Blood volume distribution during head-up tilt induced central hypovolaemia in man.
We evaluated regional electrical impedance (Z degree) at 2.5 and 100 kHz to separate intra- and extracellular fluid changes and correlated Z degree 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 +/- SE) increase in TI100 kHz after 60 min. In 24 experiments pre-syncopal symptoms were induced after 43 +/- 2 min, when TI100 kHz had increased 4.2 +/- 0.2 Ohm. Head-up tilt instantly decreased the activity of technetium labelled erythrocytes (99Tcm) over the thorax by 24 +/- 2%, and increased 99Tcm over the thigh by 68 +/- 10% (P less than 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 less than 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 degree 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 99Tcm and haematocrite (r = 0.90, P less than 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 degree.
Department of Anaesthesia, Rigshospitalet, Copenhagen, Denmark.
S
S
Matzen
G
G
Perko
S
S
Groth
D B
DB
Friedman
N H
NH
Secher
D000328:Adult; D001810:Blood Volume / Q000502:physiology*; D001826:Body Fluids / Q000502:physiology; D004560:Electricity; D004911:Erythrocyte Volume / Q000502:physiology; D005260:Female; D006257:Head; D006339:Heart Rate / Q000502:physiology; D006400:Hematocrit; D006801:Humans; D007866:Leg / Q000502:physiology; D008297:Male; D016684:Prone Position / Q000502:physiology; D013575:Syncope / Q000503:physiopathology; D013669:Technetium Tc 99m Medronate; D013909:Thorax; D014690:Venous Pressure / Q000502:physiology
D013669:Technetium Tc 99m Medronate