Abstract
Background. Nephropathia epidemica (NE) is a haemorrhagic fever with renal syndrome (HFRS) caused by Puumala hantavirus (PUUV). Acute infection causes transient kidney injury, permeability disorder, and fluid retention, for example.
Methods. B-type natriuretic peptide (BNP) and N-terminal peptide (NT-proBNP) during NE were investigated; disease severity and development of clinical symptoms were considered.
Results. Mean concentrations were 80.2 pg/mL and 55.2 pg/mL for BNP, and 2362.5 pg/mL and 1057.0 pg/mL for NT-proBNP in males and females, respectively. Hospitalization was 6.3 versus 5.2 days (P = 0.01) and 5.9 versus 4.4 days (P = 0.01) for patients with elevated BNP (> 100 pg/mL) or NT-proBNP (> 300 pg/mL), respectively, compared to those with normal peptide concentrations. Weight change during hospitalization was –2.8 or –0.3 kg (P <0.05) in patients with elevated or normal BNP, respectively. Heart rate (r = –0.46, P = 0.001 and r = –0.37, P = 0.01), creatinine clearance (r = –0.46, P = 0.001 and r = –0.56, P = 0.000), blood haemoglobin concentration (r = –0.55, P = 0.000 and r = –0.52, P = 0.000), and C-reactive protein (r = –0.47, P = 0.001 and r = –0.36, P = 0.01) measured when the peptide samples were collected correlated with BNP and NT-proBNP, respectively. In addition, anterior chamber depth of eye and plasma BNP (r = –0.39, P < 0.05) displayed a correlation.
Conclusions. BNP and NT-proBNP levels are associated with severity of several clinical features of acute NE.
Key messages
Acute nephropathia epidemica (NE) affects the cardiovascular system.
BNP and NT-proBNP plasma levels are elevated in NE, most likely caused by reduced elimination due to kidney failure.
The peptide concentrations correlate with the severity of NE, and they may be involved in the development of several typical clinical features of the illness.
Introduction
Puumala hantavirus (PUUV) carried and spread by bank voles (Myodes glareolus) causes an emerging infectious disease called nephropathia epidemica (NE) (Citation1). The human PUUV infection is acquired from aerosolized excreta of the rodents. This haemorrhagic fever with renal syndrome (HFRS) has an acute onset of fever, gastrointestinal symptoms, reduced urine output, blurred vision, and occasionally haemorrhagic complications typically 2–5 weeks after the exposure. The central nervous system (CNS) may also be affected, and many patients experience headache, insomnia, vertigo, nausea, nuchal rigidity, and confusion (Citation2). The most distinctive laboratory findings of the acute illness include thrombocytopenia, signs of transient renal failure, and haematuria. Generalized tissue oedema due to increased permeability and acute renal insufficiency in many patients is also obvious.
B-type natriuretic peptide (NP) (BNP) is a vasoactive hormone with an important role in blood pressure regulation and volume homeostasis (Citation3–6). The peptide is produced in the myocardium as a 108-amino acid prohormone (proBNP) which is further cleaved in biologically active 32-amino acid C-terminal fragment (BNP) and inactive 76-amino acid N-terminal peptide (NT-proBNP). The BNP decreases systemic vascular resistance and central venous pressure and regulates natriuresis. BNP expression is stimulated by both volume overload and elevated blood pressure. BNP is actively removed from the bloodstream, although passive clearance through kidneys, for example, is possible (Citation5). NT-proBNP is cleared passively by organs with high blood flow such as the kidney.
The cardiovascular system is affected in acute NE. Acute kidney injury and tissue oedema are obvious in most patients, although only a minority of them develop significant shock and hypotension during their acute illness (Citation1). Management of the acute-phase kidney failure, tissue oedema, and permeability disorder is mainly dependent on optimal control of the fluid and electrolyte balance. Among effectively treated patients, for example, haemodialysis is rarely required. In this study we analysed biologically active BNP and the inactive NT-proBNP concentrations in the plasma of acute-phase NE patients. Our aim was to test if the peptide levels are associated with severity of the illness or severity of involvement of a specific organ in acute NE. We also considered the possibility that these peptides may be associated with development of the most typical clinical features of NE. In summary, our goal was to understand the physiology and biology of the natriuretic peptides in acute NE; understanding the potential role of these peptides may, for example, assist in guiding and optimizing fluid therapy during acute NE.
Methods
Study design
The patient population of this prospective study has been previously described in detail (Citation2). The patients were hospitalized because of acute NE in the Oulu University Hospital in Northern Finland during September 2005 to February 2008. For this study, we included 48 patients from whom acute-phase samples were available for determination of plasma BNP and NT-proBNP concentrations. The study protocol had been explained to each patient, and they had the right to participate, refuse, or withdraw from the study according to the Helsinki declaration. The Ethics Committee of the Oulu University Hospital has approved the study, and all participants have signed an informed consent form.
Virological and immunological analysis
The laboratory diagnosis of NE was based on PUUV serology that was initially analysed using a commercial enzyme-linked immunosorbent assay of IgM antibodies (Reagena Puumala IgM EIA Kit, Reagena, Toivala, Finland). In selected cases (inconclusive IgM test result, patients with a travel history to geographical locations with risk of other hantavirus species), the diagnosis was also confirmed with an indirect immunofluorescence test for PUUV-IgG which displayed a granular staining pattern in cases of typical acute infection (Citation7).
Clinical examination and laboratory data
The microbiology laboratory informed the clinician immediately whenever a positive acute-phase PUUV antibody test result was found, and the clinician contacted the newly diagnosed patient. Routine practice of all hospitalized patients included daily recordings of blood pressure, heart rate, body temperature, and body weight. Degree of acute kidney injury caused by NE was analysed: daily urine output, plasma creatinine, plasma sodium, and plasma potassium concentrations were recorded. In addition, creatinine clearance was calculated according to Cockcroft–Gault formula. Severity of NE was evaluated using the following parameters: length of hospitalization (days), lowest measured thrombocyte count, and highest plasma C-reactive protein (CRP) concentration. Degree of tissue permeability and tissue oedema was estimated: first, cerebrospinal fluid (CSF) samples were collected, and CSF/serum albumin ratio (mg/mL / g/L) was calculated; second, weight change during hospitalization was measured; third, chest radiographs were analysed. In addition, daily measurements of blood haemoglobin (Hb) concentration and blood thrombocyte as well as white cell counts were recorded. The patients were also examined by an ophthalmologist as described in detail in our recent study (Citation8).
Plasma BNP and NT-proBNP measurements
Biologically active plasma BNP concentration was determined as previously described in detail (Citation9). Plasma NT-proBNP was analysed (Vitalaboratories, Helsinki, Finland) with a commercial electrochemiluminescence immunoassay (Elecsys, Roche Diagnostics, Basel, Switzerland). The BNP and NT-proBNP concentrations were determined when the NE diagnosis was confirmed; the samples were collected 4–12 days after onset of fever.
Data analysis and statistical methods
For the data analysis, the clinical and laboratory parameters were divided into two groups: general parameters () were considered to indicate disease severity. Acute parameters () were thought to indicate the physiological condition of the patient at the time when the BNP and NT-proBNP samples were collected.
Table I. A: Summary of selected general parameters which may indicate the severity of the acute nephropathia epidemica. Mean age, gender distribution, duration of hospitalization, maximum plasma creatinine, minimum creatinine clearance, minimum thrombocyte count, maximum C-reactive protein, cerebrospinal fluid (CSF)/serum albumin ratio, weight change during the period of hospitalization, and presence of pleural effusion in chest radiograph are shown. These clinical and laboratory parameters are divided according to plasma BNP and NT-proBNP (pg/mL) normal values. The statistical significance is shown. B: Summary of acute clinical and laboratory parameters which were measured when the BNP and NT-proBNP samples were collected. Systolic blood pressure, heart rate, urine output during the day the NP samples were collected, plasma creatinine, creatinine clearance, plasma sodium, plasma potassium, thrombocyte count, blood haemoglobin, haematocrit, and C-reactive protein. Correlation with the plasma BNP and NT-proBNP levels and the statistical significance are shown.
Multivariate analysis of the results was not possible due to the low number of patients. Instead, Spearman bivariate correlation test, Mann–Whitney U test, and chi-square test were used to analyse associations between the parameters. A P value of < 0.05 was considered significant. In case of ophthalmological parameters, the results measured from the right eye of each patient were used. PASW Statistics 18 (SPSS, Inc., Chicago, IL, USA) was used for statistical analysis.
Results
Properties of the patients
A total of 48 patients were included to the study. shows a summary of general clinical and laboratory parameters which are thought to indicate the severity of the NE. indicates the acute clinical and laboratory condition of the patient when the samples were collected. Only a small number of the patients were hypotonic, and none suffered with tachycardia () despite their acute-phase NE. The samples were collected, and the patients were examined immediately after confirmation of the diagnosis; many were in the oliguric phase, although some patients were in the polyuric phase of their illness already (). We found a correlation between plasma sodium concentration and creatinine clearance (r = 0.47, P < 0.000) () due to dilution and excess of body fluids. Thrombocytopenia was observed in 45 patients (94%), and the duration of hospitalization was affected by the severity of thrombocytopenia (r = –0.41, P = 0.01) (). None of the patients required haemodialysis treatment.
Figure 1. A: Summary of the patients’ heart rate (beats per minute) and systolic blood pressure measured when the NP samples were collected (BP, mmHg) is shown (no statistical correlation). B: The systolic blood pressure measured when the NP samples were taken displayed a correlation (r = 0.38, P < 0.01) with amount of urine collected over a 24-hour period (mL). C: Patients with low creatinine clearance (mL/min) were hyponatraemic (r = 0.47, P <0.000). D: The correlation (r = –0.41, P = 0.01) between the lowest measurement of blood thrombocyte count (× 109/L) and the duration of hospitalization is shown. Solid symbol: male; open circle: female.
Plasma BNP and NT-BNP levels
The mean plasma BNP concentrations were 80.2 pg/mL (n = 38) and 55.2 pg/mL (n = 10) in male and female patients, respectively. The values were more often above normal level (> 100 pg/mL) in male patients (11 patients, 28.9%) compared to females (1 patient, 10%, P = 0.001). The mean plasma NT-proBNP concentrations were 2362.5 pg/mL (n = 38) in male and 1057.0 pg/mL (n = 10) in female patients; elevated values (> 300 pg/mL) were observed in 29 male (76.3%) and 5 female (50%) patients (P < 0.01). BNP levels were found to be above normal level (>100 pg/mL) in patients from whom the samples were collected on days 7 to 10 from the onset of fever. Elevated NT-proBNP values (> 300 pg/mL) were observed throughout the study period. There was a correlation between the plasma NT-proBNP and BNP levels (r = 0.47, P <0.001). An inverse correlation between the age of a patient and plasma NT-proBNP concentration was observed (r = –0.29, P < 0.05).
Disease severity and inflammation
Selected general parameters of disease severity are shown in . Length of hospitalization can be regarded as a marker of disease severity. Plasma BNP and NT-proBNP concentrations had a positive correlation with the length of hospitalization (r = 0.481, P = 0.001 and r = 0.408, P < 0.001, respectively). We also found that the patients with the most severely affected kidneys exhibited high peptide values, and pleural effusion was associated with elevated NT-proBNP concentrations (). In addition, large weight change during hospitalization occurred in patients with high BNP levels. We also evaluated the association of the plasma CRP concentration as a marker of degree of inflammation; we observed an inverse correlation between the plasma CRP and the BNP or the NT-proBNP levels (). In male patients, the blood thrombocyte count correlated with the plasma NT-proBNP (r = 0.32, P <0.05).
Haemodynamic parameters
Systolic blood pressure measured when the blood samples were collected correlated with plasma BNP level (r = 0.30, P < 0.05). In addition, heart rate measured when the blood samples were collected and plasma BNP or NT-proBNP concentration displayed an inverse correlation (). Urine output and the systolic blood pressure level presented a correlation (r = 0.38, P < 0.01)
Tissue permeability and oedema
Weight change during the hospitalization may reflect the degree of tissue oedema and degree of permeability disorder in each patient. There was a correlation between the plasma NT-proBNP level and the weight change (r = –0.41, P = 0.01). In addition, patients in whom the plasma BNP was above normal level (> 100 pg/mL) lost a greater amount of their body weight (mean –2.8 kg) compared to those with normal plasma BNP level (mean –0.3 kg) (P < 0.05) (). Acute-phase chest radiographs were also available for analysis from 44 patients. The radiograph was considered normal in 23 patients, 10 radiographs indicated pleural effusion, and 11 patients had any abnormality without pleural effusion. The mean BNP and NT-proBNP levels were 74.7 versus 69.6 pg/mL (n.s.) and 2070.0 versus 1915.7 pg/ml (n.s.) in patients with normal chest radiograph compared to those with pleural effusion, respectively. However, in patients with pleural effusion the NT-proBNP was above normal level (>300 pg/mL) more commonly compared to those without pleural effusion (). We also found that the patients with pleural effusion (mean age 52.3 years, n = 10) were older (P < 0.05) than those with normal chest radiograph (mean age 43.1 years, n = 23) and the duration of hospitalization was longer, 6.7 days versus 5.0 days (P < 0.05), among patients with effusion. All other clinical and laboratory parameters were similar regardless of the chest radiograph finding. Another marker of tissue permeability, the CSF/serum albumin concentration ratio, was also determined. We found no association between the peptide values and the CSF/serum albumin ratio (). However, we found that patients with a high CSF/serum albumin ratio also had a high peak plasma creatinine level (r = 0.686, P = 0.001).
Kidney function and electrolyte balance
Acute-phase NE patients in general have a low urine output and they develop tissue oedema. In our study, patients with low urine output also had a high plasma creatinine level (r = –0.30, P < 0.05), they had low systolic blood pressure (r = 0.35, P < 0.01), and their plasma sodium concentration was low (r = 0.495, P < 0.001). Patients in the polyuric phase (daily urine output > 2500 mL) had higher plasma sodium level (137 mmol/L) compared to those with low or normal urine output (< 2500 mL) (133 mmol/L, P < 0.01). Both plasma BNP (r = –0.45, P < 0.01) and NT-proBNP (r = –0.51, P < 0.000) had an inverse correlation with creatinine clearance (). Plasma potassium levels were surprisingly constant regardless of the kidney function (data not shown).
Figure 2. Plasma BNP level (A) (pg/mL) (r = –0.45, P < 0.01) and plasma NT-proBNP (B) (pg/mL) (r = –0.51, P < 0.000) had an inverse correlation with creatinine clearance (mL/min). Solid symbol: male; open circle: female.
Ocular findings
The patients were carefully evaluated for their ophthalmological properties during acute NE, and they were also examined one month later as previously described (Citation2,Citation8). The acute-phase anterior chamber depth displayed a negative correlation with the plasma BNP concentration (r = –0.39, P <0.05). In addition, the NE patients were found to have a significant difference in the depth of their anterior chamber when they were first analysed during their acute NE and then re-examined one month later (Citation8). The difference between the acute and control measurements correlated with the acute-phase plasma sodium concentration (r = –0.42, P < 0.01); those patients with low acute-phase plasma sodium level had the greatest difference in their anterior chamber depth between the acute and control measurements. Other ocular parameters, including the intraocular pressure, were not dependent on the plasma natriuretic peptide concentrations or plasma electrolyte levels.
Discussion
Kidney failure, fluid retention, and tissue oedema may potentially affect the heart and the circulation during acute NE (Citation1). Previous reports have revealed cardiac involvement in NE; electrocardiography disorders, mild left ventricular dysfunction, and myocarditis have been observed (Citation10,Citation11). It is also possible that cardiovascular mortality during or soon after the acute NE may be increased (Citation12). In general, elevated BNP levels may indicate increased mortality in coronary heart disease, bacterial sepsis, and acute or chronic kidney dysfunction (Citation6,Citation13–15). In our study, none of the patients died because of NE. However, the elevated peptide levels were associated with a greater amount of tissue oedema and longer duration of hospitalization, for example. The patients with high BNP values also suffered from severe kidney dysfunction. Based on these observations, it can be concluded that the plasma BNP or NT-proBNP concentration may be a marker of disease severity also in acute hantavirus infection.
Most of our acute-phase NE patients suffered from hyponatraemia due to fluid retention and dilution. Patients with low plasma sodium level had a high plasma NT-proBNP, low creatinine clearance, high creatinine concentration, and they also suffered from low urine output. On the other hand, patients in the polyuric phase of the illness had normal plasma sodium levels. Those with an elevated BNP level also had higher blood pressure compared to patients with low plasma BNP. None of the patients, however, suffered from significant hypotonia or tachycardia. The acute-phase febrile NE patients with elevated plasma BNP or NT-proBNP levels actually had lower heart rate and higher blood pressure compared to those with low peptide concentrations. These observations may indicate that elevated BNP levels in acute NE can actually be associated with the beneficial actions of the natriuretic peptides; they have been shown to exhibit, for example, protective effects against fluid volume overload (Citation6). We also saw low plasma CRP concentrations in patients with high plasma BNP levels. BNP has been shown to participate in the regulation of inflammatory reactions, and we can only speculate that our observation may be associated with the previously suggested anti-inflammatory properties of the BNP (Citation16,Citation17).
BNP is actively cleared (half-life 22 minutes) by receptor-mediated binding, although the peptide is also degraded by the activity of neutral endopeptidases in the bloodstream. To some extent, passive clearance through kidneys, for example, is also possible (Citation5). NT-proBNP is cleared passively (half-life 120 minutes) by organs with high blood flow such as the kidney (Citation5). In our study, the plasma NT-proBNP levels were elevated in nearly all patients, and the NT-proBNP concentration was highly dependent on the degree of severity of the kidney failure. The plasma BNP levels were also dependent on the kidney function. However, the plasma NT-proBNP values were considerably higher compared to those of BNP which was elevated in patients with the most severe kidney failure only. Based on these results, it seems probable that the reduced elimination of the NT-proBNP due to kidney failure mostly explains the high peptide values seen in our patients. It is also possible that increased BNP/NT-proBNP production may partly explain the elevated plasma levels. This is supported by the fact that many of our patients had fluid retention due to kidney failure. In addition, we observed an association between the degree of tissue oedema and the plasma BNP/NT-proBNP concentrations. However, the patients with elevated BNP and NT-proBNP values were haemodynamically stable, and they did not suffer from typical symptoms of cardiac dysfunction. It seems possible that the increased tissue permeability and tissue oedema, rather than increased blood volume overload, dominates in most acute NE patients. Further conclusions on the association between the cardiac properties and the peptide levels are not warranted; a major limitation of our study is the lack of acute-phase echocardiography analysis.
Reduced vision in acute NE may be explained at least partly by shallowing of the anterior chamber and alterations in the lens of the eye (Citation8,Citation18). Reduction in the intraocular pressure during acute NE has also been observed. We noticed an association between the plasma sodium concentration and the anterior chamber depth in our patients. In addition, acute-phase anterior chamber depth was also affected by the plasma BNP level. Our results indicate that systemic disturbances in electrolyte balance may contribute to these most typical ocular disturbances seen during acute NE. The results are in agreement with previous observations that suggest a role for systemic electrolyte balance and kidney function as regulators of intraocular conditions (Citation19,Citation20).
We conclude that acute Puumala hantavirus infection activates compensatory cardiovascular mechanisms. Disturbed vascular permeability, tissue oedema, acute kidney failure, and disturbed electrolyte balance may all be associated with the increased natriuretic peptide values seen in our patients. The degree of fluid and electrolyte imbalance in NE can be affected by rehydration, and appropriate fluid therapy may lead to, for example, avoidance of unnecessary haemodialysis. We suggest that the role of BNP and NT-proBNP determinations should be investigated for their potential to guide and optimize the fluid therapy in acute NE.
Declaration of interest: The authors report no conflicts of interest.
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