Middle cerebral artery blood flow velocity in response to lower body positive pressure

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.