Participants
The cross-sectional survey was conducted between January and August 2008. Twenty-three community-based populations from Beijing and Hebei Province were chosen by simple random sampling, such that these populations represented the average standards of living, education, and health care in North China. Those (n = 5,770) who were community-dwelling residents aged 18 years or older in these 23 areas, were located on the basis of the local resident registry of each community. Being clearly informed of the study protocol, 3,093/5,770 (53.6%) agreed to participate in this survey. The most common reason for non-participation (46.7%) was due to refusal of blood test or examination. Written informed consent was obtained for each participant, who was required to correspond to an interviewer-led questionnaire with questions on smoking habits, occupation, reproductive history, and medical histories. Trained physicians performed as interviewers. Except for oral reports, detailed information regarding medical histories of all participants was provided by community health service of each community. Approval of this study was granted by the Institutional Review Board, Capital Medical University.
Those (241 of 3,093) who had one or more of the following factors were excluded from this study: a) aortic valve diseases and/or aortic aneurysm, for they are frequently associated with abnormalities of the systemic arterial system, and, in particular, with reduced arterial compliance, which may have important influence on PWV; b) history of percutaneous coronary intervention and/or coronary artery bypass grafting, for these patients may suffer from potential injuries of arteries due to puncture or artery harvesting, which might affect the test result of the automatic device used in the present study; c) ABI of 0.9 or less, for this indicates the probability of obstructive arteriosclerosis, which could lead to a decreased PWV value; and d) ejection fraction < 30%, for patients with severe heart failure tend to have an increased PWV.
Finally 2,852 eligible participants were involved. From these people, a healthy reference sample (n =1,201) was established, for participants of this sample had none of following issues:
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1)
hypertension (defined as systolic blood pressure [SBP] ≥ 140 mmHg and/or diastolic blood pressure [DBP] ≥ 90 mmHg, and/or antihypertensive drug(s) usage);
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2)
diabetes (defined as level of fasting blood glucose ≥ 126 mg/dL or receiving insulin therapy and/or taking anti-diabetic medications);
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3)
dyslipidemia (defined as serum total cholesterol [TC] level > 6.2 mmol/L and/or triglycerides [TG] > 2.3 mmol/L or taking lipid-lowering medicines);
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4)
CVDs (including coronary heart disease, heart failure, stroke, transient ischemic attack, and/or intermittent claudication);
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5)
current smoking (defined as smoking within the past twelve months);
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6)
obesity (defined as body mass index [BMI] ≥ 30 kg/m2).
Measurement of baPWV
An automatic device (ModelBP-203PRE, Colin Inc., Komaki, Japan) was used to perform baPWV measurements. Each participant was asked to take five minutes of rest and then to take examinations in the supine position with cuffs wrapped both on brachia and on ankles, electrocardiogram electrodes placed on both wrists, and a microphone for detecting heart sounds placed on the left edge of the sternum. The pulse volume waveforms were recorded via a plethysmographic sensor connected to the cuffs. Volume waveforms for the brachium and ankle were stored, then an automatic gain analysis with quality adjustment would be done after ten seconds’ sampling. Time interval between the brachial and ankle (Tba) was defined as time between the initial rise in the brachial and tibial pressure waveforms. The distance between two sampling points of baPWV could be calculated automatically based on the subject’s body stature. Both length and height were measured to the nearest of 1 centimeter (cm).The length from aortic valve to ankle (La) was calculated via the following equation: La (cm) = 0.8129 × height (cm) + 12.328; while that from the aortic valve to the brachia (Lb) was computed using another equation: Lb (cm) = 0.2195 × height (cm) – 2.074. Finally, based on the equation: baPWV = (La–Lb) / Tba, value of baPWV was obtained. In the meantime, bilateral baPWV, brachial and ankle blood pressures, electrocardiogram, and heart sounds were also obtained via automatically computation by this device. In this study, the mean value of the bilateral baPWV, and left brachial blood pressure were used.
Laboratory measurements
After 12–15 hours of fasting, a venous blood sample of 10 ml was taken from each subject. The blood samples were preserved in tubes containing 0.1% EDTA. Plasma was isolated by centrifugation at 2500 rpm, 4°C, for 20 minutes and then shipped in the frozen state using dry ice to the central laboratory. Specimens were all stored at −70°C before laboratory assays was carried out. Measurements of high density lipoprotein cholesterol (HDL-C) level complied with that described in Manual of Laboratory Operations of the Lipid Research Clinics Program [9], where the intra-assay coefficient of variation (CV) and inter-assay CV are required to be 1.4% and 2.0% respectively. Levels of TC and TG were measured using standard enzymatic methods [10], where intra-assay CV and inter--assay CV are 0.9% and 1.8% respectively for TC, 1.5% and 1.9% respectively for TG. Levels of fasting glucose were measured using the hexokinase method, where intra-assay and inter-assay CV are required to be 1.9% and 2.6% respectively. For those with TG < 400 mg/dl, low-density lipoprotein cholesterol (LDL-C) levels were calculated using the Friedewald equation [11]: . No participant with TG ≥ 400 mg/dl was found in this investigation. The central laboratory of The Military General Hospital of Beijing PLA was responsible for all the blood tests in this study. The internal and external quality controls procedures were performed in accordance with regulations of Chinese Laboratory Quality Control.
Statistical analysis
Statistical analyses were performed using the SPSS for Windows statistical software package version 15.0 (SPSS Inc., Chicago, IL, USA). Continuous variables were expressed as mean ± standard deviation (SD) unless otherwise indicated. Differences in continuous variables between the subjects with and without high baPWV were tested with Student’s t test, including age, BMI, SBP, DBP, HR, and levels of TC, HDL-C, LDL-C, TG and fasting glucose. Differences in binomial categorized variables between the two groups were analyzed with Person’s Chi-square test, including hypertension (yes/no), diabetes (yes/no), smoking (yes/no, defined as smoking regularly over the prior 12 months), ischemic heart disease (yes/no), stroke (yes/no), angiotensin converting enzyme inhibitors (ACEIs) and/or angiotensin receptor blockers (ARBs) usage (yes/no), statins usage (yes/no), calcium-channel blockers (CCBs) usage (yes/no). Considering the difference between men and women, we performed multiple analyses in two genders separately. Age-specific cut-off points for high baPWV were defined as the 90th percentiles in each age group of the healthy reference sample respectively.
Multivariate analyses based on multiple backward stepwise logistic regression were also performed to test the associations between potential risk factors and increased arterial stiffness defined as high baPWV, in men and women separately. Variables were considered for entry into the multiple logistic models were age, BMI, HR, SBP, DBP, HR, TC, HDL-C, TG, fasting glucose, diabetes, ischemic heart disease, hypertension, stroke, smoking, statin usage, ACEIs and/or ARBs usage, and CCBs usage. The standard used for a variable entering or not entering the multivariate analysis was based on p < 0.10 yielded by univariate analysis, and/or its potential clinical significance on the basis of previous studies and clinical practice. The criterion for inclusion of variables was p < 0.05, and that for exclusion was p > 0.10. All p values reported are two-tailed and p < 0.05 was considered to be statistically significant.
