The role of cardiac biomarkers for predicting left ventricular dysfunction and cardiovascular mortality in acute exacerbations of COPD

Introduction

Chronic obstructive pulmonary disease (COPD) is a major cause of chronic morbidity and mortality worldwide, and acute exacerbations of COPD (AECOPD) are the main reason for hospitalisation of patients with COPD. While most exacerbations are due to viral or bacterial infections, up to one-third have an unknown aetiology There is an abundance of literature, which implicates abnormalities of the cardiovascular system as an important factor in the prognosis of patients with COPD.1

Recent research has suggested that there may be greater overlap between the pathophysiology of the lungs and the heart than was previously appreciated. As both chronic lung disease and cardiac disease are extremely common, there is a large cohort of patients with both conditions. It was recently shown by Andell et al2 in a previous issue of Open Heart that patients with COPD, who constitute 6% of patients diagnosed with myocardial infarction (MI), had a higher mortality and a greater risk of new-onset heart failure. Several biomarkers are currently in routine clinical use for diagnosis, prediction and risk stratification in cardiac disease (table 1) The predictive value of such cardiac biomarkers in AECOPD has not yet been systematically reviewed. Both troponin and brain natriuretic peptide (BNP) are markers of myocardial stress, which can be measured easily and relatively cheaply as a bedside test.

Troponins are widely used as biomarkers to aid the diagnosis of MI and become present in the blood at higher levels when cardiac myocytes are damaged. BNP is currently used as a biomarker in cardiac failure, as it, along with other natriuretic peptides, is released in response to increased atrial pressure. It has long been known that lung pathology can directly lead to cardiac disease, such as in cor pulmonale, in which right-sided heart failure is a result of increased pulmonary arterial pressure; this increase in pressure may be caused by a panoply of diseases, including pulmonary fibrosis and COPD.3 More specifically, the autoregulation of the pulmonary circulation, such as hypoxic pulmonary vasoconstriction, becomes maladaptive when there is widespread rather than localised hypoxia.3

Epidemiological evidence suggests that left ventricular (LV) failure is a common comorbidity in patients with COPD4 and that outcomes for patients with both LV failure and COPD are worse than those for patients with only COPD. In a long-term study of patients with COPD in Nordic countries, mortality in 36% of patients with COPD was due to cardiovascular causes. In patients with COPD, there is a significant burden of morbidity and mortality related to congestive heart failure, and patients with COPD have been noted to have higher in-hospital mortality.

The aim of this systematic review was to evaluate both BNP and cardiac troponins and their potential prognostic value as markers for LV dysfunction and cardiovascular mortality in patients admitted to hospital with AECOPD.

Methods

A search of PubMed, MEDLINE and Cochrane Library up to January 2013 was performed without limitations using the search terms ‘Cardiac Biomarker COPD’, ‘Cardiac biomarkers COPD’, ‘Cardiac biomarkers acute COPD’, ‘Cardiac biomarker COPD exacerbation’, ‘Troponin COPD’ and ‘BNP COPD’. Additional literature was identified through the review of references found in the primary literature search. Identified randomised, placebo-controlled trials, meta-analyses and systematic reviews were reviewed.

A total of 219 titles and abstracts were identified which were then reviewed.

The search criteria is in the online supplementary appendix. The following predetermined criteria were used to determine which studies to evaluate further: written in English, related to AECOPD, use of BNP and/or troponins only, primary literature. On this basis, 35 full texts were obtained and reviewed. Two independent reviewers evaluated the studies against predetermined exclusion criteria. The exclusion criteria were as follows: not including mortality, admission to intensive care unit (ITU), recurrent hospitalisation or diagnosis of LV dysfunction as outcomes. After reviewing the full-text articles to assess relevance, a total of 14 were included (figure 1). The risk of bias was evaluated at the outcome level for all included studies using the QUADAS-2 tool.5 As the review includes both prognostic and diagnostic studies, we used this tool for all studies for consistency, marking fields relating to the reference standard as N/A where appropriate. Two authors used predefined parameters and a standardised collection method to extract data from the papers. One author conducted the risk of bias analysis (see online supplementary appendix 1).

Results

Conclusion

In summary, it appears that there may be a link between an elevated level of BNP or NT-proBNP and increased cardiovascular mortality in AECOPD, although the data currently available are not conclusive.

Two of the included studies looked at whether an elevation in BNP or NT-proBNP was associated with LV dysfunction in patients with COPD, and both of these found a significant association. These limited data suggest that BNP or NT-proBNP can be a useful tool in the diagnosis of LV failure concomitant with COPD.

Of the 10 included papers that measured troponins, 7 measured mortality, and in all of these, a significant association was found between elevated levels of troponins and increased mortality. Across the studies, a variety of time points were used, but these were all generally long term (more than 30 days after measurement). In addition, increased levels of troponins were predictors of readmission to hospital after discharge. A recent paper in Open Heart has suggested that not only the troponin level but also whether it rises then falls or remains elevated during an acute exacerbation may also have prognostic value.19

Three studies measured both BNP/NT-proBNP and a troponin in their study population; this provides us with information about the possible combinatorial value of these biomarkers. Interestingly, Abroug et al8 found a significant correlation between increased levels of both biomarkers. Chang et al6 did find that elevations of both biomarkers were associated with greater mortality than elevation of either biomarker alone. At present, this systematic review has identified a gap in terms of information about the combined value of these biomarkers.

Some of the limitations of this review are that the studies did not all define COPD or AECOPD in the same way. The methods used to define COPD include: spirometry; the combined use of ECG, chest X-ray, heart rate, blood pressure, respiratory rate, arterial blood gas results, PaO₂, chest pain and use of accessory muscles; agreement of two physicians on the basis of history and examination.

Another issue is the wide range of biomarkers, time points at which they were measured and end points measured, precluding meta-analysis. Accordingly, future research should aim to address which of the specific biomarkers offer the most value in the setting of AECOPD in terms of influencing clinical decision making and management.

Troponins used as biomarkers can encompass a wide range of molecules, including troponin T, troponin I and cardiac-specific isoforms of troponins. While we did not exclude studies on the basis of which type of troponin they measured, this variety precludes direct comparison of studies or pooling of data.

Given that patients with COPD commonly have cardiovascular comorbidities, the development of biomarkers that can help identify LV dysfunction could guide treatment, improving both prognosis and quality of life. BNP and cardiac troponins are currently in widespread clinical use for patients with cardiac disease, and would be a simple, cheap test to aid COPD investigation and management. They also appear to have a predictive value in terms of prognosis, and could help identify patients with AECOPD who are at greater risks of poor outcomes and allow targeted interventions to improve outcomes.

The results presented above do suggest that NT-proBNP could be a useful biomarker for the diagnosis of LV failure, and also a predictor of mortality, particularly in the short term. However, more research is needed in order to determine the clinical utility of BNP or NT-proBNP as a biomarker in AECOPD.