Left ventricular remodeling during and after 60 days of sedentary head-down bed rest

Abstract

Short periods of weightlessness are associated with reduced stroke volume and left ventricular (LV) mass that appear rapidly and are thought to be largely dependent on plasma volume. The magnitude of these cardiac adaptations are even greater after prolonged periods of simulated weightlessness, but the time course during and the recovery from bed rest has not been previously described. We collected serial measures of plasma volume (PV, carbon monoxide rebreathing) and LV structure and function [tissue Doppler imaging, three-dimensional (3-D) and 2-D echocardiography] before, during, and up to 2 wk after 60 days of 6° head down tilt bed rest (HDTBR) in seven healthy subjects (four men, three women). By 60 days of HDTBR, PV was markedly reduced (2.7 ± 0.3 vs. 2.3 ± 0.3 liters,P< 0.001). Resting measures of LV volume and mass were ∼15% (P< 0.001) and ∼14% lower (P< 0.001), respectively, compared with pre-HDTBR values. After 3 days of reambulation, both PV and LV volumes were not different than pre-HDTBR values. However, LV mass did not recover with normalization of PV and remained 12 ± 4% lower than pre-bed rest values (P< 0.001). As previously reported, decreased PV and LV volume precede and likely contribute to cardiac atrophy during prolonged LV unloading. Although PV and LV volume recover rapidly after HDTBR, there is no concomitant normalization of LV mass. These results demonstrate that reduced LV mass in response to prolonged simulated weightlessness is not a simple effect of tissue dehydration, but rather true LV muscle atrophy that persists well into recovery.

Keywords: cardiac atrophy; cardiovascular physiology; echocardiography; space flight.

Fig. 1.

Left ventricular (LV) end systolic and LV end-diastolic volume at baseline, after 60 days of head-down tilt bed rest (HDTBR), and after 14 days of active recovery in seven subjects. Bars represent mean group values; lines represent individual subject responses.

Fig. 2.

Absolute LV mass at baseline, after 60 days of HDTBR, and after 14 days of active recovery in seven subjects. Bars represent mean group values; lines represent individual subject responses.

Fig. 3.

Percent change in LV mass from baseline with 95% confidence intervals, measured across 60 days of HDTBR and 14 days of physical reconditioning using three-dimensional echocardiography. *P < 0.01 compared with values before bed rest.

Fig. 4.

Mean percent change in LV mass and LV end-diastolic volume in bed rest studies of varying duration. See Table 4 for details about each study with regard to bed rest duration, sex of subjects, and measurement methodology.