ENHANCED SLOW CAUDAD FLUID SHIFTS IN ORTHOSTATIC INTOLERANCE AFTER 24-H BED-REST

To evaluate mechanisms of late orthostatic intolerance, slow fluid shi fts along the body axis were studied during deconditioning by 24-h bed -rest and during 13-min upright tilts before and after this manoeuvre. In 11 healthy male subjects the fluid volumes of a thorax and a calf segment (impedance plethysmography) as well as tissue thickness at the forehead and the tibia (miniature ultrasonic plethysmograph) were rec orded. Cardiovascular performance was monitored by recording heart rat e (electrocardiogram), brachial and finger arterial pressure (by the R iva Rocci method and by the Finapres technique) as well as stroke volu me (by impedance cardiography). Bed-rest led to a cephalad fluid shift with a mean interstitial leg dehydration of 2.2 ml.100 ml(-1) with no changes in body mass and plasma volume. No syncope during the tilt oc curred before bed-rest, while after bed-rest 8 subjects fainted betwee n min 2.1 and 9.0 of the tilt. Bed-rest resulted in an augmented initi al heart rate response to tilting which was similar in all subjects. I n later orthostasis, bed-rest caused two- to threefold faster caudad f luid shifts with higher calf filtration rates in fainters (prior to hy potension) than in nonfainters. Through bed-rest the estimated extrava sation within 10 min into general lower body tissue spaces increased b y 192 ml in (late) fainters as opposed to only 23 ml in nonfainters. I t was concluded that contributing factors to orthostatic intolerance m ay be slow transcapillary fluid shifts which are easily underestimated and whose quantity and time course call for further investigation aft er various deconditioning manoeuvres. In particular, the postflight fl uid shifts in astronauts who will have markedly dehydrated legs, may i mpose a circulatory stress which needs to be evaluated. In general, th e filtration rate in relevant areas appears to be an integrative and e asily determined parameter, reflecting hormonal and neurogenic vascula r as well as local interstitial control of the Starling forces.