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P874Mitral and aortic flow adaptation to 58-days head-down bed-rest assessed by PC-MRI, and effectiveness of high-intensity jump training countermeasure

Aims: Prolonged immobilization generates cardiac deconditioning, a cardiovascular disease risk factor, and efficient countermeasures (CM) are needed to prevent it. We aimed to assess by Phase-Contrast (PC) MRI the effects of long-term strict head-down (-6 degrees) bed-rest (BR) deconditioning and the effectiveness of high-intensity jump training CM on aortic and mitral flow. 
Methods: 23 males (29±6 years, 181±6 cm, 77±7 kg) were enrolled. The experiment was conducted at Envihab research facility as part of the European Space Agency BR studies. Participants were randomly allocated to the jump training group (JUMP, n=12) or to the control group (CTRL, n=11). A typical training session consisted of 4x10 countermovement jumps and 2x10 hops in a horizontal sledge jump system, with 5–6 sessions/week. PC-MRI images (3T Biograph mMR) with interleaved 3-directional velocity encoding (VENC: x and y: 80 cm/s; z: 150 cm/s) were obtained (spatial resolution 1.4x1.4 mm2) at the level of the aortic root, and of the mitral plane, before (PRE) and after 58-days (HDT58) of BR. The resulting planar magnitude data and 3-directional velocity images were semi-automatically analysed with previously validated custom software to compute the following parameters: cardiac output (CO), stroke volume (SV), flow rate (Qpeak), systolic duration (Sys) and heart beat duration (RR), rapid filling (Efill) and inflow rate (Epeak). 
Results: In CTRL, compared to baseline values, at HDT58 a significant (p<.05, paired t-test) RR (14%) and systolic (10%) shortening, with a decrease in CO (8%), SV (22%), Qpeak (12%), Efilling (26%), Epeak (26%) were observed. In CM, only RR was shortened (8%), together with a decrease in SV (12%), Qpeak (7.5%), Efilling (11%), Epeak (15%). 
Conclusions: This is the first study addressing aortic and mitral flow using PC-MRI during BR. Cardiac adaptation to deconditioning due to prolonged immobilization resulted in a reduction of aortic outflow and mitral inflow. The applied CM appeared only partially effective in opposing this phenomenon. This information could be useful to better understand physiologic changes in patients undergoing long periods of immobilization, and to improve countermeasures to reduce cardiac deconditioning. 
Acknowledgement/Funding: This research was supported by the Italian Space Agency (contract 2013-064-R.0, recipient EG Caiani)