Arterial pressure in humans during weightlessness induced by parabolic flights

Results from our laboratory have indicated that, compared with those of the l-G supine (Sup) position, left atrial diameter (LAD) and transmural centr al venous pressure increase in humans during weightlessness (0 G) induced b y parabolic flights (R. Videbaek and P. Norsk. J. Appl. Physiol. 83: 1862-1 866, 1997). Therefore, because cardiopulmonary low-pressure receptors are s timulated during 0 G, the hypothesis was tested that mean arterial pressure (MAP) in humans decreases during 0 G to values below those of the 1-G Sup condition. When the subjects were Sup, 0 G induced a decrease in MAP from 9 3 +/- 4 to 88 +/- 4 mmHg (P < 0.001), and LAD increased from 30 +/- 1 to 33 +/- 1 mm (P < 0.001). In the seated position, MAP also decreased from 93 /- 6 to 87 +/- 5 mmHg (P < 0.01) and LAD increased from 28 +/- 1 to 32 +/- 1 mm (P < 0.001). During l-G conditions with subjects in the horizontal lef t lateral position, LAD increased compared with that of Sup (P < 0.001) wit h no further effects of 0 G. In conclusion, MAP decreases during short-term weightlessness to below that of l-G Sup simultaneously with an increase in LAD. Therefore, distension of the heart and associated central vessels dur ing 0 G might induce the hypotensive effects through peripheral vasodilatat ion. Furthermore, the left lateral position in humans could constitute a si mulation model of weightlessness.