Blood volume distribution during head-up tilt induced central hypovolaemia in man

Summary. We evaluated regional electrical impedance (Z°) at 2.5 and 100 kHz to separate intra- and extracellular fluid changes and correlated Z° over the thorax (TI) to relative changes in the central blood volume (CBV) induced by head-up tilt. In nine experiments head-up tilt resulted in normotensive central hypovolaemia associated with a 3·7 & 0·4 Ohm (mean f SE) increase in TI_{100 kHz} after 60 min. In 24 experiments presyncopal symptoms were induced after 43±2 min, when TI_100kHz had increased 4·2 ± 0·2 Ohm. Head-up tilt instantly decreased the activity of technetium labelled erythrocytes (⁹⁹Tc^m) over the thorax by 24 ± 2%, and increased ⁹⁹Tc^m over the thigh by 68±10% (P< 0·01, n = 8) with no further changes during the sustained tilt. Haematocrite increased during head-up tilt from 43·1 ± 0·3 to 47·9 ± 0·6% (P<0·01, n= 8). Accordingly, the increase in TI (6·3 ± 0·6 vs. 4·5 0·4 Ohm, n= 6) and the decrease in Z° through one leg (7·2 ± 1·2 vs. 2·8 ± 0·5 Ohm, n= 6) at 2·5 kHz was more pronounced than at 100 kHz. Also the changes in TI were correlated to CBV as calculated from ⁹⁹T_c^m and haematocrite (r = 0.90, P < 0·01). The results suggest that: (1) Hypovolaemic shock is associated with a faster increase of TI than normotensive head-up tilt. (2) Head-up tilt is characterized by an initial decrease in CBV followed by a further decrease in plasma volume, which eventually leads to hypovolaemic shock. (3) Blood volume changes during head-up tilt are reflected in regional Z°.