Perhonen M A, Zuckerman J H, Levine B D
Institute for Exercise and Environmental Medicine, Presbyterian Hospital and University of Texas Southwestern Medical Center, Dallas, Texas, USA.
Circulation. 2001 Apr 10;103(14):1851-7. doi: 10.1161/01.cir.103.14.1851.
Orthostatic intolerance after bed rest is characterized by hypovolemia and an excessive reduction in stroke volume (SV) in the upright position. We studied whether the reduction in SV is due to a specific adaptation of the heart to head-down tilt bed rest (HDTBR) or acute hypovolemia alone.
We constructed left ventricular (LV) pressure-volume curves from pulmonary capillary wedge pressure and LV end-diastolic volume and Starling curves from pulmonary capillary wedge pressure and SV during lower body negative pressure and saline loading in 7 men (25+/-2 years) before and after 2 weeks of -6 degrees HDTBR and after the acute administration of intravenous furosemide. Both HDTBR and hypovolemia led to a similar reduction in plasma volume. However, baseline LV end-diastolic volume decreased by 20+/-4% after HDTBR and by 7+/-2% after hypovolemia (interaction P<0.001). Moreover, SV was reduced more and the Starling curve was steeper during orthostatic stress after HDTBR than after hypovolemia. The pressure-volume curve showed a leftward shift and the equilibrium volume of the left ventricle was decreased after HDTBR; however, after hypovolemia alone, the curve was identical, with no change in equilibrium volume. Lower body negative pressure tolerance was reduced after both conditions; it decreased by 27+/-7% (P<0.05) after HDTBR and by 18+/-8% (P<0.05) after hypovolemia.
Chronic HDTBR leads to ventricular remodeling, which is not seen with equivalent degrees of acute hypovolemia. This remodeling leads to a greater decrease in SV during orthostatic stress after bed rest than hypovolemia alone, potentially contributing to orthostatic intolerance.
卧床休息后的直立不耐受表现为血容量不足以及直立位时心搏量（SV）过度降低。我们研究了心搏量降低是由于心脏对头低位卧床休息（HDTBR）的特定适应性改变还是仅由于急性血容量不足。
我们在7名男性（25±2岁）-6度HDTBR 2周前后以及静脉急性给予速尿后，根据肺毛细血管楔压和左心室舒张末期容积构建左心室（LV）压力-容积曲线，并根据肺毛细血管楔压和下体负压及生理盐水负荷期间的心搏量构建斯塔林曲线。HDTBR和血容量不足均导致血浆量出现类似程度的减少。然而，HDTBR后基线左心室舒张末期容积降低了20±4%，血容量不足后降低了7±2%（交互作用P<0.001）。此外，HDTBR后直立应激期间心搏量降低更多，斯塔林曲线更陡峭。压力-容积曲线显示HDTBR后向左移位，左心室平衡容积减小；然而，仅血容量不足后，曲线相同，平衡容积无变化。两种情况后下体负压耐受性均降低；HDTBR后降低了27±7%（P<0.05），血容量不足后降低了18±8%（P<0.05）。
慢性HDTBR导致心室重塑，同等程度的急性血容量不足则未出现这种情况。这种重塑导致卧床休息后直立应激期间的心搏量降低幅度大于仅血容量不足时，这可能是直立不耐受的原因之一。
