Measurement of Leg Blood Volume Change by Impedance Plethysmography with Special Reference to Microgravity Simulation Testing

Abstract

Exposure of the human circulatory system to microgravity causes a headward shift of blood and leads to an increase in venous return, cardiac output, heart rate and systolic and mean pressures, together with a decrease in diastolic pressure. In the resultant adjustment, the total blood volume is decreased. The adaptation of the circulatory system to microgravity proceeds fairly smoothly, but some symptomatic cardiovascular dysfunctions are reported to occur when astronauts return to earth. The causes which provoke these symptoms are considered to be not only blood volume reduction, but also the change of cardiovascular control mechanisms, such as modification of the baroreceptor reflex. Orthostatic intolerance is known as one of the marked symptoms and this is considered to be provoked primarily by the blood shift to the lower extremities upon standing in the gravitational state. Thus, in order to analyse the adaptation and the readaptation of the circulatory system to microgravity and to normal gravity, it is important to study the dynamics of the blood shift to the lower extremities (Gaffey, 1985; Thornton et al., 1977). A multi-voltage-channel impedance plethysmograph has been developed for this purpose and applied in an orthostatic intolerance test following a microgravity simulation experiment.