Title: “The 52 in 52 Review: POCUS for Identification of Low CVP”
Article Citation: Nagdev AD, Merchant RC, Tirado-Gonzalez A, Sisson CA, Murphy MC. 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. PMID: 19556029
What we already know about the topic: CVP is physiological variable measured by placing a catheter into a central vein and transducing a continuous pressure off it, much like an arterial line. Ultrasound, by contrast, is an easily available, non-invasive, and less time consuming process by which the anatomic structures of interest can be visualized in real time.
Why this study is important: The River’s Protocol (the subject of a prior 52 in 52) changed sepsis care around the world, but required the measurement of CVP. This was a cumbersome process in many emergency departments, and researchers were desperate for ways of making this live-saving protocol (or care similarly based in its physiological reasoning) more practically feasible. Some thought that ability to look directly at the vessel in question with POCUS might be one way to obviate the need for the placement of central venous access.
Brief overview of the study: An ultrasonographic calculation based in respirophasic IVC diameter variation, the caval index (maximal diameter – minimal diameter)/(maximal diameter) was calculated on a convenience sample of emergency department patients undergoing central venous catheterization as part of their care. A CVP was transduced off of these patients by a nurse after the physician had made their ultrasonographic measurements. Patients were stratified as low CVP if they were < 8 mmHg and high CVP otherwise. Sensitivity/Specificity were calculated at the caval index of 50% for predicting low vs high CVP and were found to be 91% and 94% respectively.
Limitations: Ultrasound, like life, is operator dependent. This paper made no effort to quality control its image acquisition in real time, or assess inter-rater reliability in the same patients. As a practical point, they tried not to interrupt patient care, and did not account for, or standardize the amount of time or interventions patients received between the ultrasonographic data collection and the CVP collection. Sick patients have dynamic physiologies, and it would have been ideal have the two data points collected at the same time.
Take home points: Ultimately, this paper is a product of a bygone era. The Rivers Protocol was revolutionary, but has convincingly been shown to be equivalent to modern sepsis care. Furthermore, our understanding of CVP and its (non)-relationship to volume status has matured. For both cultural and scientific reasons, the questions asked an conclusions reached in this paper no longer should effect practice in the modern ED, but it’s a fascinating and import piece of our specialty’s history. Since this paper, we’ve still considered questions about IVC diameter on US, but the conversation has moved on from its original motivation here, but this is a major early contributor to the conversation.