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Diastolic suction is impaired by bed rest: MRI tagging studies of diastolic untwisting

¹Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, Texas; ²Division of Cardiology, Department of Internal Medicine, Johns Hopkins Medical Institutions, Baltimore, Maryland; ³Department of Radiology, and ⁴Division of Cardiology, Departments of Internal Medicine and Radiology, and ⁵Division of Cardiology, Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas

Submitted 3 August 2006 ; accepted in final form 11 January 2008

Bed rest deconditioning leads to physiological cardiac atrophy, which may compromise left ventricular (LV) filling during orthostatic stress by reducing diastolic untwisting and suction. To test this hypothesis, myocardial-tagged magnetic resonance imaging (MRI) was performed, and maximal untwisting rates of the endocardium, midwall, and epicardium were calculated by Harmonic Phase Analysis (HARP) before and after –6° head-down tilt bed rest for 18 days with (n = 14) and without exercise training (n = 10). LV mass and LV end-diastolic volume were measured using cine MRI. Exercise subjects cycled on a supine ergometer for 30 min, three times per day at 75% maximal heart rate (HR). After sedentary bed rest, there was a significant reduction in maximal untwisting rates of the midwall (–46.8 ± 14.3 to –35.4 ± 12.4 °/s; P = 0.04) where untwisting is most reliably measured, and to a lesser degree of certainty in the endocardium (–50.3 ± 13.8 to –40.1 ± 18.5 °/s; P = 0.09); the epicardium was unchanged. In contrast, when exercise was performed in bed, untwisting rates were enhanced at the endocardium (–48.4 ± 20.8 to –72.3 ± 22.3 °/ms; P = 0.05) and midwall (–39.2 ± 12.2 to –59.0 ± 19.6 °/s; P = 0.03). The differential response was significant between groups at the endocardium (interaction P = 0.02) and the midwall (interaction P = 0.004). LV mass decreased in the sedentary group (156.4 ± 30.3 to 149.5 ± 27.9 g; P = 0.07), but it increased slightly in the exercise-trained subjects (156.4 ± 34.3 to 162.3 ± 40.5 g; P = 0.16); (interaction P = 0.03). We conclude that diastolic untwisting is impaired following sedentary bed rest. However, exercise training in bed can prevent the physiological cardiac remodeling associated with bed rest and preserve or even enhance diastolic suction.

cardiac atrophy; bed rest deconditioning; magnetic resonance imaging with myocardial tagging; exercise; spaceflight