Show me the POCUS
Inferior Vena Cava Evaluation
The goal of this section is to use the evaluation of the inferior vena cava and right atrial pressure as the starting point to help us understand fluid responsiveness which we will need to review in a later section.
Estimating right atrial pressure
With the Inferior Vena Cava (IVC) view we can make estimations of the central venous pressure. In a spontaneous breathing patient the diameter of the IVC decreases in response to inspiration. This happens as a result of the negative intrathoracic pressure generated and leads to an increase in the RV filling from the systemic veins. The opposite happens with spontaneous expiration and the diameter of the IVC. The percentage decrease in the diameter of the IVC during inspiration correlate with right atrial (RA) pressures. Keep in mind that the IVC is commonly dilated and may not collapse in patients on ventilators so its use to estimate RA pressure is not recommended.
Although FoCUS does not require you to make any measurement, we will take two measurements of the IVC since they correlate specifically with right atrial pressure. This pressure can then be used for determination of systolic pulmonary artery pressure using CWD (beyond FoCUS). In all likelihood you will be asked to determine if the CVP is high or not and low or not by visual estimation alone.
Measurement technique
The diameter of the IVC should be measured in the subcostal view with the patient in the supine position at <2.0 cm from the junction with the right atrium. We ask the patient to perform a sniff on interrogating the IVC. The diameter of the IVC decreases in response to inspiration when the negative intrathoracic pressure leads to an increase in RV filling. Note that the IVC is commonly dilated and may not collapse in patients on mechanical ventilators and so this estimating CVP based on IVC is not routinely used in such cases.
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IVC Size | Changes w respiration or sniff maneuver | Estimated CVP |
|---|---|---|
> 2.1 cm | < 50% | 15 (range 10-20) |
< 2.1 cm | > 50% | 3 (range 0-5) |
< 2.1 cm | < 50% | 8 (range 5-10) |
> 2.1 cm | > 50% | 8 (range 5-10) |
IVC measurement. On the left, IVC measurement. Red arrow head indicating approximate location of the measurement. On the right, estimated RA or CVP based on size and collapsibility of the IVC. Blue arrow indicating low CVP and red indicating high CVP. In short, a >2.1 cm measurement and minimal collapsibility is high CVP, a <2.1cm measurement and major collapsibility is low CVP and anything is between is a CVP of 8mmHg. See collapsibility index chart on the right where estimated CVP is given as the intermediate value and the parenthesis shows the ranges of pressures in mmHg.
Examples
High CVP
This patient's IVC is dilated >2.1cm and does not collapse when performing sniff test.
Not high or low CVP ("Normal CVP")
The IVC appears more than 2cm. It does collapse more than 50% on inspiration. Thus his CVP is estimated at 8mmHg without having a specific measurement
High CVP
This patient's IVC is dilated >2.1cm and does not collapse when performing sniff test.
Low CVP
Small IVC <2.1cm with more than 50% collapsability
Error in measurement
The image on the left displays a pulsatile structure, the aorta, and vertebral bodies posterior to it. On the right the liver encases the IVC so you see tissue anterior and posterior to it.