From Echocardiography in ICU
How to visualize the Inferior vena cava (IVC)?
The IVC can be visualized from the subcostal window:
From subcostal 4 chamber view, translate the probe medially to visualize the right atrium on the right of the screen, and to see a large part of the liver: keep the same depth and direction, translate the tip of the probe toward the right shoulder.
Then rotate the probe counter-clockwise until you see the long axis of the IVC merging into the right atrium.
You will see the IVC passing through the liver and merging with the right atrium. Often, you can visualize the sub-hepatic veins merging in the IVC
+++ THE MOST IMPORTANT IS TO VISUALIZE THE MERGING OF THE IVC WITH THE RA +++
It is the only way to distinguish the IVC from the abdominal aorta and from the sub-hepatic veins, which can have the same direction.
|Direction||Goes through the liver||Goes through the liver|
|Relationships with heart||Merges with RA||Continues down the heart|
|Flow||Continuous, changes with respiration||Pulsatile|
|Respiratory variations||+ or -||No|
|Collateral vessels||Sub-hepatic veins merge with the IVC||Not visible from this approach|
Picture of the IVC going through the liver and merging with RA Picture of the abdominal aorta going through the liver and behind the heart
How to measure the IVC diameter and respiratory variations?
You will measure the diameter of the IVC 2 to 3 centimeters before its merging with the right atrium, where the IVC walls are parallel:
In normal conditions in spontaneously breathing patients, the IVC diameter varies with the respiratory cycle, due to the variations in intra-thoracic pressures. You will find further explanations about IVC physiology in the "estimation of patients' volume status" chapter.
You will take the largest measure of IVC as reference (expiration in spontaneously breathing patients, insufflation in patients with positive pressure ventilation).
The best way to describe the evolution of the IVC diameter through the respiratory cycle is to use Mmode. You will place the ultrasound beam on the cross-section of the IVC, activate Mmode. You will see the structures on this beam line moving depending on the time. It is then easy to measure the smallest and largest diameter of the IVC.
There is one main pitfall in using this technique: with the respiration and movements of the diaphragm, the IVC can get out of the imaging plane, and disappear from your image. You may falsely conclude that it is fully collapsible. To avoid this problem, you should check in 2D first that the IVC is visible troughout the whole respiratory cycle, and then activate Mmode.
Significance of IVC diameter and respiratory variations
The assessment of the IVC is the cornerstone of the patients' volume status evaluation. The interpretation of these parameters will be different depending on the patient's respiratory mode (spontaneously breathing or mechanical ventilation).
Spontaneously breathing patients
In these patients, the IVC diameter and respiratory variation reflects the pressure in the right atrium (RA).
Patients with mechanical ventilation
In these patients, the presence of respiratory variations of the IVC will help you to predict responders to volume challenge.