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

Summary 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 ( MCA v) 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 MCA v and blood pressure, partial pressure of end‐tidal carbon dioxide (P ET CO 2 ) 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 ( M odel flow) and P ET CO 2 remaining unchanged (all P >0·05) throughout. MCA v 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 MCA v and MAP , yet no change in MCA v was observed at the higher LBPP of 40 mm Hg despite a further increase in MAP .