Differences in circulatory control in normal subjects who faint and who do not faint during orthostatic stress

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Abstract
We have determined if there are differences in normal subjects who fainted and those who did not faint during prolonged standing. We studied the short-term orthostatic responses in relation to heart rate, blood pressure measured by Finapres, left ventricular stroke volume analysed by pulse contour method, cardiac output and systemic vascular resistance, and also postural blood pressure and heart rate variability as assessed by spectral analysis. Thirteen healthy males without a history of syncope were studied. Three fainted after 10–13 min standing; the ten non-fainters remained upright for 20 min. The initial (first 30 s) postural circulatory adjustment was comparable for blood pressure but the rebound bradycardia was smaller in the fainters (heart rate at 22 s amounted to +13 ±10 beats/min above control vs. +1 ±5 beats/min in the non-fainters). Upright heart rate at 2 min standing was higher in the fainters (+31 ±2 beats/min vs. +20 ±5 beats/min), and blood pressure at 7 min standing was lower (−2/+5/+8 ± 5/5/5 mmHg vs. +11/+13/+16 ± 10/6/5 mmHg). The responses of stroke volume and cardiac output were comparable but systemic vascular resistance gradually decreased in the fainters from 5 min standing to the onset of fainting (+4 ±13% vs. +33 ±19% at 7 min standing). In fainters, the variability in upright blood pressure around 0.1 Hz was larger (8.8 mmHg2/Hz for diastolic blood pressure vs. 5.7 ±1.5 mmHg2/Hz in non-fainters). In conclusion, the circulatory adjustments to active standing in fainters is different from non-fainters, as they lose vasoconstrictor tone after 5 min of standing despite an increase in blood pressure variability and heart rate which suggest increased sympathetic activity. Whether there are other opposing factors, which include vasodilator substances, is discussed.