Studies of the Human Cardiovascular System by Ultrasonic Methods in Space Missions: Main Results and Frontiers

Integral ultrasonic investigations of the cardiovascular system (echocardiography, 2D echography, and Doppler ultrasound of vessels) at relative rest and studies of the hemodynamic response to a lower body negative pressure (LBNP) (Doppler ultrasound of the cerebral and femoral arteries) were performed in 26 cosmonauts before launch, during missions (8 to 438 days in duration), and after landing. Overall, more than 200 investigations were made at rest and 124, during the LBNP test. It was found that the antigravity regulation of the vascular tone was gradually lost as unnecessary in microgravity. Because of the lack of gravitational stimuli, arterial resistance weakened in all vascular regions below the heart level, the blood was redistributed to areas with a lower vascular resistance, venous congestion developed in the cervicocephalic and abdominal regions, and leg veins increased in capacity and compliance. However, vital hemodynamic parameters, such as the pump function of the heart, blood supply to the brain, blood pressure, and heart rate, remained stable even during long-term exposure to microgravity. Ultrasound studies conducted during the LBNP test, which imitates the orthostatic blood distribution, showed that the resistance of the femoral arteries to blood displacement toward leg vessels degraded progressively during long-term missions. Degradation of the vasoconstriction potential of leg arteries and an increase in the capacity and compliance of leg veins contributed to the orthostatic instability developing in space. A simultaneous recording of the blood flow in cerebral and femoral arteries helps to evaluate the hemodynamic response to real and simulated orthostatic effects and to detect initial symptoms of orthostatic instability. The technique can be useful in designing novel methods to prevent orthostatic instability and assessing their efficiency.