Study objective: We determine the effect of abdominal compression on common femoral vein cross-sectional area. The effects of Valsalva maneuver and reverse Trendelenburg’s position are also studied.Methods: Prospective, observational, comparative evaluation performed on sequential patients presenting to an adult emergency department. Using ultrasonography, common femoral vein area was measured with patients in supine and reverse Trendelenburg’s positions. In each position measurements were taken at baseline while abdominal compression was performed and while the patient performed the Valsalva maneuver. Abdominal compression involved pressing with an examining hand over the patient’s right upper abdomen. Results: Fifty patients were enrolled in the study. The average common femoral vein area measured 0.89 cm2 at baseline in the supine position. It increased 0.47 cm2 (95% confidence interval [CI] 0.38 to 0.57 cm2) to 1.36 cm2 with abdominal compression and by 0.90 cm2 (95% CI 0.74 to 1.07 cm2) to 1.79 cm2 with the Valsalva maneuver. In the reverse Trendelenburg’s position, the common femoral vein area averaged 1.10 cm2, 0.22 cm2 bigger than the supine baseline area (95% CI 0.13 to 0.30 cm2). With additional abdominal compression, the vein area increased by 0.71 cm2 to 1.59 cm2 (95% CI 0.59 to 0.83 cm2). With the patient performing the Valsalva maneuver in the reverse Trendelenburg’s position, the vein increased in area 1.07 cm2 (95% CI 0.91 to 1.24 cm2) to 1.96 cm2. Conclusion: Abdominal compression increases the cross-sectional area of the common femoral vein, and the reverse Trendelenburg’s position has a further additive effect. Abdominal compression may be useful when femoral venous cannulation is attempted. In patients who can comply, the Valsalva maneuver may be even more effective. [Ann Emerg Med. 2008;52:446-452.] Copyright © 2008 by the American College of Emergency Physicians.