Assessment of Right Atrial Pressure From IVC Dimensions by Echocardiography

Assessment of inferior vena caval dimensions by echocardiography is very useful in knowing the volume status of the individual and indirectly assessing the right atrial pressure. When the IVC is collapsed, that means right atrial pressure is low and person is likely to be hypovolemic. Especially in a person with hypotension this is very important in guiding fluid management. On the other way round, when the IVC is enlarged, or rather plethoric, then you know right atrial pressure is high and even if there is hypotension, you cannot give much fluid to such a case. So this is the importance of assessing IVC, which we will see a little more in detail.

Assessment of inferior vena cava by echocardiography is usually done from the subcostal view. This is the subcostal view. You can see the inferior vena cava, right atrium and part of the hepatic vein. This is the liver parenchyma. Imaging of the IVC from the right axillary region is resorted to in persons who have undergone surgery and the anterior portion is covered with dressing as in thoracic surgery, that is sternotomy surgeries and also when abdominal surgeries have been done and the epigastric region where you keep the probe for subcostal view is also covered by dressings.

While estimating right ventricular systolic pressure or RVSP, from tricuspid regurgitation gradient, usually we add 10 mm of mercury to the observed tricuspid regurgitation gradient to get the estimated right ventricular systolic pressure or RVSP. But this nominal 10 may not be always correct. When the IVC is less than normal in size, that is less than 1.5 cm, and collapses with inspiration, the right atrial pressure may have to be taken as 5 mm of mercury. And normal IVC, 1.5 to 2.5 cm, with an inspiratory decrease in diameter more than 50%, the right atrial pressure is in the range of 5 to 10 mm of mercury. When the inspiratory decrease is less than 50%, that is part of the plethora criteria, right atrial pressure is taken as 10 to 15 mm of mercury. So if the observed tricuspid regurgitation jet velocity (gradient) is 40, then you will have to add slightly higher when the inspiratory decrease is less than 50%, even if the IVC size is apparently normal. And, when the IVC is dilated more than 2.5 cm, and inspiratory decrease in 50%, less than 50%, that is the usual feature for IVC plethora, then you may have to add 15 to 20 mm of mercury. And this is the extreme situation where IVC is dilated, no inspiratory decrease in diameter, and associated with dilated hepatic veins, then right atrial pressure may be more than 20 mm of mercury. Of course, in a sitting person, or a standing person you can also assess this from the jugular venous pressure. But in a sick person in the ICU, you may have to go by these criteria to assess the right atrial pressure.

Inspiratory decrease in decrease in dimensions of IVC can be as IVC collapsibility index. That is, you calculate the maximum dimension in expiration and minimum dimension in inspiration. The difference, is divided by the maximum IVC diameter and it is multiplied by 100 to get a percentage. This is known as IVC collapsibility index.

This is an important point to be noted regarding IVC dimension in a critically ill person in the ICU. During positive pressure ventilation, inspiration is generated by positive pressure. Hence IVC expands, rather than collapses in inspiration. So your assessment of IVC plethora may be wrong when you have a positive pressure ventilation.