Cohort of study
In March and April, 1987, we established the cohort of Hanzhong Adolescent Hypertension Study [9, 10]. Based on a baseline survey of 4,623 adolescents aged 6-15 years old in over 20 schools of three towns (Qili, Laojun and Shayan) in Hanzhong, Shaanxi, China. General information, medical history, blood pressure, height, body mass index, waistline, arm circumference, pulse, etc. were collected. BMI was calculated as kilograms per meters squared (kg/m2). Salt sensitivity was measured in a randomly selected sample of 310 adolescents using the tests of oral saline load and furosemide sodium-volume depletion . Salt sensitivity (SS) was diagnosed in 101 subjects(mean 9.3 years)while 209 (mean 9.2 years)were determined as non-salt sensitive (NSS). The profile of baseline was exhibited in Table 1. There was no difference between the two groups in terms of age, family history of hypertension, body mass index, blood pressure, resting heart rate, etc.
We made a 18-years follow-up of the subjects in the above cohort in March and April, 2005. Through baseline survey and follow-up examinations, the subjects excluded the secondary hypertension patients and patients with other severe diseases. The baseline survey plan was approved by the Academic Committee of the First Affiliated Hospital of Xian Medical University, and parents of the surveyed adolescents had all signed the informed consent form. The follow up plan was approved by the Ethics Committee of the First Affiliated Hospital of Medical College, Xian Jiaotong University, and the followed subjects had all signed the informed consent form.
Tests to determine SS
Oral saline load test and natriuretic test were used to determine salt sensitivity of the subjects. Fasting adolescents were sent to Shaanxi Hanzhong Cardiovascular Disease Institute in early morning. After 30 minutes of resting, their BP was measured for three times, the average of the three measurements taken as the “presaline” mean arterial blood pressure (MABP, it was derived from the following formula: diastolic pressure + 1/3 pulse pressure). The subjects then drank up a certain volume (in dose of 100 ml per age) of 1% saline within 30 minutes, and BP was again measured 2 hours after salt loading to be considered as “postsaline” MABP. Then natriuretic test were accomplished by administration of a 40 mg doses of furosemide just after BP measurement. The BP was for the third time measured 2 hours after and the “postfurosemide” MABP was calculated. If the summation from the increase of “postsaline” MABP 2 hours after salt loading comparing with preloading and the “postfurosemide” MABP decrease 2 hours after sodium depletion comparing with prefurosemide was equal to or exceeding 10 mm Hg, the subject was deemed to be sodium sensitive (SS), and if the summation <10 mm Hg, the subject was judged as non-sodium sensitive (NSS).
Blood pressure measurement
Medical practitioners measuring blood pressure all received professional trainings based on WHO standards, and passed relevant examinations . The environment for BP measuring was quiet and comfortable for subjects, who sat and rested quietly and did not drink anything. Cuffs of various sizes were chosen for subjects based on sizes of their upper arms, to measure their right brachial pressure in sitting position. The pulse disappearing pressure was taken first; it plus 30 mmHg would be the Peak Inflating Pressure. The formal measurement of BP started with increasing the pressure to Peak Inflating Pressure, and then the operator deflated at a speed of 2 mmHg per second. Following the Korotkoff sound method, the operator took the number when the first sound appeared as the SBP, and the number when the fifth sound appeared as the DBP. BP was measured three times for each subject, with interval of 30 seconds between each measurement; average SBP and DBP of the three measurements were calculated. MABP = 1/3SBP + 2/3 DBP.
Statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS 13.0). One-way ANOVA was used as appropriate to evaluate differences in the mean values of specified variables between groups, differences in proportions between two groups were tested by χ2 analysis. Age, sex and body-mass index (BMI) were adjusted in multivariable analysis. Two-tailed values of P < 0.05 were taken as statistical significant. All data were expressed in terms of mean ± s.d.