Inferior vena cava percentage collapse during respiration is affected by the sampling location: an ultrasound study in healthy volunteers.

OBJECTIVES Physicians are unable to reliably determine intravascular volume status through the clinical examination. Respiratory variation in the diameter of the inferior vena cava (IVC) has been investigated as a noninvasive marker of intravascular volume status; however, there has been a lack of standardization across investigations. The authors evaluated three locations along the IVC to determine if there is clinical equivalence of the respiratory percent collapse at these sites. The objective of this study was to determine the importance of location when measuring the IVC diameter during quiet respiration. METHODS Measurements of the IVC were obtained during quiet passive respiration in supine healthy volunteers. All images were recorded in B-mode, with cine-loop adjustments in real time, to ensure that maximum and minimum IVC dimensions were obtained. One-way repeated-measures analysis of variance (ANOVA) was used for comparison of IVC measurement sites. RESULTS The mean (+/-SD) percentage collapse was 20% (+/-16%) at the level of the diaphragm, 30% (+/-21%) at the level of the hepatic vein inlet, and 35% (+/-22%) at the level of the left renal vein. ANOVA revealed a significant overall effect for location of measurement, with F(2,35) = 6.00 and p = 0.006. Contrasts showed that the diaphragm percentage collapse was significantly smaller than the hepatic (F(1,36) = 5.14; p = 0.03) or renal caval index (F(1,36) = 11.85; p = 0.002). CONCLUSIONS Measurements of respiratory variation in IVC collapse in healthy volunteers are equivalent at the level of the left renal vein and at 2 cm caudal to the hepatic vein inlet. Measurements taken at the junction of the right atrium and IVC are not equivalent to the other sites; clinicians should avoid measuring percentage collapse of the IVC at this location.