• COPD
• determinats
• Exercise tolerance

SUMMERY
Chronic Obstructive Pulmonary Disease (COPD) is a preventable and treatable disease characterised by airflow limitation that is not fully reversible with some significant extrapulmonary effects that may contribute to the severity in individual patients. Progressive decline in airflow is associated with an abnormal inflammatory response of the lung to noxious particles or gases (1).
Limitation in the exercise capacity is a major characteristic of COPD, particularly in moderate-severe disease. This may be expressed as dyspnoea occurring during an effort of different intensity, and as reduction in the possibility to perform a normal or almost normal active life. In more severe patients, exercise limitation is so relevant to avoid the patient to leave home and also limiting them in the normal home daily activity. Limitation in exercise capacity in COPD patients has been demonstrated to be related to different mechanisms, which may be differently represented in the same patient. These mechanisms are: a) abnormal mechanical properties of the respiratory system, leading to ventilatory limitation; b) abnormal cardiovascular response to the exercise (cardiac limitation); c) reduction in the peripheral muscle strength and endurance, leading to muscular limitation.
Cardiopulmonary exercise testing (CPET) is considered the gold standard for exercise intolerance evaluation. During the test, patients are subjected to symptom limited incremental exercise, breath-by-breath monitoring of cardiopulmonary variables (e.g. pulmonary O2 uptake, pulmonary CO2 output, minute ventilation (Ve ), heart rate (HR), assessment of perceptual responses (e.g. dyspnea, leg discomfort) and measurements such as exercise-related arterial oxygen desaturation, dynamic hyperinflation and limb-muscle strength.
Other methods for assessing exercise limitation in COPD are: endurance time , incremental shuttle walking test (ISWT), endurance shuttle walking test (ESWT).To quantify the physical activity in daily life through motion sensors like Armband SenseWear becomes important in the valuation of COPD patients.
Several studies have attempted to correlate the tolerance effort of COPD patients with various physiological parameters, clinical, metabolic and inflammatory. However, few studies have attempted to simultaneously consider many different factors that can contribute to determine the limitation exercise in these patients. This is important to determine which of several pathophysiological, biological and metabolic mechanisms are primarily responsible for limiting the effort in individual patients with COPD.
The aim of the present study was to assess in a large group of patients with COPD of different severity, the relative role of the different components of the disease which may have influence on the response to the exercise.
In this attempt, we chose the response to the CPET as the more accurate method for evaluating the pulmonary, cardiovascular and metabolic responses to the exercise, and we tried to correlated the response to CPET with several baseline findings. We included a large, although not exhaustive, number of measurements, including: a) clinical findings (rate of dyspnoea, exacerbation rate, comorbidities); b) pulmonary function measurements; c) respiratory muscle strength measurements; d) metabolic findings (BMI, FFM, blood haemoglobin); e) markers of airway and systemic inflammation (derived from induced sputum, exhaled breath or peripheral blood) or cardiac involvement (pro-BNP). We tried also, in a multivariate analysis performed of all examined patients and in subgroups of patients according to the main factor limiting exercise, to assess the main determinants, among those already mentioned, which may have influence on the exercise limitation.

Study design
The study is a cross-sectional study and took place in three visits, along a study period of 2 weeks.
We studied fifty-one outpatients with stable COPD between July 2008 and May 2010, recruited among those attending to the clinics of our Respiratory Pathophysiology Unit of the Cardio-Thoracic and Vascular Department of the University of Pisa.
Three patients didn’t perform the cardiopulmonary exercise test (CPET), two patients dropped out before performing the CPET, and one patient was defined at the examination as affected by a relevant asthma-like component of the disease. Forty-five patients were therefore included in the analysis.
The patients performed: pulmonary function test, arterial blood analysis, maximal inspiratory (MIP) and expiratory (MEP) pressures, incremental shuttle walking test, cardiopulmonary exercise test (CPET). Physical activity was measured over at least 7 consecutive days, using a multisensory Armband. Metabolic status was evaluated: body mass index (BMI), Fat Free Mass (FFM) and Fat Free Mass Index (FFMI). The Saint George’s Respiratory Questionnaire (SGRQ) was used for the assessment of the health-related quality of life. Charlson index and CIRS index (Cumulative illness rating scale ) were used as valid and reliable methods to measure comorbidity. In a sample of venous peripheral blood, the following biomarkers were measured: C-reactive protein (CRP), total and differential inflammatory cells, haemoglobin and N-terminal pro–B-type natriuretic peptide (NT-pro-BNP) concentration, as a systemic biomarker of heart failure. Sputum was induced and measured: macrophage, lymphocyte, neutrophil, eosinophil percentages were expressed as percent of total inflammatory cells, excluding squamous cells. In the sputum were measured supernatant, TNF-alfa, IL-8 and neutrophilic elastase (NE). Exhaled nitric oxide (NO) were measured and exhaled breath condensate (EBC) was collected.
Statistical analysis was performed on the data obtained in the 45 patients who performed all measurements. All statistical analyses were performed using a SPSS 16.0 statistical package.

Results
We considered as dependent variable VO2peak/kg, which may be considered the best way for expressing exercise limitation during CPET, and as independent variable all these findings which may potentially have some influence in determining the amount of the exercise limitation. In an univariate single regression analysis, V02peak/Kg was significantly correlated with MRC score, FEV1% FVC %, SVC % FEV1/SVC % IC % RV/TLC %, DLCO %.
As regards the correlation with biomarkers measured in the blood, sputum and exhaled condensate we can see that there was no significant correlations of VO2peak/kg for many biomarkers, except for some indices of systemic inflammation blood neutrophil %, serum IL-8, blood microparticles, blood haemoglobin and serum N-Pro-BNP.
In a stepwise regression analysis we observed that the only variable that explains the model was blood haemoglobin and total variance explained by this model was 31.5%.
Excluding haemoglobin aspecific parameters, the variable that explains the model is MRC score, with total variance explain of 20.5 %.
If we consider one independent variable for each clinical , functional and biological field, DLCO % is the variable that explains the model and total the variance explain was 19.6%.
Other correlations for VO2peak/kg with: ISWT, as expected, steps/day and SGRQ.
Divided the patients in flow limitation (FL) and no flow limitation(NFL), according Hyatt technique, only in the FL group the VO2peak/Kg correlated in univariate analysis with a several clinical functional and biological indices (MRC, FEV1%, IC%, DLCO%; steps/day, SGRQ tot %). In stepwise regression analysis MRC and IC% were the parameters that enter in the analysis, with cumulative R2= 0.454.
Considering the patients that had interrupted the CPET for ventilator limitation (30 patients out of 45) in univariate analysis VO2peak/Kg correlated with MRC, FEV1%, IC%, DLCO%, MIP, serum pro-BNP, TEE, ISWT, SGRQ tot %. In stepwise regression analysis were excluded ISWT and parameters of Armband; only MRC entered in the model (p= 0.021, R= 0.589) and explained the 34.7% of variance of exercise tolerance.
In the patients that interrupted the CPET for muscular limitation (15 patients), the exercise tolerance not correlated with functional parameters but several indices: age, blood neutrophils %, blood eosinophils %, sputum neutrophils %, ISWT. This patients had less compromise in FEV1% and higher MRC score than the 30 patients that stopped CPET for ventilator limitation.

Conclusion
In the present cross sectional study, we tried to examine the determinants of the exercise limitation which is a characteristic of the patients with moderate-severe COPD. We included a large, although not exhaustive, series of clinical, functional and biological measurements, in order to find which variable, among those considered, might have the predominant role in determining the limitation in exercise capacity. We chose the maximum specific oxygen consumption (VO2peak/kg), obtained during a CPET, as a good marker of the exercise limitation in our COPD patients. We found that several clinical, functional and biological variables were significantly correlated with VO2peak/kg. Many of these correlations were expected according to the pathophysiology of the disease (like MRC, IC, DLCO), but some of them were fairly unexpected (like some biomarkers). Furthermore, our study was not able to show significant correlations with other variable which may be considered as possible determinants of the exercise capacity (like respiratory muscle strength, BMI or FFM). When me included the variables which resulted significantly correlated with VO2max/kg in a multivariate step-wise regression (according to different models), we found that the man determinants were haemoglobin and, in a second position, MRC dyspnoea score and diffusing capacity. This was particularly true when only subjects with pulmonary mechanical factors which may limit exercise (like the presence of flow limitation), or with a respiratory limitation to the CPET, were considered. This suggest that the main factors limiting the peak oxygen consumption during exercise are represented by non-pulmonary mechanisms in all unselected COPD patients, and by pulmonary mechanisms in COPD patient with clear limitation to the exercise related to the pulmonary disease.

Limitation of the study
Our study has some limitation. Firstly, we have not directly measured the strength of lower limb muscle, which are main determinants of the maximum work load sustained during the CPET.
Secondly, we have no independent measure of the heart function (like ejection fraction or other indices derived from echocardiography). We know that exercise limitation is strongly related to the performance of the cardiovascular system.