I think 微量返流 here means that the back flow of blood to the tricuspid valve is less than 10%.

Tricuspid regurgitation was graded as follows: 1 = trace, <10%; 2 = mild, 10%-24%; 3 = moderate, 25%-50%; and 4 = severe, >50%.
http://jdc.jefferson.edu/surgeryfp/10/

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Note added at 1 hr (2008-06-09 13:01:29 GMT)
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［摘 要］目的：观察三尖瓣返流（TR）患者的股总静脉（DFV）多谱勒血流频谱变化，探讨其与TR程度的关系。方法：对TR患者32例（其中轻度18例，中度8 例，重度6例）及21例正常人进行股总静脉超声多普勒探测。测量CFV血流频谱参数：最低（逆向血流）流速（Va）、逆向血流持续时间（Ta）。结果：轻度TR组血流频谱参数与正常组比较无显著意义。中、重度TR组Va、Ta与正常组及轻度TR组比较差异有显著性（P<0.05）。结论：股总静脉的 Vmin、Ta与TR的程度密切相关，可作为判定右房压力升高的可靠依据。

Abstract:Objective To explorethe Doppler Flow Spectrum of Common Femoral Veins in patients with tricuspid regurgitation,Study on relationship between changes of it and degrees of TR.Methods To detect the characteristics of Doppler Flow spectrum of common femoral veins (CFV) 21 normal subjects and 32 adults with different degrees of tricuspid regurgitation (TR) The Doppler parameters of CFV:the minimum flow velocity (Va) and duration time (Ta) were obtained.Results The CFV flow spectrum of mild TR group was similar to that of control group and was modulated by respiratory movement.There is in moderate,severe TR group Va and Ta compared with normal and mild TR group (P<0.01).Conclusion There is veryrelationship between changes of the CFV Va Ta and degrees of TR.It seems to be the dependent methods to judge of the increasing of right atria pressure.
http://www.51lunwen.com/details/lw200803191237188543.html

Therefore, "mild regurgitation" should be 轻度返流 instead of 微量返流.

micro: 有“微量”之意。在医学方面，“微量测定”译为“microdetermination”、“微量呼吸计”译为“microrespirometer”、“微量滴定管”译为“microburet”等等。医学方面，“微量”译为“micro”比比皆是。