(Circulation. 1997;96:517-525.)
© 1997 American Heart Association, Inc.
Articles |
From the Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas (Tex) and the University of Texas Southwestern Medical Center at Dallas.
Correspondence to Benjamin D. Levine, MD, Director, Institute for Exercise and Environmental Medicine, 7232 Greenville Ave, Dallas, TX 75321. E-mail Levineb{at}wpmail.phscare.org
Background The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characterized by an excessive fall in stroke volume (SV) in the upright position. We hypothesized that this large fall in SV is due to a change in cardiac mechanics.
Methods and Results We measured pulmonary capillary wedge pressure (PCWP), SV, left ventricular end-diastolic volume (LVEDV), and left ventricular mass (by echocardiography) at rest, during lower-body negative pressure, and after saline infusion before and after 2 weeks of bed rest with -6° head-down tilt (n=12 subjects aged 24±5 years). Pressure (P)-volume (V) curves were modeled exponentially by P=aekV+b and logarithmically by P=-Sln[(Vm-V)/(Vm-V0], where V0 indicates volume at P=0, and the constants k and S were used as indices of normalized chamber stiffness. Dynamic stiffness (dP/dV) was calculated at baseline LVEDV. The slope of the line relating SV to PCWP during lower-body negative pressure characterized the steepness of the Starling curve. We also measured plasma volume (with Evans blue dye) and maximal orthostatic tolerance. Bed rest led to a reduction in plasma volume (17%), baseline PCWP (18%), SV (12%), LVEDV (16%), V0 (33%), and orthostatic tolerance (24%) (all P<.05). The slope of the SV/PCWP curve increased from 4.6±0.4 to 8.8±0.9 mL/mm Hg (P<.01) owing to a parallel leftward shift in the P-V curve. Normalized chamber stiffness was unchanged, but dP/dV was reduced by 50% at baseline LVEDV, and cardiac mass tended to be reduced by 5% (P<.10).
Conclusions Two weeks of head-downtilt bed rest leads to a smaller, less distensible left ventricle but a shift to a more compliant portion of the P-V curve. This results in a steeper Starling relationship, which contributes to orthostatic intolerance by causing an excessive reduction in SV during orthostasis.
Key Words: cardiac output pressure cardiac volume diastole syncope elasticity
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