Limited Effect of 60-Days Strict Head Down Tilt Bed Rest on Vascular Aging

Figures

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  FIGURE 1. Illustration of the study design. During 14-day baseline data collection (BDC), subjects were ambulatory but supine for measurements. Shortly before the onset of the 60-day head down tilt period (HDT), subjects were assigned to three different intervention groups named control (Ctr, no artificial gravity), continuous artificial gravity (cAG), and intermittent artificial gravity (iAG). We obtained magnetic resonance imaging (MRI) measurements 9 days before (BDC-9), and on the 56th day of HDT (HDT56). We conducted echocardiography and assessed hemodynamics (blood pressure, pulse wave velocity, etc.) on BDC-6 and on HDT60.

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  FIGURE 2. Data acquisition for vascular measurements. Left panel: Recording sites and length (L) determination for brachial-femoral pulse wave velocity (bfPWV) assessment. Right panel – top: ECG and Doppler flow curves in the left ventricular outflow tract. We measured isovolumetric contraction time (ICT) from the R-peak to aortic valve opening indicated by flow onset. Right panel – bottom: Pulse wave arrival time (PAT) is defined as the time from the ECG R-Peak to pulse wave arrival at a certain landmark. By dividing L by pulse transit time (PTT) between arm and thigh cuffs, we obtained bfPWV. We obtained measurements from every heartbeat recorded during a 60-s period. For the sake of clarity, PAT and PTT are shown for two consecutive heart beats. We calculated corrected PAT (cPAT) by subtracting ICT from PAT.

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  FIGURE 3. Line graph for individual brachial-femoral pulse wave velocities (bfPWV) 6 days before (PRE, dot symbol) and after 60 days HDTBR (HDT, square symbol). Scaled-up single symbols to the left and right represent corresponding mean values and error bars indicate the first standard deviation. bfPWV did not change with 60 days HDTBR (9.0 ± 1.2 vs. 9.2 ± 1.3 m/s, p = 0.864).

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  FIGURE 4. Pulse wave arrival times (PAT) increase with greater topological distance from the heart to arm, finger, thigh, and toe, all indicated by subscripted acronyms. PATArm and PATThigh significantly increased after 60 days HDTBR (black bars) compared to baseline recordings 6 days before HDTBR (gray bars). Isovolumetric contraction time (ICT) also increased with HDTBR. Corrected PAT did not change with HDTBR. Results are represented as mean values from 24 subjects with error bars indicating the first standard deviation; * < 0.05.

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  FIGURE 5. Exploratory analysis relating the HDTBR response to age. We stratified participants in three age groups (AgeGr.): AgeGr. 1 = 24–27 years; AgeGr. 2 = 29–36 years, and AgeGr. 3 = 37–55 years. (A) heart rate, (B) systolic blood pressure (SBP), (C) aortic compliance (AOC), and corrected pulse wave arrival times to the (D) arm (cPATArm), (E) thigh (cPATThigh), and (F) finger (cPATFinger). The p-values indicate the interaction between AgeGr. and bed rest. Results are represented as mean values ± standard deviation from base line recordings (PRE, dot symbol) and HDT (square symbol). Significant difference between these time points is indicated by †(p < 0.05) and ‡(p < 0.001).