Breathing affects venous return from legs in humans.

It is conventionally believed that venous return to the heart increases during inspiration through the combined effect of the fall in intrathoracic pressure and the squeezing of the splanchnic veins by diaphragm descent. In the present study we have tested the hypothesis that in humans venous return from the legs decreases, rather than increases, during inspiration as a result of the rise in abdominal pressure. Ten normal subjects were examined in recumbent posture during various respiratory maneuvers. Venous return from the legs was monitored using an ultrasonic flow recorder (Doppler effect) placed over the femoral veins. Quiet inspiration was associated with a fall in femoral venous blood flow which, for the 10 subjects, averaged (mean +/- SE) 65 +/- 11% of the end-expiratory value. There was, however, a substantial variability between subjects, which could be largely explained by differences in abdominal pressure swings (delta Pab). In fact when the subjects voluntarily changed their patterns of inspiratory muscle use, the amount of fall in femoral blood flow during inspiration varied in proportion to the diaphragmatic contribution to tidal volume and delta Pab; i.e., flow decreased more as the diaphragm contributed more to inspiration and the rise in Pab was greater. During "pure" diaphragmatic breathing flow always ceased completely, whereas during predominantly rib cage inspiration flow increased rather than decreased. Isovolume belly-in maneuvers and gentle external compression of the abdomen also caused cessation of femoral blood flow, indicating that diaphragmatic contraction is not mandatory for venous return from the legs to be impeded during inspiration.(ABSTRACT TRUNCATED AT 250 WORDS)