Friedman P J, Peters R M, Botkin M C, Brimm J E, Meltvedt R C
Crit Care Med. 1986 Mar;14(3):182-7. doi: 10.1097/00003246-198603000-00002.
Gravity has significant effects on the microvascular pressure in the lungs and thus on regional fluid filtration in the lungs. Below the level of the heart, gravity increases the microvascular pressure with respect to the left atrial pressure; above the level of the heart, microvascular pressure is less than atrial pressure. To assess the contribution of gravity to fluid filtration in the lungs independent of left-heart filling pressure, the distribution of lung volume above and below the left atrium must first be determined. To permit calculation of the contribution of hydrostatic pressure per unit of lung volume, 26 computed tomographic scans of the entire chest were traced and measured, marking the level of the center of the left atrium (LAL) on each slice. The intrathoracic volume above and below the left atrium was measured by multiplying scan slice thickness by the area of the lung above or below the LAL measured on each scan slice. On 16 scans, intrathoracic volumes of 1-cm horizontal layers of lung above and below the LAL were also calculated by measuring corresponding segments of the transverse scan slices. The calculations indicate that in the supine posture more lung is dependent than when upright, and that elevation of a patient to 30 degrees reduces the volume of lung below the LAL nearly as much as does the upright posture.
重力对肺部微血管压力有显著影响，进而影响肺部局部的液体滤过。在心脏水平以下，相对于左心房压力，重力会增加微血管压力；在心脏水平以上，微血管压力低于心房压力。为了评估重力对肺部液体滤过的影响，且不受左心充盈压的干扰，必须首先确定左心房上下肺容积的分布情况。为了能够计算每单位肺容积中静水压力的贡献，对26例全胸部计算机断层扫描图像进行了描记和测量，在每个层面上标记出左心房中心（LAL）的位置。通过将扫描层厚乘以每个扫描层面上LAL上方或下方肺的面积，来测量左心房上下的胸腔容积。在16次扫描中，还通过测量横向扫描层面的相应节段，计算了LAL上下1厘米水平肺层的胸腔容积。计算结果表明，仰卧位时比直立位时更多的肺处于低垂部位，并且将患者抬高至30度时，LAL以下的肺容积减少量几乎与直立位时相同。
