What are the signs to look for on Point-of-Care Ultrasound (POCUS) of the Inferior Vena Cava (IVC) to assess volume status?

From the Guidelines

When assessing volume status using Point-of-Care Ultrasound (POCUS) of the Inferior Vena Cava (IVC), look for an IVC diameter < 2.1 cm that collapses > 50% with a sniff, which suggests normal RA pressure, whereas an IVC diameter > 2.1 cm that collapses < 50% with a sniff suggests high RA pressure. The signs to look for on POCUS of the IVC to assess volume status include changes in IVC diameter and collapsibility during respiration, with a plethoric IVC (diameter >2.1 cm) with minimal respiratory variation (<50% collapse during inspiration in spontaneously breathing patients) suggesting volume overload or right heart failure, and a small IVC (<1.5 cm) with significant collapsibility (>50% collapse during inspiration) indicating hypovolemia 1. Some key points to consider when evaluating the IVC with POCUS include:

• Measuring the IVC diameter in the subcostal view with the patient in the supine position at 1.0 to 2.0 cm from the junction with the right atrium, using the long-axis view 1
• Evaluating the inspiratory response, which may require a brief sniff, as normal inspiration may not elicit this response 1
• Calculating the collapsibility index, which provides a quantitative measure, with values >50% suggesting fluid responsiveness in spontaneously breathing patients 1
• Interpreting the IVC assessment in clinical context, as factors like increased intra-abdominal pressure, severe tricuspid regurgitation, or pericardial tamponade can affect measurements 1. It is also important to note that the IVC should be measured in long-axis view, approximately 2-3 cm from the right atrial junction, and both maximum and minimum diameters should be recorded during a respiratory cycle 1.

From the Research

Signs to Look for on POCUS of the IVC to Assess Volume Status

The following signs can be looked for on Point-of-Care Ultrasound (POCUS) of the Inferior Vena Cava (IVC) to assess volume status:

• A dilated IVC with decreased collapsibility (< 50%) is a sign of increased right atrial pressure, which may indicate volume overload 2, 3
• A collapsed IVC may be indicative of hypovolemia 2
• The IVC collapsibility index (cIVC) reflects the decrease in the diameter upon inspiration, and can be used to assess volume status 2, 4
• The inferior vena cava/abdominal aorta (IVC/AO) ratio may be a suitable parameter to assess the volume status in pediatric patients, although age-based thresholds need to be defined 2
• A combination of vascular, lung, and cardiac POCUS can be a valuable supplementary tool in the assessment of intravascular volume in several clinical scenarios 2

IVC Diameter and Collapsibility Index

The IVC diameter and collapsibility index can be easily assessed using POCUS, but their role in predicting dehydration in pediatric age has not been fully demonstrated, and an estimation of volume status only by assessing the IVC should be interpreted carefully 2

• A dilated IVC (> 2.5 cm) with minimal collapsibility (less than 50%) had a sensitivity of 85.7% and specificity of 86.4% with an area under the curve (AOC) of 0.92 for predicting a volume overloaded fluid status 3
• The IVC-CI can provide a useful guide for noninvasive intravascular volume status assessment of critically-ill patients, with a significant correlation between CVP and IVC-CI (r = -0.612, p < 0.001) 4

Anatomical Variations of the IVC

Anatomical/developmental variations of the IVC can vary widely in prevalence, and awareness of these variations is necessary for POCUS users to prevent misinterpretation of aberrant vessels and avoid diagnostic pitfalls 5

• A left-sided IVC with azygos continuation is a rare anatomical variation that can be detected using POCUS 5

Comparison with Other Veins

The diameters and respiratory variability of the subclavian vein (SCV), internal jugular vein (IJV), and femoral vein (FV) have been compared to the IVC, but the IVC-CI was not correlated with the SCV-CI, the IJV-CI, or the FV-CI in volume-overloaded patients 6

• The median values of the maximum and minimum diameters of the IVC, SCV, IJV, and FV in volume-overloaded patients were statistically higher compared to normovolemic patients 6