title: Middle cerebral artery blood flow velocity in response to lower body positive pressure.
abstract: Lower body positive pressure (LBPP) has been used in the treatment of haemorrhagic shock and in offsetting g-force induced fluid shifts. However, the middle cerebral artery blood flow velocity (MCAv) response to supine LBPP is unknown. Fifteen healthy volunteers (mean ± SD: age, 26 ± 5 year; body mass, 79 ± 10 kg; height, 174 ± 9 cm) completed 5 minutes of 20 and 40 mm Hg LBPP, in a randomized order, separated by 5 minutes rest (baseline). Beat-to-beat MCAv and blood pressure, partial pressure of end-tidal carbon dioxide (PET CO2 ) and heart rate were recorded and presented as the change from the preceding baseline. All measures were similar between baseline periods (all P>0·30). Mean arterial pressure (MAP) increased by 7 ± 6 (8 ± 7%) and 13 ± 7 mm Hg (19 ± 11%) from baseline during 20 and 40 mm Hg (P<0·01), respectively. The greater MAP increase at 40 mm Hg (P<0·01 versus 20 mm Hg) was mediated via a greater increase in total peripheral resistance (P<0·01), with heart rate, cardiac output (Model flow) and PET CO2 remaining unchanged (all P>0·05) throughout. MCAv increased from baseline by 3 ± 4 cm s(-1) (5 ± 5%) during 20 mm Hg (P = 0·003), whilst no change (P = 0·18) was observed during 40 mm Hg. Our results indicate a divergent response, in that 20 mm Hg LBPP-induced modest increases in both MCAv and MAP, yet no change in MCAv was observed at the higher LBPP of 40 mm Hg despite a further increase in MAP.
authors.affiliation: School of Sport and Exercise, Massey University, Palmerston North, New Zealand.
authors.forename: Blake G
authors.initials: BG
authors.lastname: Perry
authors.forename: Zachary J
authors.initials: ZJ
authors.lastname: Schlader
authors.forename: Aaron
authors.initials: A
authors.lastname: Raman
authors.forename: Darryl J
authors.initials: DJ
authors.lastname: Cochrane
authors.forename: Samuel J E
authors.initials: SJ
authors.lastname: Lucas
authors.forename: Toby
authors.initials: T
authors.lastname: Mündel
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D062186:Arterial Pressure; D001783:Blood Flow Velocity; D002560:Cerebrovascular Circulation; D005260:Female; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D016683:Supine Position; D013997:Time Factors; D018617:Ultrasonography, Doppler, Pulsed; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance; D055815:Young Adult
keywords: arterial blood pressure; cerebral autoregulation; cerebral blood flow; cerebral haemodynamics; sympathetic nervous system