Research on the effects of 15-day of head-down tilt bed rest on arterial hemodynamics and blood supply using Doppler ultrasound technology

Prolonged exposure to microgravity would cause cardiovascular deconditioning in astronauts, leading to physiological discomfort, decreased cognitive function, and reduced work efficiency. This study aims to explore the hemodynamic effects of short-term -6 degrees head-down tilt bed rest (HDBR) on the human circulatory system and the regulation mechanisms of blood supply to the neck and extremities. An HDBR experiment with a duration of 29 days was conducted. Doppler ultrasound was employed to quantify the blood flow spectra of the left carotid (CA), brachial (BA), radial (RA), and femoral (FA) arteries in 14 volunteers before and after HDBR. Blood flow velocity curves were obtained through edge contour extraction technology, to calculate hemodynamic parameters. After HDBR, the FA diameter significantly decreased by 0.2 mm. The resistance index (RI) of the RA, pulsatility index (PI) of the RA and PI of the FA significantly increased. The minute blood flow volume (MBF) in the CA, BA, RA, and FA significantly decreased. The proportion of total blood flow volume (PTBF) to the CA significantly increased by 4.8 %, while the PTBF to the FA significantly decreased by 4.1 %. After HDBR, the blood flow velocity, MBF, and total blood supply in the CA and extremities arteries decreased. Vasoconstriction and increased resistance in the FA led to a decreased blood supply ratio to the lower extremities and an increased ratio to the neck. This study provides a theoretical basis for the prevention of cardiovascular deconditioning and the establishment of targeted countermeasures, which are significant for enhancing astronauts' physical performance.