Study Protocol
The Chronic Arterial Disease, quality of life and mortality in chronic KIDney injury (CADKID) -study (http://www.ClinicalTrials.gov NCT04223726) is an ongoing prospective follow-up study protocol assessing arterial disease, quality of life and mortality in patients with CKD stage 4–5. 210 consecutive patients referred to the predialysis outpatient clinic of Kidney Center, Turku University Hospital between 2013 and 2017 were recruited to the study. Enrolled patients had CKD stage 4–5 defined as glomerular filtration rate (eGFR) < 30 ml/min/1.73 m2 and estimated using the Chronic Kidney Disease Epidemiolgy Collaboration (CKD-EPI) formula. [16] The study design was approved by Medical Ethics Committee of the Hospital District of Southwest Finland. All procedures were in accordance with the Helsinki Declaration. All patients gave written informed consent before entering the study.
All 210 patients were invited to attend stress ergometry. 176 patients agreed to attend, and 174 patients underwent a standard maximal bicycle stress test. For two patients stress ergometry was not possible due to patient related issues. Abdominal aortic calcification was assessed from plain lateral lumbar radiograph, and echocardiography in addition to biochemical studies were examined at baseline.
Maximal Stress Ergometry
Maximal Stress Ergometry was performed as an incremental, symptom-limited cycling exercise test in accordance with clinical standards. Each patient started with a 30 s warm-up phase during which the target speed of 60 rpm was reached. Primary workload was determined according to an estimated maximum workload and a targeted test duration of 6 to 10 minutes. An increase in workload per minute (10, 15 or 20 W) was accomplished automatically by the ergometer software until symptom limitation within 6 to 10 minutes. Participants were informed to cycle at a speed of 60 rpm, and were encouraged to continue cycling until exhaustion. Perceived strain was reported as the highest rating on the Borg Scale from 1 to 20. The mean proportional workload of the last 4 minutes of the age, sex and body size predicted value (WMAX%), was used in the analyses in addition to the corresponding workload in watts (WMAX). The values considered normal for expected maximal exercise performance measured as watts are derived and extrapolated from large data sets of the Mini Suomi –study. [17] The algorithm for expected exercise performance is incorporated in cycle ergometer software and used in day-to-day clinical work. The study population was divided into two groups according to WMAX% <50% versus ≥ 50% of the expected normal value.
Echocardiography
A comprehensive echocardiographic examination was performed at rest before the exercise test at the Department of Clinical Physiology of Turku University Hospital. Data collected included the systolic and diastolic dimensions and function of the left ventricle (LV), left ventricular wall thickness, aortic and left atrial dimensions, LV mass index (LVMI), LV ejection fraction (LVEF), global longitudinal strain (GLS), and early maximal ventricular filling velocity and the late filling velocity (E/A-ratio). For E/e’ the transmitral early diastolic inflow velocity (E wave) was measured using pulse-wave Doppler in the apical four-chamber view, and the peak (e’) diastolic mitral annular velocity was measured using tissue Doppler imaging at the septal mitral annulus. Ultrasound examinations were performed using a commercially available ultrasound system (Vivid E9; GE Vingmed Ultrasound, Horten, Norway) with a 3.5-MHz phased-array transducer (M5S).
Assessment of AAC
Abdominal aortic calcification (AAC) score was calculated for each subject. Lateral lumbar radiography with standard equipment was performed in a standing position. A validated 24-point scale, as described previously, was used. [18] Calcific deposits of the anterior and posterior wall of the aorta, adjacent to first through fourth lumbar vertebrae, were assessed at each vertebral segment, and were graded on a scale 0–3 as follows: 0 = no calcific deposits, 1 = small scattered calcific deposits filling less than one-third of the longitudinal aortic wall, 2 = one-thirds to two-thirds of the longitudinal aortic wall calcified, 3 = at least two-thirds of the longitudinal aortic wall calcified. The grades of eight segments were summed ranging from 0 to 24 points. Two independent observers recorded AAC scores of all lateral lumbar X-rays, and mean was used for analysis.
Statistical Analysis
Data are presented only for the 174 patients that underwent stress ergometry. We compared the patients not attending stress ergometry (n = 36) to those who did (n = 174). Those not attending were more often women, had a higher proportion of coronary artery disease (CAD), were older and had higher TnT but no differences were observed in AAC or proportion of diabetics (data not shown).
Results are presented as mean ± standard deviation (SD) for the normally distributed variables and as median [inter-quartile range (IQR)] for skewed variables. Skewed variables were loge-transformed to normalize distributions. Normality in continuous covariates was tested with Kolmogorov-Smirnov and Shapiro-Wilk tests. Student’s t-test was used to compare continuous normally distributed covariates and Chi-square test for categorical covariates in the study subgroups. For some skewed variables a suitable transformation was not found and thus the comparisons between groups were done using a non-parametric Kruskal-Wallis test.
Univariable associations between the study variables were analyzed by calculating Spearman’s correlation coefficients. Multivariable analysis was done using linear regression technique. Variables with significant univariable correlations with WMAX%, as well as diabetes and previous coronary artery disease were included as covariates in a stepwise multivariable linear regression model. The multivariable associations between exposure variables and WMAX were also studied using the same analyses. However, for WMAX (expressed in Watts), age, height and gender were also included as covariates in the multivariable model. To make the multivariable results easily comparable, variables were included in the models without transformation.
Receiver operating characteristics (ROC) curve analyses were conducted to estimate the area under the curve (AUC) as a measure of discriminative capacity of TnT and AAC for WMAX%<50%.
All statistical analyses were performed using statistical analysis system, SAS version 9.3 (SAS Institute Inc., Cary NC). P < 0.05 was considered statistically significant.