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A review of the literature revealed conflicting reports on the physiologic variations of the caliber of the normal inferior vena cava as seen by sonography. In this study, 25 normal volunteers were examined using dynamic scanning, and the upper parts of their inferior vena cavas were observed for changes during various phases of respiration as well a Valsalva maneuver. Optimum distension and, therefore, optimum visualization of the inferior vena cava were achieved after simple breath holding or at end expiration. Inspiration and Valsalva maneuver decreased the size of this vessel in most subjects, making if difficult to see.
Grant et al. (Fri,) studied this question.