Haemodynamic changes during vasodepressor syncope in children and autonomic function.
We investigated postural haemodynamic changes in 24 boys and 30 girls, aged 8-16 years, with orthostatic intolerance. During the orthostatic test, nine boys and seven girls (30%) developed a vasodepressor attack (orthostatic-positive group). The orthostatic-positive group had a more marked increase in heart rate (HR) with slightly lower systolic blood pressure than the orthostatic-negative group during standing, with no significant difference in cardiac output (CO). The shift of blood volume from the intrathoracic region to the lower part of the body determined by electrical impedance and plasma catecholamines on rising were identical in the two groups. At the onset of the vasodepressor attack, CO did not change significantly, indicating that sudden arterial vasodilatation was the cause. In a pharmacological study on autonomic function, higher sensitivity of alpha-adrenoceptor in the resistance vessels was found in the orthostatic-positive group, but there was no significant difference in the basal level of cardiac vagal and sympathetic activity and in cardiac beta-adrenoceptor sensitivity between the two groups. The stepwise regression analysis showed that the magnitude of HR increment during standing was the most useful indicator in predicting the occurrence of a vasodepressor attack. These results suggest that low sympathetic activity in the resistance vessels together with an exaggerated rise in HR during upright posture is strongly associated with vasovagal and vasodepressor attacks in children.