The utility of maximal oxygen uptake testing as cardiovascular disease risk marker in female patients with rheumatoid arthritis without associated lung disease

Aim The aim of this study was to evaluate maximal oxygen uptake (VO2 max) as a marker of cardiovascular disease (CVD) in rheumatoid arthritis (RA) and its relation to the CVD risk factors in a cohort of female patients with RA without associated lung disease. Patients and methods A total of 132 female patients with RA were assessed for cardiopulmonary fitness with a VO2 max testing. Moreover, 100 healthy female individuals were recruited as control group. Exclusion of patients with pulmonary fibrosis/nodules by using high-resolution computed tomography was done. Traditional CVD risk factors and disease characteristics and their correlation with VO2 max level were assessed in all patients. Results Based on VO2 max mean, patients were classified into three groups: unfit (<16.72 ml/kg/min), fairly fit (16.73–25.6 ml/kg/min), and with average fitness (>25.6 ml/kg/min). Patients had significantly worse VO2 max mean (21.28±6.96 ml/kg/min) compared with control (30.88±7.36 ml/kg/min). Patients with poor VO2 max level were more likely to be older, hypertensive, with family history of CVD, with high BMI, and with high mean of Framingham risk score. Significant differences were detected between the fitness subgroups in mean of carotid intima–media thickness and presence of carotid plaques. Long duration of RA, uncontrolled disease activity, high health assessment questionnaire, high C-reactive protein, and positive anticyclic citrullinated protein antibodies were correlated significantly with reduced VO2 max level. Conclusion VO2 max test can be used as a surrogate CVD marker in patients with RA. VO2 max can be used as a noninvasive test to detect and quantify fitness defects in patients with RA at increased risk of CVD.


Introduction
Rheumatoid arthritis (RA) affects ∼1% of the general population and is associated with mortality rates ranging from 1.28 to 3 [1]. The increased mortality in RA is explained by accelerated coronary artery and cerebrovascular atherosclerosis besides other cardiovascular (CV) complications [2].
The morbidity and mortality attributed to CV risk are increased in RA. The traditional risk factors including smoking, hypertension, dyslipidemia, insulin resistance (IR), diabetes mellitus, physical inactivity, and obesity do not explain the increased CV risk in RA [3]. Inflammation is the important link between RA and cardiovascular disease (CVD), as it plays a key role in atherosclerosis [3]. RA is an autoimmune systemic disease; ∼40% of patients have extra-articular manifestations during the course of the disease [4]. Pulmonary complications are directly associated with RA including, pulmonary nodules, interstitial lung disease (ILD), pleural disease, and airway disease [5]. The extra-articular pulmonary complications of RA were evaluated using open lung biopsy or highresolution computed tomography (HRCT), which demonstrated ILD in ∼40% of these patients [6]. The clinically evident RA-ILD occurs in ∼10% of patients with RA in addition to considerable amount of subclinical disease [7]. The findings of HRCT in RA are distinct [7] and generally correlate with the histological findings.
The gold standard method for assessment of cardiorespiratory competence is the maximal oxygen uptake (VO 2 max) examination [8]. The level of VO 2 max is significantly low in patients with RA compared with the healthy individuals [9]. Nevertheless, studies cannot determine if the low level of VO 2 max is owing to the effect of RA itself, to CVD, or to the pulmonary manifestations.

Objective
The objective of this study was to evaluate VO 2 max as a marker of CVD in RA and its relation with CVD risk factors in a cohort of female patients with RA without associated lung disease using appropriate power calculations with control of several potential confounders such as sex and associated RA lung disease.

Patients and methods
Recruitment of patients with RA cohort began in 2016; 132 female patients with RA from Rheumatology Outpatient Clinic and Rheumatology In-Patient Department of Alnoor Specialized Hospital, a Tertiary Care Teaching Institutes in Makkah, Saudi Arabia, were enrolled in this prospective study. Moreover, 100 healthy female participants of matched age were recruited as a control group. Written informed consent was taken from patients with RA and volunteers after providing them with detailed verbal and written description.
Inclusion criteria were fulfillment of RA criteria according to the new American College of Rheumatology/European League against Rheumatism panel [10] and absence of any pulmonary manifestations associated with RA approved by HRCT and pulmonary function tests.
Exclusion criteria were recently diagnosed valve disease, cardiomyopathies and arrhythmias, renal disease on hemodialysis, amputation, cerebrovascular disease, known malignancy, current infection, pregnancy, recent joint surgery, and functional comorbidities incompatible with VO 2 max testing.
The participants underwent the following: Demographic data, full medical history (especially diabetes mellitus history, medication history and comorbidities), clinical examination (especially assessment of blood pressure and BMI), and Framingham risk score (FRS) were calculated to assess the 10-year risk of fatal CVD [11]. The 28-joint Disease Activity Score (DAS28) with its components [12] was recorded for each participant.
After fasting for 12 h, the following laboratory investigations were carried out for all the patients: complete blood count for hemoglobin and white cell count, fasting venous plasma glucose, fasting serum insulin level (ELISA method), serum lipids profile, rheumatoid factor, and anticyclic citrullinated protein antibody (ACCP). The homeostasis model assessment was used to evaluate IR (calculator available from http://www.OCDEM.ox.ac.uk), which is based on fasting plasma glucose and serum insulin concentrations [13]. The atherogenic index (AI), which corresponds to the ratio of total cholesterol (TC) to high-density lipoprotein (HDL), was calculated.
High-resolution computed tomography analysis HRCT (Aquilion 16; Toshiba Medical Systems, Otawara, Tochigi Perfecture, Japan) of the chest without contrast was carried out during breathholding and inspiration. Computed tomography images with low dose and thin section were obtained while the patient was lying in the supine position. The images were reconstructed at a 1.25-mm section thickness at 10-mm intervals with the use of a high-spatial-frequency (bone) algorithm. Images were reviewed on a Picture Archiving and Communication System screen by an experienced chest physician. Patients with abnormal HRCT findings were excluded from this study.

Carotid ultrasonography
Bilateral B-mode ultrasound examination of the carotid arteries was performed using a 12-MHz linear matrix array transducer (Mylab 70; Esaote, Genoa, Italy). Intima-media thickness (IMT) measurements were performed bilaterally in the distant wall of the common carotid artery, from ∼10 mm proximal to the start of the carotid bulb [14]. Carotid artery plaques were identified bilaterally as recommended in the Mannheim consensus [15].

Maximal oxygen uptake analyses
All participants were invited to undergo an individualized VO 2 max test protocol with electrocardiography, after taking into consideration their physical abilities and the American Heart Association guidelines [16] and specific contraindications to terminate the test [17]. The VO 2 max test was performed on a treadmill using a titrated breath-by-breath system (Metalyzer 3B; Cortex, Leipzig, Germany). A prediction equation including age, self-selected walking speed (km/h), and work heart rate was used to calculate the outcome of the treadmill test for each participant in ml/kg/min [18]. Based on the results of the VO 2 max test, patients with RA were divided into three groups: unfit, fairly fit, and with average fitness.

Statistical analysis
The collected data were tabulated and statistically analyzed using statistical package for the social sciences (SPSS) version 17.0 (Software by IBM corporation, South Africa). Quantitative variables were described as mean and SD. The unpaired t-test was used to compare two groups regarding quantitative variables. The χ 2 -test was used to compare qualitative variables between groups. One-way analysis of variance test was used to compare more than two groups regarding quantitative variables. Spearman's correlation test was used to rank different variables against each other positively or inversely. P value of up to 0.05 was considered statistically significant, and P value of up to 0.001 was considered highly statistically significant.

Results
In total, 232 participants (132 female patients with RA and 100 matched female control participants) were included. Table 1 shows that patients and control participants did not differ regarding age and CV risk factor. However, patients scored significantly worse on VO 2 max mean (21.28±6.96 ml/kg/min), with P value of less than 0.001.

Cardiovascular disease risk factors
As shown in Table 2, individuals with unfit VO 2 max were more likely be older, with hypertension, with family history of CVD, with high BMI, and with high mean of FRS (10-year CVD risk). However, there was no clear association between VO 2 max and other variables such as smoking, DM, IR, lipid profile, WBC count, and hemoglobin level. Significant differences were detected between the fitness subgroups regarding carotid IMT and presence of carotid plaques ( Table 2, P<0.001).
Association between maximal oxygen uptake and clinical characteristics of rheumatoid arthritis As illustrated in Table 2, most unfit and fairly fit VO 2 max patients had significantly longer duration of RA, active disease (DAS28 of >3.2) (P<0.001). Among these patients, 51.51% were positive for rheumatoid factor and 52.27% were positive for ACCP antibodies (P=0.001). Significant differences were detected between the fitness subgroups in terms of C-reactive protein (CRP), health assessment questionnaire (HAQ), and HAQ disability index. Pharmacologic therapies used in the treatment of patients with RA are presented in Table 2. There was statistically significant difference between the groups regarding NSAIDs (P<0.001). No significant difference emerged among the three groups in either traditional disease-modifying antirheumatic drugs (DMARDs), cumulative steroid dose, antitumor necrosis factor therapy, interlukin-6 receptor inhibitor (tocilizumab), or abatacept.
Correlation between maximal oxygen uptake levels and age, cardiovascular disease risk factors, lipid profiles, insulin resistance, and rheumatoid arthritis disease characteristics There were inverse correlations between VO 2 max levels and age, RA disease duration, TC level, HDL levels, low-density lipoprotein However, there was no significant correlation between VO 2 max levels and other variables (Table 3 and Fig. 1).

Discussion
The accurate evaluation of CV risk among patients with RA remains an area of diligent research. A significantly lower VO 2 max levels were repeatedly demonstrated in patients with RA compared with healthy counterparts, most likely attributed to their low levels of physical activity [9,19] and increased disease activity/severity. Accordingly, we also recorded that VO 2 max level found to be significantly lower in patients with RA compared with control group.
In this analysis, we eliminated the probable bias by excluding patients with pulmonary fibrosis or other significant pulmonary features. Moreover, patients with poor mobility and functional comorbidities incompatible with VO 2 max testing were excluded. This would suggest that we have chosen the most fit patients with RA here; even so, we clarified that their VO 2 max levels were significantly low.
Another finding of our study was that older RA women seemed to have lower VO 2 max level, which is in accordance with previous results that state lower rates of physical fitness among women and a much larger reduction in older age [20]. One reason for these findings might be that older RA women having an inactive lifestyle and not being motivated to become more physically active [21], which represents a challenge to all health professionals.
Broad evidence demonstrates a strong inverse correlation between VO 2 max level and CVD morbidity and mortality [22,23]. In accordance to that, our results clarified the significant inverse association between cardiovascular risk profile and VO 2 max levels (TC level, HDL levels, low-density lipoprotein levels, FRS, BMI, and carotid IMT), highlighting the crucial role of fitness to conserve good health and prevent early CV morbidity in RA population. A randomized trial showed that high cardiorespiratory fitness level can be achieved through regular moderate-intensity lifestyle activities such as walking in sedentary individuals [24].
The overall explanation of low VO 2 max levels in our results only by demographic and traditional CVR measures in RA group might indicate that other factors are involved. Our results demonstrated an inverse association between VO 2 max level and clinical markers of disease activity. Patients with poor VO 2 max level had long disease duration, uncontrolled disease activity according to DAS28, high CRP level, and high score of HAQ. The low overall VO 2 max levels observed in the present study is an alarming finding that could be linked with inappropriate rheumatoid disease control. Another result in this study was the significant correlation between ACCP and lower VO 2 max level. In new research, tissue samples were obtained from lung and synovial biopsies of patients with RA. Investigating the protein proportions in these tissues discovered identical citrullinated peptides in both sites [25]. ACCP had been linked to the development of ILD, particularly when increasingly present [26,27] Regarding comparing the line of medication, no significant difference emerged between the three groups, except for NSAIDs. Unlike other pain relievers, NSAIDs seem to be more effective in treating symptoms of RA. This is because they reduced inflammation, pain, and fever. NSAIDs enable patients with RA to perform some activities, which accelerates their fitness level. There is considerable controversy as to whether biological agents improve or worsen CV risk in RA population. To date, there have been no randomized controlled trials comparing biological medications for the treatment of RA associated with CV mortality and morbidity. Adverse effects of biologic DMARDs on lipids have been reported previously [28,29]. Therefore, risks and benefits of biologic DMARD must be weighed carefully. This study is not limited by evaluating VO 2 max levels in an equivalent age-matched population and its case-control design but by the noninclusion of patients with pulmonary manifestations and patients with poor mobility possibly, introducing a positive bias in the current study.

Conclusion
This study provided a new insight into less explored test (VO 2 max test) which can be used as a surrogate CVD marker in patients with RA. VO 2 max test can be used as a noninvasive test to detect and quantify fitness defects in patients with RA at increased risk of CVD. We therefore recommend referral of patients with RA with an enhanced risk of cardiac manifestations for VO 2 max testing to better define their fitness level.

Financial support and sponsorship
Nil.

Conflicts of interest
There are no conflicts of interest.