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±5year; body mass, 79±10kg; height, 174±9cm) completed 5minutes of 20 and 40mmHg LBPP, in a randomized order, separated by 5minutes 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±7mmHg (19±11%) from baseline during 20 and 40mmHg (P<0·01), respectively. The greater MAP increase at 40mm Hg (P<0·01 versus 20mmHg) 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±4cms(-1) (5±5%) during 20mmHg (P=0·003), whilst no change (P=0·18) was observed during 40mmHg. Our results indicate a divergent response, in that 20mmHg LBPP-induced modest increases in both MCAv and MAP, yet no change in MCAv was observed at the higher LBPP of 40mmHg despite a further increase in MAP.