Prognostic Value of the Fat-free Mass Index on a Multidimensional Scale in Chronic Obstructive Pulmonary Disease

BACKGROUND AND RATIONALE Chronic obstructive pulmonary disease (COPD) is a respiratory disease characterized by chronic airflow limitation with significant systemic effects, which is often associated with one or more comorbidities such as malnutrition with negative effects on the evolution of the disease and increased risk of mortality. To determine the prognostic severity of COPD, Celli et al. proposed in 2004 the BODE index, a multidimensional scale that integrates information on body mass index (BMI), forced expiratory volume in the first second (FEV1), dyspnea and exercise capacity, evaluated by the 6-minute walking test (6MWT). Except for BMI, to which it assigns a maximum value of 1 point, it assigns from 0 to 3 points in the rest of the items, so the highest possible score is 10. The BODE reflects the progressive modification of the disease and is useful for predicting hospitalization and the risk of death among patients with COPD, and an increase at one point is associated with an increase of 34% of all causes of mortality and 62% due to respiratory causes.

The BODE index uses BMI as a variable of the nutritional status of subjects with COPD, however, alterations in body composition may occur, in the absence of clinically significant weight loss. In patients with COPD, a lower fat-free mass index (FFMI) contributes to worsening lung function, quality of life and exercise capacity, as well as a negative impact on exacerbations and survival, suggesting that FFMI may be more accurate than BMI as a prognostic factor for mortality in these patients. In a recent study conducted by Luo Y et al., the nutritional status of patients with stable COPD was evaluated, showing that 48.5% patients had a low FFMI (≤15 kg/m2 in women and ≤16 kg/m2 in men) with a significantly greater decrease in those with more advanced disease. The FFMI (Fat free mass index) can be objectively measured by electrical bioimpedance analysis (BIA), dual-energy X-ray absorptiometry, computed tomography, ultrasonography or magnetic resonance imaging. The BIA is based on the Relationship between the electrical properties of the human body, the body composition of the different tissues and the total water content in the body. It is a simple, cheap, easy to perform and non-invasive method, based on the resistance to the passage of electric current through the body compartments and very useful for performing body composition analysis.

On the other hand, the 6MWT is a predictor of mortality in patients with COPD. Although it is a widely used test that impresses to be simple, its forecasting reliability depends on strict repeatability conditions and qualified human resources which presents obvious difficulties in people with walking disabilities and elders. To facilitate the multidimensional approach to this disease, other simpler, adequately validated and accepted clinical tools have been developed to estimate the severity of symptoms and their impact on quality of life. The COPD Assessment test (CAT) is the most widely used in clinical practice guidelines and is a questionnaire consisting of eight domains with a score of 1 to 5, which measure the health status of patients with COPD in such a way that the higher the score, the worse the patient's condition. According to the latest GOLD guide, it is an indispensable variable to classify the severity of the disease with a cut-off point in the 10 points. Its prognostic value has been widely demonstrated, and a recent study showed that patients with severe COPD and a CAT ≥ 15 have a significantly higher risk of exacerbations. Another multidimensional scale proposed in the literature is the BODEx, which also addresses the need to simplify the tests to be performed, considering the technical and administrative difficulties of the 6MWT, and proposed in 2009 by Soler-Cataluña et al., in which they found a prognostic mortality value similar to the BODE without the need to perform the 6MWT because it would be replaced by the number of serious exacerbations (those that required hospital care) in the previous year. Although the BODE index is widely used in clinical practice, it is believed that its performance can be improved. For example, with respect to its application as an indication for lung transplantation, the previous BODE has not demonstrated a prognostic value of the overall post-transplant results except in the subgroup of patients with a BODE ≥ 7. Therefore, with the hypothesis of being able to implement other items other than the scale, more representative in terms of nutritional status (FFMI instead of BMI) and simpler (the CAT instead of the 6MWT), the main objective is to compare the prognostic value of the multidimensional FODE scale (F of fat-free mass index), which replaces the BMI with the FFMI, compared to the BODE scale. As a secondary objective to analyse the variants in which the 6MWT is replaced by the CAT questionnaire.

STUDY HYPOTHESIS

• Conceptual Hypothesis On the multidimensional BODE scale, the Fat Free Mass Index is a better prognostic factor for exacerbations and mortality in COPD patients, compared to the Body Mass Index.
• Operational Hypothesis The Fat Free Mass Index better predicts exacerbations and mortality of COPD patients compared to the Body Mass Index, increasing by 10% of the area below the ROC curve of the multidimensional BODE scale.