In clinical practice, size, and collapsibility of the IVC is used as a surrogate for right atrial pressure (RAP). In spontaneously breathing patients, inspiration causes negative intrathoracic pressure and collapses the IVC. An IVC diameter ≤ 2.1 cm and collapsibility >50% with a sniff indicates normal RAP of 3 mm Hg (0-5 mm Hg); IVC diameter > 2.1 cm with < 50% inspiratory collapse indicates high RAP of 15 mm Hg (10-20 mm Hg) and an intermediate value of 8 mm Hg (5-10 mm Hg) is assigned to scenarios in between. However, isolated IVC ultrasound is subject to several limitations and the below case illustrates one

A patient on maintenance hemodialysis presents to the hospital with shortness of breath and weight gain after missing dialysis treatment. She was soon started on dialysis assuming volume overload. However, the patient developed hypotension during the session. A quick IVC ultrasound showed complete collapse of the vessel with inspiration.

The doctor concluded possible hypovolemia and stopped ultrafiltration. Blood pressure (BP) improved. A repeat POCUS was done by another physician which showed a significant pericardial effusion + a large left pleural effusion. IVC was plethoric and not collapsing with respiration.

What was the problem with initial scan? It was artifactual collapse as the IVC moved out of plane of the ultrasound beam (could be due to the movement of the vessel itself & also due to movement of the operator’s hand). Here are 2 more examples showing such ‘strange’ collapse of the IVC. Example 2 shows transverse image to better demonstrate side-to-side excursion of IVC with a sniff.

Why did the BP improve with stopping fluid removal initially? because fluid removal caused decrease in preload – intrapericardial pressure > pressure in cardiac chambers – drop in cardiac output. Large pleural effusion could have contributed as well.

Take-home points:

– Do not rely on single-point POCUS for volume status assessment

– Clinical integration is the key: question yourself – in this case, why weight gain? why sob? why hepatic vein appears bigger than usual? Etc.

– Don’t hesitate to scan again when in doubt