1.1 Study population
The Kailuan Study is a study of cardiovascular, cerebrovascular, and related disease risk factors and interventions based on functional community populations in the Kailuan community in Tangshan, China. This study began in 2006 and participants were followed up every 2 years thereafter. National Physical Fitness Monitoring is a project in China that began in 2000 and is conducted every 5 years to systematically assess the nationwide physical fitness of individuals with sample surveys. The fifth iteration of National Physical Fitness Monitoring included four coal mining companies of the Kailuan community for sampling in 2020. Using the information database of employees that was established by these four companies, 1200 men aged 20–49 years were selected by random methods for physical testing. These tests included those for body morphology, physical function, and physical fitness. We also measured brachial–ankle pulse wave velocity (baPWV) before and after a two-stage load test. This study was performed in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of the Kailuan General Hospital. All participants provided written informed consent.
1.2 Inclusion and exclusion criteria
In this study, individuals who participated in the sixth follow-up visit of the Kailuan Study and were selected to participate in the fifth National Physical Fitness Monitoring were included in our study. We excluded participants who did not have the two-stage load test performed or did not complete baPWV measurements (Figure 1).
1.3 Data collection
1.3.1 General information
The components of the investigation, anthropometric measurements, and biochemical testing have been previously described in detail[18]. Anthropometric indicators, including height, weight, body mass index (BMI), and other indicators, were derived from National Physical Fitness Monitoring data.
1.3.2 Experimental procedures
All subjects wore light clothes and conducted a two-stage load test on a power bicycle (GMCS-GLC3). All inspections were carried out in a quiet room with a comfortable temperature. Smoking and drinking were prohibited within 12 hours before exercise. The whole process was maintained for 7 minutes. The first 30 seconds was the zero-load warm-up stage. A load level (25 w) was then increased every 3 minutes for two times. The end of 30 seconds was the zero-load recovery stage. The pedaling speed was maintained at approximately 60 r/min during the whole process. The heart rate (HR) of the subjects was monitored in real time by an HR monitor worn on the inner side of the midpoint of the upper right arm. The maximum oxygen uptake was calculated by the power bicycle’s system.
1.3.3 Measurement of baPWV, BP, and HR
Specially trained nurses used the BP-203 RPE III networked arterial stiffness detection device (Omron Health Medical [China] Co., Ltd.), as previously described[19, 20], and measurement of baPWV, BP, and HR was performed before and after the two-stage load test in all subjects. For the first measurement, participants in light clothes had not been smoking and were seated for at least 15 minutes in a room with the temperature controlled between 22°C and 25°C. The participants were then asked to lie down on an examination couch in the supine position and remain quiet during the measurement. Cuffs were wrapped on both arms and ankles. The lower edges of the arm cuffs were positioned 2–3 cm above the transverse striation of the cubital fossa, while the lower edges of the ankle cuffs were positioned 1–2 cm above the superior aspect of the medial malleolus. One heart sound detector was placed at the left and right edges of the sternum. Measurements were repeated twice for each subject, and the result of the second time was used as the final result.
All subjects repeated the measurements of baPWV, HR, BP, and mean arterial pressure (MAP) immediately after completing the exercise. Systolic blood pressure (SBP) and MAP measurements were taken from the right branchial artery, and the maximum of the left and right sides of baPWV was used for analysis.
1.4 Smoking status
The smoking status of the subjects (i.e., never smokers, former smokers, and current smokers) was assessed with a questionnaire. We ensured the accuracy of smoking information through a telephone follow-up survey. Never smokers were defined as having no history of smoking, former smokers[21] as not smoking for at least 6 months, and current smokers [22]as any who had smoked at least one cigarette per day for 1 year. Additionally, to calculate pack-years in current smokers, data on the average number of cigarettes smoked per day and the years of cigarette smoking were collected. Pack-years of smoking were defined as the average number of packs of cigarettes smoked per day multiplied by the duration of smoking in years[23]. On the basis of the number of cigarettes smoked per day, current smokers were classified into the two following groups: light-intensity (1–10 cigarettes/day) and heavy- intensity (>10 cigarettes/day) [24]. To further confirm our findings, we divided the smokers into low-intensity (≤4 pack-years) and high-intensity groups (>4 pack-years) in accordance with the pack-years as described in Celermajer et al.’s [25]study as well.
1.5 Related definitions
Hypertension was defined as (i) systolic blood pressure (SBP) ≥140 mmHg or diastolic blood pressure (DBP) ≥90 mmHg, or (ii) formerly diagnosed hypertension or using antihypertensive drugs[26]. Diabetes was defined as either a fasting blood glucose (FBG) level ≥7.0 mmol/L, self-reporting of a physician’s diagnosis, or self-reported use of antidiabetic medication[27]. Changes in baPWV, HR, SBP, DBP, and MAP were calculated as follows: measurement value after exercise − measurement value before exercise.
1.6. Statistical analysis
SAS 9.4 (Version 9.4; SAS Institute, Cary, NC) was used for data analysis. Continuous variables that conformed to a normal distribution are presented as the mean ± standard deviation ( ±s). Data with a non-normal distribution are expressed as the median (interquartile range). Categorical variables are expressed as the number of samples (percentage). Demographic factors and hemodynamic parameters were compared between the groups using the Student’s t test or the Wilcoxon rank sum test, as appropriate. Categorical variables were assessed using the χ2 test for independence. The paired-sample t test was used to compare baPWV, HR, and BP values in each group before and after exercise.
A generalized linear model was established to evaluate between-group differences in hemodynamic parameters postexercise. When we evaluated the response to exercise, the model was adjusted for baseline HR, baseline MAP, baseline baPWV and age, BMI (continuous variables), triglyceride (TG) levels, education level (less than senior college or senior college or above), alcohol consumption (current drinker, yes or no), the time interval of baPWV measurement after exercise (time interval).
Generalized linear models were used to analyze differences in the change in baPWV (dependent variable) between the groups (i.e., different smoking statuses as independent variables). All analyses were adjusted for age and baseline baPWV (Model 1), and were further adjusted for BMI, TG levels, time interval, education level, alcohol consumption, baseline HR, and baseline MAP (Model 2). Analyses were also further adjusted for the change in MAP and the change in HR (Model 3). Between-group differences are described by the β-value and 95% confidence interval (CI).
We performed a sensitivity analysis as follows. 1) We repeated the above-mentioned generalized linear model for sensitivity analysis of smokers classified by pack-years. 2) To exclude the effect of inconsistencies in the time interval, hypertension, diabetes, taking antihypertensive drugs, stenosis, or blockage of the lower limbs, we excluded the variables of a time interval exceeding 26.3 minutes (95th quartile), hypertension, diabetes, taking antihypertensive drugs, and an ankle–brachial index ≤0.9 in the generalized linear model.
A P value <0.05 was considered statistically significant with a two-sided test.