Aims: Prolonged immobilization generates cardiac deconditioning, a risk factor for cardiovascular disease. Our aim was to assess the effects of long-term strict head-down (-6 degrees) bed-rest (BR) deconditioning on ascending aortic flow and cerebral blood flow by Phase Contrast (PC) MRI, capable to provide in vivo quantitative blood flow assessment.
Methods: Twelve healthy male volunteers (mean age 28±6) were enrolled. The experiment was conducted at: envihab (Koln, DLR, Germany) as part of the European Space Agency BR studies. PC-MRI images (3T Biograph mMR, Siemens) with interleaved three-directional velocity encoding (VENC: x and y: 80 cm/s; z: 150 cm/s) were obtained transecting the ascending aorta, on top of the aortic root (spatial resolution 1.4 x 1.4 mm2), and after the carotid bifurcation, before (PRE) and after 58-days (HDT58) of BR.
The resulting planar magnitude data and three-directional velocity images were semi-automatically analysed with previously validated custom software to segment frame-by-frame the aortic area lumen (AL), thus providing the region in which to compute the following parameters from velocity images: cardiac output (CO), stroke volume (SV), mean flow velocity (mFV), flow rate (Qpeak), time-to-peak flow, systolic duration and heart beat duration (RR).
Results: After 58-days BR, compared to baseline values, a significant (p<.05, paired t-test) shortening of the RR (14%) and systolic phase (10%), with a decrease in CO (8%), SV (22%), Qpeak (12%), mFV (9%) were observed. Interestingly, CO and SV to the brain, computed as difference between flow before and after carotid bifurcation, did not change (2.3±0.8 vs 2.5±0.6 l/min, and 37±13 vs 36±7 ml, respectively).
Results
| RR | Systole | CO | SV | Qpeak | Mean flow velocity | Aortic lumen area | |
|---|---|---|---|---|---|---|---|
| (msec) | (msec) | (l/min) | (ml) | (l/min) | (cm/s) | (mm2) | |
| PRE | 988±114 | 390±31 | 7.01±1.2 | 115±18 | 33.4±4.4 | 26±3 | 686±84 |
| HDT58 | 846±113* | 352±47* | 6.45±0.8* | 90±9* | 29.4±2.5* | 24±3* | 667±91 |
| RR | Systole | CO | SV | Qpeak | Mean flow velocity | Aortic lumen area | |
|---|---|---|---|---|---|---|---|
| (msec) | (msec) | (l/min) | (ml) | (l/min) | (cm/s) | (mm2) | |
| PRE | 988±114 | 390±31 | 7.01±1.2 | 115±18 | 33.4±4.4 | 26±3 | 686±84 |
| HDT58 | 846±113* | 352±47* | 6.45±0.8* | 90±9* | 29.4±2.5* | 24±3* | 667±91 |
*p<0.05 paired t-test.
Conclusions: This is the first study addressing aortic morphology and flow using PC-MRI during BR experiments. Cardiac adaptation to deconditioning due to immobilization resulted in a reduction of aortic outflow, while cerebral blood flow resulted preserved. This information could be useful to test effectiveness of potential countermeasures to reduce cardiac deconditioning in space research, and for better understanding physiologic changes in patients undergoing long periods of immobilization.
Acknowledgement/Funding: We acknowledge the support of Italian Space Agency (contracts 2013.032.R.0 and 2013.033.R.0, recipient E.G.Caiani)
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