Mechanical deconditioning of the heart due to long-term bed rest as observed on seismocardiogram morphology

Abstract

During head-down tilt bed rest (HDT) the cardiovascular system is subject to headward fluid shifts. The fluid shift phenomenon is analogous to weightlessness experienced during spaceflight microgravity. The purpose of this study was to investigate the effect of prolonged 60-day bed rest on the mechanical performance of the heart using the morphology of seismocardiography (SCG). Three-lead electrocardiogram (ECG), SCG and blood pressure recordings were collected simultaneously from 20 males in a 60-day HDT study (MEDES, Toulouse, France). The study was divided into two campaigns of ten participants. The first commenced in January, and the second in September. Signals were recorded in the supine position during the baseline data collection (BDC) before bed rest, during 6° HDT bed rest and during recovery (R), post-bed rest. Using SCG and blood pressure at the finger, the following were determined: Pulse Transit Time (PTT); and left-ventricular ejection time (LVET). SCG morphology was analyzed using functional data analysis (FDA). The coefficients of the model were estimated over 20 cycles of SCG recordings of BDC12 and HDT52. SCG fiducial morphology AO (aortic valve opening) and AC (aortic valve closing) amplitudes showed significant decrease between BDC12 and HDT52 (p < 0.03). PTT and LVET were also found to decrease through HDT bed rest (p < 0.01). Furthermore, PTT and LVET magnitude of response to bed rest was found to be different between campaigns (p < 0.001) possibly due to seasonal effects on of the cardiovascular system. Correlations between FDA and cardiac timing intervals PTT and LVET using SCG suggests decreases in mechanical strength of the heart and increased arterial stiffness due to fluid shifts associated with the prolonged bed rest.

Fig. 1. HDT schematic of sensor placement.

SCG (yellow rectangle) placed on the xiphoid process. Blood pressure measured at the finger (orange rectangle). ECG Lead II shown RA lead (gray circle) on right clavicle, RL lead (dark blue circle) on lower right rib cage and LL (light blue circle) on lower left rib cage.

Fig. 2. Blood pressure and seismocardiogram waveforms with annotations.

Pulse Transit Time (PTT) is the time interval between the aortic valve opening (AO) peak of SCG and RP (Foot) of BP. LVET is the time interval between the AO and AC peak of the SCG.

Fig. 3. FDA analysis of SCG signals containing AO and AC basis function coefficient sets pre- and post-HDT.

Average AO and AC peak decrease over 20 cardiac cycles after 52 days head-down tilt bed rest compared to pre-HDT. Morphology spline coefficient sets corresponding to the AO (sets 4, 5 and 6) and AC (sets 14, 15, and 16) peak complexes show distributions from pre-HDT to day 52 HDT furthering the peak changes. Coefficient sets define the spline basis functions shown to describe the SCG signal morphology. Boxes represent ±1 interquartile range, whiskers represent ± 1.5 interquartile range and center lines are medians.

Fig. 4. Cardiovascular function through the three phases of bed rest.

Cardiovascular timing intervals of PTT and LVET were taken from the relationships of SCG and show decreasing trends. PTT has a drastic average decrease that does not recover after 8 days post bed rest. LVET has a variable adjustment to fluid shifts of bed rest but decreases towards the end with a slight recovery. Blood pressure values adjust to fluid shifts with an initial decrease but stabilize towards the end of bed rest. Upper values in the plot represent systolic BP and lower values, diastolic BP. Boxes represent ± 1 interquartile range, whiskers represent ± 1.5 interquartile range and center lines are medians. * Denotes significant differences compared to BDC12 and † denotes significance between campaigns at each test day (p < 0.05).

Fig. 5. PTT comparison through bed rest phases and campaign differences.

Rapidly quickening of PTT to the finger upon entrance into HDT through the end of bed rest suggesting increased arterial stiffness due to responses of fluid shifts. The value does not recover 8 days post bed rest. Due to seasonal temperature changes campaign 1 had shown more drastic decrease in pulse transit time due potentially to vascular vaso-controlled responses. Data is represented as mean ± standard deviation. * Denotes significant differences compared to BDC12 and † denotes significance between campaigns at each test day (p < 0.05).