There’s a common notion among nephrologists and the broader medical community: “I looked at the IVC, so I assessed volume status.” But let’s clear this up- IVC POCUS is not a surrogate for intravascular volume. It only gives an estimate of right atrial pressure (or CVP), which doesn’t directly correlate well with intravascular volume. Don’t believe me? Check out this figure from a 1984 paper (link) showing the correlation between CVP and volume is all over the place.
So, equating IVC ultrasound with volume status can lead to management errors. For instance, a patient with a small, collapsible IVC might be euvolemic, hypovolemic, or have a high cardiac output state (like cirrhosis-related hyperdynamic circulation). Accurate differentiation relies on simultaneous cardiac output measurement, which you can do at the bedside with Doppler ultrasound, though it’s not foolproof. This is also important when managing hyponatremia. Misinterpreting euvolemia as hypovolemia based on IVC POCUS and giving normal saline could worsen serum sodium levels in some patients. On the flip side, a plethoric IVC might suggest excess intravascular volume or issues like pericardial effusion, pulmonary embolism, chronic pulmonary hypertension, or even a pneumothorax.
Check out this excerpt from a must-read paper by Parkin et al. (link), showing what determines right atrial pressure. Pay attention to the fact that RAP measures ‘volume status’ only when the heart is stopped!
If you’re mathematically challenged like me, I made a figure illustrating the factors to consider when interpreting IVC ultrasound.
Below is an infographic from our editorial (link) advocating for moving away from isolated organ POCUS. Fun fact: I originally submitted an abridged version of this as a letter to the editor in response to a ‘teaching article’ published in a major nephrology journal that used IVC POCUS synonymously with volume status. It got rejected, saying there was nothing wrong with the original paper. This is concerning because we’re not properly teaching the next generation how to integrate physiology with clinical medicine.
Just creating check mark nephrology POCUS curricula focused on isolated organ POCUS, merely to fill fellowship spots, could have negative impacts on patient care. We might not kill patients by giving or removing a bit more fluid, but it’s not ideal practice and can lead to problems like patient discomfort, missed or delayed diagnosis, longer ICU stays, unnecessary hospital visits, or procedures.
The point of this post isn’t to trash IVC ultrasound but to emphasize using it responsibly, always in conjunction with focused cardiac ultrasound and with cardiovascular physiology in mind. Also, it’s smarter to use the term hemodynamic status instead of volume status since it reminds you to assess the entire hemodynamic circuit. Remember, POCUS doesn’t directly measure volume; instead, it evaluates flows and pressures, from which we infer volume or other pathophysiological processes.
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- Source: https://nephropocus.com/2024/06/17/ivc-ultrasound-and-volume-status-assessment-are-they-the-same/