Circulatory adaptation to orthostatic stress in healthy 10-14-year-old children investigated in a general practice.

1. The magnitude and time course of circulatory adaptation to active standing were investigated in healthy premenarchic girls and boys (n = 24; 10-14 years old) by non-invasive measurement of heart rate and continuous finger blood pressure (Finapres). 2. Four subjects (two girls, two boys) showed presyncopal symptoms after 4-9 min of free standing. 3. In the 20 non-fainting subjects, changes in blood pressure and heart rate upon standing did not differ between girls (n = 10) and boys (n = 10). In the initial phase of standing (first 30 s) systolic and diastolic blood pressures dropped by 22 +/- 14 (mean +/- SD) and 16 +/- 7 mmHg, respectively, at 8 +/- 2 s. Blood pressure subsequently recovered and showed an overshoot in all subjects. The transient drop in blood pressure was accompanied by an increase in heart rate of 40 +/- 7 beats/min. These characteristic transient changes were not observed with passive head-up tilt. During the early steady-state phase (2 min), systolic blood pressure was similar to the supine value and diastolic blood pressure rose by 11 +/- 5 mmHg. Heart rate increased by 25 +/- 11 beats/min. In six of the subjects (three girls, three boys) the increase in heart rate exceeded 30 beats/min (postural tachycardia). Little further changes were observed during prolonged (10 min) standing.(ABSTRACT TRUNCATED AT 250 WORDS)