One day after HUTT in the VVS group, 10 ml morning urine was retained, and the USG was detected by polyelectrolyte ionization method.

To complete baseline head-up tilt test (BHUT) and/or sublingual nitroglycerin head-up tilt test (SNHUT).

Subjects should be discontinued from any cardiovascular active drugs for more than 5 half-lives prior to the test, stop taking any diet that affected autonomic function, and abstaining from eating and drinking for at least 4 h before the experiment. The test requires quiet environment, dim light and suitable temperature. The subjects and their family members (guardians) should be introduced to the precautions before the test and the possible risks during the test, and a written informed consent from the subject or his/her family (guardian) should be obtained.

The examine time was arranged from 8:00 to 12:00. And the ECG and blood pressure of the right upper limb were monitored by SHUT-100 tilt test monitoring software system of Beijing Standard Medical Technology Co., Ltd. Subjects lay supine on the electric tilting bed in a quiet environment for at least 10 min with band fixed to avoid buckling of ankle joint and knee joint. Then Baseline SBP, DBP, HR and ECG were recorded. Next, the electric tilting bed was operated to place the subject at a 60 degree angle of head-up tilt position within 15 s. Changes of ECG and blood pressure were continuously monitored at the beginning of tilt and throughout HUTT process until a positive reaction occurred or the prescribed 45 min was reached. The test should be terminated and subjects should be return to the supine position as soon as a positive reaction occurred [1].

If a positive reaction never occurs within prescribed 45-min BHUT, the subject should be kept the same tilting angle on the tilting bed, simultaneously with a sublingual nitroglycerin tablet [(4–6) μg/kg (maximum≤300 μg)]. HR, BP and ECG were continuously monitored, and the test should be terminated and subjects should be return to the supine position as soon as a positive reaction occurred; If no positive reaction occurs, it should be observed until 20 min after the subject taking the drug [1].

Children with syncopal episodes or presyncope with any of the following responses in HUTT were considered positive patients: 1) fall of BP, 2) decrease of HR, 3) sinus arrest occurs and is replaced by junctional escape rhythm, 4) transient, secondary or more atrioventricular block and cardiac arrest for 3 s. The standard of BP decrease was SBP ≤ 80 mmHg (1 mmHg = 0.133 kPa) or DBP ≤ 50 mmHg or mean blood pressure decrease≥25%, HR decrease refers to sinus bradycardia, HR < 75 beats/min for 4–6 years old children, HR < 65 beats/min for 6–8 years old children, HR < 60 beats/min for those older than 8 years. VVS was classified into three types according to the changes in BP and HR associated with positive reactions during HUTT: 1) VVS-VI: BP drops dramatically and no significant change in HR, 2) VVS-CI: HR decreases obviously and no significant change in BP, 3) VVS-M: HR and BP both are significantly decreased [1].

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