Orthostatic stress by lower body negative pressure and its body fluid distribution kinetics under microgravity

To support the hypothesis that interstitial fluid loss is one of the components of the "cardiovascular deconditioning" induced by space flight, a series of three experiments was performed in one subject, after 5 days in space: 1) standard thigh cuffs were inflated up to suprasystolic pressure values (CUFF I) for two minutes, 2) Half an hour later a lower body negative pressure test (LBNP) followed with -15 mmHg for 15, -30 mmHg for 5, and -40 mmHg for 15 minutes, 3) Twenty minutes after that, the CUFF maneuver was repeated (CUFF II). Body fluid shifts were detected by quadripole segmental electrical impedance (BIM) and by electrical impedance tomography (APT). The APT electrodes were placed directly under one of the thigh cuffs. Cardiovascular reaction patterns were observed by continuous ECG-recordings and by minute-by-minute arm cuff blood pressure measurements. By means of segmental impedance the expected body fluid change in the abdominal, thigh, and calf regions could be detected. The thoracic segment showed no significant changes in microgravity. However, the calf increased its electrical impedance roughly by 50%. The interstitial fluid in the skin and musculature, detected by APT during CUFF I, was reduced more than 50%. The LBNP-maneuver was able to refill this interstitial space. However, only the higher levels (LBNP > 15 mmHg) showed the outward filtration component during the constant LBNP levels. In addition, the LBNP-experiment clearly showed the reduced ability to cope with orthostatic stress. Pulse pressure dropped and heart rate increased much more than on ground. The fluid displacement during CUFF I showed a similar hemodynamic response, which had not been seen on ground before. The CUFF II maneuver after LBNP showed that the reaction had returned to normal. This leads to the conclusions that LBNP can counteract cardiovascular deconditioning, but only higher LBNP-levels are able to transfer fluid into the interstitial spaces of the legs, if they are emptied by the loss of gravitational forces.