The Dark Art of IVC Ultrasound

When I was first taught about sonographic assessment of the Inferior Vena Cava (IVC), the following table was unveiled with great solemnity:

IVC diameter (cm)	IVCCI	Estimated RA pressure (mm Hg)
<1.7	>50%	0-5
>1.7	>50%	6-10
>1.7	<50%	11-15
‘dilated’	none	>15

We were told to learn these measurements, take them to the bedside and use them on our critically ill patients to guide resuscitation. We were commanded to use M-mode assessment in the subxiphoid ling axis, and ideally a sniff test.

IVC ultrasound was, and still is, touted as The Next Big Thing in assessment of fluid status and fluid responsiveness.

Sound familiar? It should. Every time a flash new test comes along (D-dimer, proBNP, procalcitonin) we are told by its starry-eyed devotees that this time it’s the real thing.

But is it?

This is the current state of ‘knowledge’:

IVC is not a great test for intravascular volume status, fluid responsiveness or even fluid tolerance. Then again, it might be. The problem is that no-one really knows. A lot of big statements have come out of a lot of teeny studies that don’t really back them up.

It’s probably better for ventilated patients than spontaneously breathing patients (you can remove much of the breath-to-breath variation that confounds so many of these assessment tools). It’s also probably OK at extremes (flat versus full) and serial measurements are probably a good idea.

• No-one knows where we should measure the IVC. The IVC collapses non-uniformly. Most studies measure the IVC at or around the confluence with the hepatic veins. An influential study by Wallace et al warned against measuring at the junction with the right atrium, but with no gold standard in their study they had no actual justification for this advice.
• No-one knows how to measure it. Many experts love M-mode, but this has serious flaws when measuring the IVC.
• We don’t even know where to place the probe. Most studies use the subxiphoid long axis. Some recommend the subxiphoid short axis. Others (such as ACEP) recommend the midaxillary long axis. But every approach has theoretical and actual flaws. Probably the worst is the midaxillary long axis, but then again… no-one knows.
• The sniff test is a great party trick but not validated. It seems like a waste of time to me.
• That table above is probably useless. Everyone’s IVC is different and there are plenty of confounding factors (patient size and position, manner of breathing, measurement site…)

The talk I gave at SMACC summarises the best evidence available to me and my colleagues at the time. If there’s something better out there, please let me know because like Scully and Mulder, I want to believe.

Justin Bowra: IVC Filling: The Ultimate Myth from SMACC Conference

The bottom line

• Check the evidence yourself before you change your practice.
• Be a doctor. Clinical context is more important than IVC ultrasound.
• IVC probably does help at extremes (fat & full versus flat & collapsing)… in most patients… I think.

References and Links

• ACEP, Focus On: Inferior Vena Cava Ultrasound (2011)
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• Baker, K. Review of Bedside Sonography for Guidance of Fluid Therapy in the Emergency Department. (unpublished)
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• Stanford University. http://www.stanford.edu/group/ccm_echocardio/cgi-bin/mediawiki/index.php/IVC
• Takata M, Wise RA, Robotham JL. Effect of abdominal pressure on venous return: abdominal vascular zone conditions. J Appl Physiol 1990 (69):1961– 1972
• UltrasoundPodcast.com Episode 22 — Fluid Responsiveness (2012)
• Wallace DJ, Allison M, Stone MB. Inferior vena cava percentage collapse during resuscitation is affected by the sampling location: an ultrasound study in healthy volunteers. Acad Emerg Med 2010;17:96–9.
• Weekes A, Tassone HM, Tayal VS, Babcok AJ, Norton J. Sonodynamic Comparison of Systolic Blood Pressure to Aortic Velocity Time Integral Measurements as a Measure of Fluid Responsiveness In Non-Traumatic Symptomatic Hypotensive Emergency Department Patients. Annals of Emergency Medicine 2010. Volume 56, Issue 3 Suppl, pS76
• Weekes AJ, Tassone HM, Babcock A, et al. Comparison of serial qualitative and quantitative assessments of caval index and left ventricular systolic function during early fluid resuscitation of hypotensive emergency department patients. Acad Emerg Med 2011; 18(9):912-21.
• Yanagawa Y, Nishi K, Sakamoto T, Okada Y.  Early diagnosis of hypovolemic shock by sonographic measurement of inferior vena cava in trauma patients. J Trauma 2005; 58(4): 825-829.
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