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Am J Physiol Heart Circ Physiol (January 31, 2002). doi:10.1152/ajpheart.00862.2001
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Articles in PresS, published online ahead of print January 31, 2002
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00862.2001
Submitted on October 3, 2001
Accepted on January 28, 2002

Attenuated Sympathetic Nerve Responses After 24-Hours of Bed Rest

Mazhar H Khan1, Allen R Kunselman2, Urs A Leuenberger1, William R Davidson1, Chester A Ray1, Kristen S Gray3, Cynthia S Hogeman1, and Lawrence I Sinoway3*

1 Medicine, Division of Cardiology, Penn State University, Hershey, PA, USA
2 Health Evaluation Sciences, Penn State University, Hershey, PA, USA
3 Medicine, Division of Cardiology, Penn State University, Hershey, PA, USA; Lebanon VA Medical Center, Lebanon, PA, USA

* To whom correspondence should be addressed. E-mail: lsinoway{at}psu.edu.

Bed rest reduces orthostatic tolerance (OT). 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-hours of bed rest. LBNP allows for baroreceptor disengagement in a graded fashion. We measured heart rate, cardiac output (HR x SV 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 < .023), heart rate was greater (P < .002), cardiac output was unchanged and the ability to augment MSNA at high levels of LBNP was reduced (rate of rise for 30 to 60 mmHg LBNP before bed rest .073 bursts.min-1.mmHg-1; after bed rest .035 bursts.min-1.mmHg-1; P, < .016). These findings suggest that 24 hours of bed rest reduces sympathetic nerve responses to LBNP.




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