On the Shoulders of Giants
Apologies...I had placed the abstract in as a placeholder and failed to replace it with the JournalFeed summary by the time the email sent this weekend.
In this small study, IVC collapse of >/= 50% on bedside ultrasound correlated with a CVP </= 8 cm H2O and was 91% sensitive, 94% specific. But be sure to read the discussion.
Why does this matter?
To measure CVP requires a central line, which is an invasive procedure. Ultrasound has promise as a non-invasive way to measure this parameter. The usual way to measure is to look just distal to the hepatic veins and measure the amount of IVC collapse with inspiration in spontaneously breathing patients. This can be modified for patients on positive pressure ventilation. There are tables touted to correlate with CVP based on the IVC measurements (see below). But multiple studies have called caval index into question. See the outstanding synopsis from SMACC in Another Spoonful below.
IVC(cm) IVCCI Est. RA(mm Hg)
<1.7 >50% 0-5
>1.7 >50% 6-10
>1.7 <50% 11-15
‘dilated’ none >15
Does this pass the sniff test?
This was a small prospective study of 73 patients who were to have a central line placed as part of their care. These patients had paired IVC ultrasound and CVP measurements. They found that if the IVC collapsed >/= 50% with inspiration (in spontaneously breathing patients, opposite if intubated) that this had a sensitivity for a CVP </= 8 cm H2O of 91%, specificity 94%. But take this with a degree of skepticism based on subsequent studies. It may not be as neat and tidy as it seems. IVC measurements at extremes (flat or plethoric) are probably helpful. But there is much debate over where and how to measure the IVC, and most studies are very small like this one, with only 23 people who had CVP </= 8.
Emergency department bedside ultrasonographic measurement of the caval index for noninvasive determination of low central venous pressure.Ann Emerg Med. 2010 Mar;55(3):290-5. doi: 10.1016/j.annemergmed.2009.04.021. Epub 2009 Jun 25.
- Don't miss this current review of the state of the art on LITFL, The Dark Art of IVC Ultrasound.
- Also, see this meta-analysis which concludes, "There are no data to support the widespread practice of using central venous pressure to guide fluid therapy. This approach to fluid resuscitation should be abandoned."