Attenuated sympathetic nerve responses after 24 hours of bed rest

1 Division of Cardiology and 2 Department of Health Evaluation Sciences, Pennsylvania State University College of Medicine, Milton S. Hershey Medical Center, Hershey 17033; and 3 Lebanon Department of Veterans Affairs Medical Center, Lebanon, Pennsylvania 17042 Bed rest reduces orthostatic tolerance. Despite decades of study, the cause of this phenomenon remains unclear. In this report we examined hemodynamic and sympathetic nerve responses to graded lower body negative pressure (LBNP) before and after 24 h of bed rest. LBNP allows for baroreceptor disengagement in a graded fashion. We measured heart rate (HR), cardiac output (HR × stroke volume obtained by echo Doppler), and muscle sympathetic nerve activity (MSNA) during a progressive and graded LBNP paradigm. Negative pressure was increased by 10 mmHg every 3 min until presyncope or completion of 60 mmHg. After bed rest, LBNP tolerance was reduced in 11 of 13 subjects ( P