Arterial diameter during central volume depletion in humans

The luminal diameter of the radial artery was followed by high frequency ultrasound during 50° head-up tilt-induced central volume depletion in ten healthy subjects of whom six were tilted twice and pretreated with the serotonin receptor antagonist methysergide or placebo following a double-blind randomized design. Eight subjects without active treatment experienced presyncopal symptoms after 16–45 (mean 32 min). Central volume depletion was indicated by an increase in mean thoracic electrical impedance from 31.5 (SEM 1.6) to 33.4 (SEM 1.7) P < 0.05]. Cardiac output decreased from 4.1 (SEM 0.3) to 2.2 (SEM 0.3) l · min^–1] and heart rate HR, from 64 (SEM 3) to 100 (SEM 7) beats · min^–1], mean arterial pressure {MAP, from 77 (SEM 4) to 89 (SEM 2) mmHg 10.3 (SEM 0.53 to 11.9 (SEM 0.27) kPa]} and total peripheral resistance {TPR, from 19 (SEM 2) to 34 (SEM 4) mmHg · min · l^–] 2.5 (SEM 0.27) to 4.5 (SEM 0.53) kPa · min^–1]} increased; but with the appearance of presyncopal symptoms, HR, MAP and TPR were reduced to 65 (SEM 8) beats · min^–1, 46 (SEM 4) mmHg 6.1(SEM 0.53) kPa] and 18 (SEM 3) mmHg · min · l^–1 2.4 (SEM 0.4) kPa · min^–1 · l^–], respectively (P < 0.05). Vascular resistance was reflected in the arterial diameter which decreased from 2.42 (SEM 0.17) to 2.27 (SEM 0.14) mm during head-up tilt and increased to 2.71 (SEM 0.14) mm with the appearance of presyncopal symptoms (P < 0.05). Methysergide reduced the resting radial (15 ± 2%) and temporal artery diameters (10 ± 3%) (P < 0.05); however, it affected neither tilt-tolerance nor the central cardiovascular response to tilt. The results suggested a serotonergic influence on arterial tone at rest, and demonstrated that vessels as large as the radial artery participated in vascular control during central volume depletion independent of such a serotonergic influence.