Objective: This study evaluated the impact of prolonged circulatory unloading and subsequent left ventricular (LV) mass decrease on LV myocardial performance. Five-week head-down tilt (-6°) bed rest (HDTBR) was used as a model of prolonged circulatory unloading.
Methods: Ten young healthy male volunteers (age 23 ± 2 years) were studied a day before and within the first 24 h after the end of HDTBR by two-dimensional Doppler echocardiography and carotid applanation tonometry. LV preload and afterload, cardiac workload, LV mass and wall stress, LV chamber function and diastolic filling were assessed. Longitudinal, radial and circumferential myocardial strain rate were evaluated by tissue tracking algorithm.
Results: After HDTBR, stroke volume (P < 0.01), stroke work (P = 0.01) and LV mass (P < 0.001) decreased, whereas relative wall thickness, peak and end-systolic wall stress and ejection fraction remained unchanged. HDTBR was also followed by a decrease in longitudinal systolic strain rate (-1.11 ± 0.05 vs. -1.00 ± 0.05 s, P = 0.02) and a prolongation of isovolumic relaxation time (IVRT) (74 ± 2 vs. 82 ± 3 ms, P < 0.01). Bed rest-induced changes in LV mass index were directly related to changes in stroke work index (r = 0.65, P < 0.05), and changes in longitudinal systolic strain rate and IVRT correlated with changes in stroke volume index, directly and inversely, respectively (r = 0.69 and -0.64, P < 0.05 for both).
Conclusion: A decrease in LV mass following HDTBR parallels the reduction in cardiac workload and is associated with an attenuation of longitudinal systolic myocardial deformation rate and prolongation of IVRT that seem to reflect a functional adaptation of cardiac muscle to lower LV volume load.