- Top of page
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.
ACADEMIC EMERGENCY MEDICINE 2010; 17:96–99 © 2009 by the Society for Academic Emergency Medicine
Determination of intravascular volume is an important step in the initial resuscitation and subsequent management of critically ill patients. Physicians are unable to reliably determine intravascular volume status through the clinical examination1 and have pursued both invasive and noninvasive means of estimating intravascular volume. Respiratory variation in the anteroposterior diameter of the inferior vena cava (IVC) has been investigated as a noninvasive marker of intravascular volume status, correlating its percentage collapse to central venous pressure. However, despite more than 30 years of interest in this parameter, there has been a lack of standardization across investigations. Studies have included healthy volunteers,2 continuous ambulatory peritoneal dialysis patients,3 hemodialysis patients,4,5 healthy volunteers at phlebotomy centers,6 and critically ill patients in intensive care units.7,8 In addition, the locations of IVC measurements differ across studies9–19 (Table 1).
Table 1. IVC Measurement Sites
|Reference||IVC Measurement Site|
|Barbier7||Just upstream of the origin of the suprahepatic vein|
|Blehar9||Immediately inferior to the confluence of the hepatic vein inlet|
|Brennan4||Within 2.5 cm of the caval–RA junction|
|Brennan10||Within 2 cm of the caval–RA junction|
|Feissel8||Approximately 3 cm from the RA|
|Grant2||Actual measurements in centimeters were not recorded|
|Kircher11||Within 2 cm of the caval–RA junction|
|Lichtenstein12||Left renal vein|
|Lyon6||2 cm distal of the IVC–hepatic vein junction|
|Minutiello13||Within 2 cm of the right atrium origin of the IVC|
|Mintz14||Inferior to the junction of the hepatic veins|
|Mitaka15||Few centimeters inferior to the hepatic vein junction with the RA|
|Moreno16||IVC diameter was measured below the level of the hepatic veins and a few centimeters inferior to its junction with the RA|
|Sakurai3||Site distal of the IVC–hepatic vein junction|
|Simonson18||Successive 10-mm IVC measurements starting at the diaphragm and continuing to 60 mm from the caval–RA junction|
|Tamaki19||Slightly peripheral point from hepatic inlet|
This study aimed to determine if the degree of IVC collapsibility (also referred to as “caval index”) varied within the same subject at commonly measured sites. In addition to providing an opportunity to compare results among existing studies, we sought to determine if the IVC percentage collapse was similar at multiple locations along its course. Such a finding would have relevance to the sonologist, as it is often difficult to obtain a clear view of the IVC along its entire course.
- Top of page
Thirty-nine volunteers were recruited for enrollment in the study. Measurements were successfully obtained in all volunteers (100%) at the junction of the right atrium and IVC and at the takeoff of the left renal vein; in two volunteers (5%) the IVC could not be adequately visualized for measurement at the level of the hepatic vein inlet.
The mean (±SD) 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. A scatterplot of percentage collapse by measurement site is shown in Figure 2. Results analyzed using one-way ANOVA repeated-measures design revealed a significant overall effect for location of measurement, with F(2,35) = 6.00 and p = 0.006. Contrasts showed that the mean percentage collapse at the diaphragm was significantly smaller than that at the hepatic vein inlet (F(1,36) = 5.14; p = 0.03) or the left renal vein (F(1,36) = 11.85;p = 0.002). There was no difference between the mean percentage collapse at the level of the hepatic vein inlet and at the left renal vein measurement sites (F(1,36) = 1.38; p = 0.25). Maulchy’s test indicated that the assumption of sphericity had not been violated (χ2 = 0.026; p = 0.98).
- Top of page
Our study supports the measurement of respiratory variation in the IVC just caudad to the hepatic vein inlet or at the level of the left renal vein. By extension, this finding suggests comparability of studies that have used these two sites in prior investigations. Our observations have practical implications, as well: either site may be used clinically in the determination of volume status, as the situation may arise where one of the views is difficult to obtain. While the hepatic segment of the IVC is usually accessible to imaging, studies have reported a 10%–11% failure rate, and as much as 30% only fair image acquisitions of the hepatic segment.4,10 In our study, only 2 of 39 subjects (5%) were unable to have the hepatic portion of the IVC adequately visualized for measurement, but were able to have the renal segment measured. Additionally, Lichtenstein12 has described a “saber profile” of the IVC, caused by the incoming hepatic vein and appearing as a bulge in the longitudinal silhouette of the vena cava. In this situation, measurement of the IVC near the hepatic vein inlet may be unreliable and should be performed at the level of the left renal vein. Finally, the collapsibility of the hepatic portion of the IVC may be influenced by the surrounding parenchyma, with liver fibrosis or cirrhosis causing impaired diminution.20 In conditions where overt liver disease is suspected, therefore, percentage collapse of the IVC may be measured more reliably at the left renal vein takeoff.
Our results also show that IVC percentage collapse at the junction of the right atrium and IVC was dissimilar to the other sites. This finding is perhaps not surprising, as the attachment of the muscular diaphragm may result in decreased compliance of the vessel at this location.
These results suggest that a similar IVC percentage collapse will be obtained at the level of either the left renal vein takeoff, or 2 cm caudal to the hepatic vein inlet, which offers clinicians some latitude for image acquisition. Further, we found that measurements at the junction of the right atrium and IVC were dissimilar to the other sites, and therefore clinicians should avoid sampling IVC dimensions at this location.