Purpose: To explore the relationship between the shape of the inferior vena cava (IVC) lumen and central venous pressure (CVP). Methods: In 60 patients undergoing mechanical ventilation and CVP monitoring in the Intensive Care Unit of Peking Union Medical College Hospital from July to October 2016, we measured with B-mode ultrasonography the transverse maximum (MXD) and minimum diameter (MID) of the IVC at end expiration, and calculated the diameter ratio (DR) as MXD/MID. Patients were divided into three groups according to CVP: low (CVP < 8 mm Hg), intermediate (8 mm Hg <= CVP <= 10 mm Hg), and high (CVP > 10 mm Hg). Results: MXD was 2.32 +/- 0.41 cm, MID was 1.41 +/- 0.40 cm, and DR was 1.76 +/- 0.49. CVP was 9.27 +/- 2.99 mm Hg. DR correlated with CVP (r = -0.527, P < .001). The low-CVP group had greater dispersion of DR values, with a large variety in IVC shape (elliptical, irregular, teardrop-shaped, partially collapsed...). The area under the ROC curve for predicting CVP with DR, with a CVP threshold of 8 mm Hg was 0.835 (95% CI, 0.726-0.945; P<.05). With a DR cutoff value of 1.76, sensitivity was 0.765 and specificity was 0.781. Conclusions: DR above 1.7 is predictive of CVP < 8 mm Hg.