©Walter Rasmussen, R.D.C.S.
Inferior Vena Cava (IVC) Examination
The inferior vena cava is a large vein located within the liver which carries blood from the abdomen in to the right atrium.
With the transducer placed just inferior to the xyphoid process, position it at mid-line and perpendicular to the abdominal wall with the index mark pointing directly toward the patient’s head. It may be necessary to make fine adjustments to the imaging plane in order to include as much of the long axis of the IVC as possible.
The main area of interest is the isthmus, (the narrowest part of the IVC), which is typically located proximal to the right atrium and is the most responsive to respiration. A dual image, one with, and one without color Doppler, as illustrated, can enhance the value of this view.
If the IVC is not initially visualized, ask the patient to inhale and hold their breath to help locate, brighten and sharpen the image. Then, the sonographer can follow it to its resting position at end-expiration as the breath is exhaled.
Color Doppler set at a low scale of about 30 cm/sec can be very helpful for finding a difficult to image IVC. IVC blood flow can be distinguished from a nearby artery by its labile, sometimes bi-directional blood flow characteristics that are highly influenced by respiration. Arterial flow in contrast, is regular, pulsatile, and unidirectional.
In some cases, the IVC is only visible with a partial or full inhalation. The challenge in this situation, is how to produce a video clip that demonstrates the changes in diameter that occur with normal respiration. This makes it necessary for the patient to breathe in a limited way, in order to assess IVC collapsibility. In this situation, two common options are either recording a long clip with the full deep breath and then having the patient exhale or having the patient inhale slightly until the IVC first becomes visible, pausing, and then finishing with a sniff.
A dual image, one with, and one without color Doppler, as illustrated, can enhance the value of this view.
Below: IVC diameter varies with respiration.
The main goal of the IVC view is to record a long clip that documents its diameter and, “colapsibility”. It will be necessary to set the clip acquisition duration for at least 4 seconds or for sufficient cardiac cycles as required to capture one respiratory cycle.
If the vessel does not collapse more than 50% with normal respiration, direct the patient to perform a long and powerful sniff through the nose which should cause a portion of the IVC to narrow or completely collapse in the presence of normal central venous pressure. With elevated venous pressure, the IVC will collapse less than 50% and in some patients the IVC will be extremely dilated and not collapse at all.
Capturing the sniff test on the video clip can be very difficult because as the patient sniffs, the IVC is pulled out of the imaging plane toward the right side of the abdomen. It can take some practice to becoming proficient at following the IVC during the sniff test and this is one of the reasons why a slower, powerful sniff is recommended over a brief sniff. It may be necessary to perform the sniffing maneuver repeatedly until it is accurately recorded. If efforts are not made to follow the IVC during the sniff, the IVC could look as if it is collapsing, while in fact, it is not.
If the patient is not able to inhale deeply or perform the sniff test, a notation should be made on the study because a sub-maximal recording can lead to an erroneous diagnosis.
After recording the collapsibility of the IVC, the next task is to perform PW Doppler for assessment of the pattern of blood flow through the IVC. If the IVC vessel is relatively parallel to the axis of the ultrasound beam, PW Doppler of IVC flow can be recorded directly. Unfortunately, the typical view of the IVC on two-dimensional echo does not produce a sufficient Doppler shift for adequate blood flow assessment because the vessel is perpendicular to the ultrasound beam. It is therefore common practice to use a hepatic vein that drains into the IVC for the assessment of IVC blood flow.
The typical location of the hepatic vein is at or near the narrowest isthmus of the IVC and extending anteriorly from it, vertically on the image. The hepatic vein itself can be quite narrow in diameter and therefore can be more easily located with the aid of color Doppler. On color Doppler, the hepatic vein will be seen as a continuous, predominantly blue line of color, which varies with the patient’s respiration and at times turns red due to the reversal of blood flow with respiration and right atrial contraction or tricuspid regurgitation. If the color Doppler display of hepatic vein flow is difficult to see, it can be helpful to decrease the color Doppler scale and increase the gain until it becomes more visible.
Place the PW Doppler sample volume within the hepatic vein and record the spectral Doppler pattern at a chart speed that allows for at least one respiratory cycle, usually 50 mm/sec or less. Since IVC and hepatic vein blood flow is slow, it is usually necessary to adjust the spectral Doppler filter so that the tracing is recorded all the way to the baseline. Adjust the spectral Doppler scale and baseline so that the velocity profile fits within the borders of the chart. Respiration often causes the hepatic vein to drift from the fixed sample volume point and it may be necessary to follow it by moving the transducer or track ball in concert with the respiratory cycle, which can be quite a difficult maneuver. When it is impossible to record spectral Doppler due to respiratory influences, it becomes necessary to have the patient suspend respiration.
Non-visualization of the IVC and hepatic vein is a rare occurrence and an annotation should be made if it could not be recorded. Occasionally, the only available hepatic vein empties directly into the right atrium, which can be recorded in the same way. A good backup view to the IVC/hepatic vein blood flow is to locate and visualize the superior vena cava from the suprasternal notch, outlined in a later section of this book.
If the patient is on mechanical ventilation, there is no negative pressure generated within the thorax to cause IVC collapse. When recording the IVC in this situation, a notation should be made on the clip such as, “On Vent”, so that the reading physician knows.