after the infusion was stopped (P < .01) (Figure 3).32 At the end of the 60-minute infusion, mean urinary flow was 7.55 ± 0.75 mL/min; it was 6.7 ± 0.65 mL/min 3 hours after the infusion was stopped. These values are 4.8 and 4.3 times higher, respectively, than the baseline urinary flow of the patients with CHF (1.56 ± 0.35 mL/min).One patient who was studied for a longer period exhibited a 2-fold increase in urinary volume and flow for 6 hours after vessel dilator infusion was stopped.
Vessel dilator also enhanced sodium excretion 3- to 4-fold (P < .01). It caused a doubling of sodium excretion within 20 minutes; and 3 hours after the vessel dilator infusion was stopped, sodium excretion was still 3-fold greater than baseline sodium excretion (P < .01).32
Vessel dilator increased fractional excretion of sodium (FENa) in patients with CHF up to a maximum of 6-fold (P < .001).32 Thus, the FENa doubled 20 minutes after vessel dilator infusion began, and it was 4.5-fold greater than baseline at the end of the infusion.32 During the infusion, systemic vascular resistance decreased 24% and pulmonary vascular resistance decreased 25% (Figure 4).32 Pulmonary capillary wedge pressure decreased 33% and central venous pressure decreased 27%. Heart rate and mean pulmonary artery pressure did not change significantly; however, vessel dilator increased cardiac output 34% (5.35 ± 0.9 to 7.9 ± 1.2 L/min) and cardiac index 35% (2.66 ± 0.01 to 3.58 ± 0.01 L · min−1 · m−2).32 It also increased stroke volume index by 24%.There were no adverse effects associated with the use of vessel dilator.
Thus, vessel dilator decreases systemic vascular resistance and increases the cardiac index. These results are beneficial for persons with CHF, whose baseline state is characterized by increased systemic resistance and a reduced cardiac index.
The decrease in systemic vascular resistance secondary to arterial vasodilation also decreases the back-pressure on the heart, which results in decreased left atrial pressure. Systemic blood pressure and systemic vascular resistance decrease following treatment with vessel dilator, which suggests improvement in both afterload and preload. In patients with CHF, when ventricular function is on the steep portion of the pressure-volume curve, preload reduction may decrease ventricular wall stress, resulting in improved cardiac output. This was found to be the case with vessel dilator administration.32
Vessel dilator is now in phase 2 trials to ascertain safety and to determine the optimal concentration of this peptide hormone for the treatment of CHF. If safety and efficacy are confirmed in phase 2 and 3 trials, vessel dilator should be available in 3 to 5 years to optimize the treatment of CHF. n
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