title: Mesenteric artery response to head-up tilt-induced central hypovolaemia and hypotension.
abstract: Superior mesenteric artery (SMA) blood flow and impedance were evaluated by duplex ultrasound during head-up tilt (HUT)-induced central hypovolaemia and hypotension in eight healthy volunteers. HUT induced a reduction in cardiac stroke volume from 88.8 +/- 6.3 to 64.7 +/- 6.3 ml (mean +/- SEM; P < 0.01) and an increase in thoracic electric impedance from 38.6 +/- 2.1 to 42.6 +/- 2.1 omega (P < 0.01) reflecting a reduced central blood volume. Maintained tilt provoked a 30% reduction in mean arterial pressure (from 87.1 +/- 3.3 to 63.4 +/- 3.6 mmHg: P < 0.01) and the appearance of presyncopal symptoms. During both the normotensive and the hypotensive phase of HUT, the SMA diameter (5.7 +/- 0.03 mm) and blood flow (514 +/- 75 ml min-1) did not change significantly, although the end-diastolic velocity increased from 9.7 +/- 4.8 to 39.7 +/- 4.0 cm s-1 (P < 0.01). The increase in diastolic velocity, despite a maintained or reduced arterial pressure, supports a reduction in the SMA impedance as it was reproduced during a meal test when a moderate reduction in mean arterial pressure (87 +/- 4 to 80 +/- 4 mm Hg; P = 0.04) was accompanied by a ninefold increase in the end-diastolic velocity (P < 0.01). The results indicate a reduction in the mesenteric vascular impedance to the extent that superior mesenteric artery blood flow is maintained during HUT-induced central hypovolaemia and hypotension.
authors.affiliation: Department of Vascular Surgery, Rigshospitalet, University of Copenhagen, Denmark.
authors.forename: M J
authors.initials: MJ
authors.lastname: Perko
authors.forename: P
authors.initials: P
authors.lastname: Madsen
authors.forename: G
authors.initials: G
authors.lastname: Perko
authors.forename: T V
authors.initials: TV
authors.lastname: Schroeder
authors.forename: N H
authors.initials: NH
authors.lastname: Secher
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity; D001810:Blood Volume / Q000502:physiology*; D017097:Electric Impedance; D005260:Female; D006257:Head; D006801:Humans; D007022:Hypotension / Q000503:physiopathology*; D008297:Male; D017538:Mesenteric Artery, Superior / Q000000981:diagnostic imaging / Q000503:physiopathology*; D011187:Posture; D013318:Stroke Volume / Q000502:physiology; D014463:Ultrasonography; D014655:Vascular Resistance / Q000502:physiology