AJEM recently published a meta-analysis of the evidence supporting making blood volume assessments in hypovolemic patients based on the ultrasound diameter of the inferior vena cava. Five studies met the authors’ selection criteria.
As the IVC is a collapsible vein that is not affected by vasoconstriction it’s diameter should be a function of fluid volume and CVP.
In every emergency ultrasound course I know of we are taught to measure IVC diameter. Then we are taught to make hemodynamic state assessments based on the numbers we come up with.
Often we are presented with a table like the one on the right.
In this case, if IVC diameter is less than 1,5 then CVP is 0-5 mmHg. Similar tables correlate IVC size and collapsibility with fluid volume status.
We all know it’s crap. For many reasons.
What gets me is the huge inter-individual variation in IVC diameter.
Then there is the study by Goldhammer (we’ve all read it you know) in 1999 that demonstrated how athletes IVC diameter was doubled compared to controls. (mean 2,31 compared to 1,14 cm in the control group).
Adding further to the confusion is wether the patient is on positive pressure ventilation or not, operator training levels, the presence of right sided heart failure, where to measure on the IVC etc.
I certainly believe observations of the extremes is important. A flat, barely visible IVC is relevant in the peri-arrest PEA patient. The patient with a ballooning IVC that doesn´t collapse at all probably won’t benefit from another fluid bolus. It is all the observations in between, in the individual patient, I suspect are useless.
And then again, if IVC diameter is related to CVP, and CVP is useless for making volume state assessments, then why would IVC diameter work any better?
Is there any evidence at all of a correlation between IVC diameter and volume status?
The authors performed an extensive literature search. Criteria for inclusion were: prospectively conducted studies that measured IVC diameter through ultrasonography in spontaneously ventilating patients and studies reporting IVC diameter with volume status or shock.
After a comprehensive search, the authors could only include five studies. They all vary significantly in design, method of IVC measurement, subject and exclusion criteria and control group. The studies are summarised in the table below.
The authors, Dipti et al, conclude ´Moderate level of evidence suggest the IVC diameter is consistently low in hypovolemia..´
Find the article here:
Am J Emerg Med. 2012 Oct;30(8):1414-1419.e1. doi: 10.1016/j.ajem.2011.10.017. Epub 2012 Jan 4. Role of inferior vena cava diameter in assessment of volume status: a meta-analysis. Dipti A, Soucy Z, Surana A, Chandra S.
It is is not too impressive but at least there is some evidence demonstrating how IVC diameter is lower in patients presenting with hypovolemia. Also, It confirms what we know about the significant variation in IVC diameter between individuals.
Still, it doesn’t change a lot for me. For me and the critically ill patient, the more extreme a measurement or IVC observation is, the more likely it is to be clinically relevant and true. Any absolute IVC measurement in-between the extremes is likely to be of little or no value in the individual patient for making a volume state assessment.