YOUNG PHYSICIANS SECTION
Demystify Volume Management in the ED with this Simple AI Echo Tool: You Don’t Have to be an Echo Nerd to Do It! Robert Whitford, MD FAAEM*
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MS brings you an elderly patent with known heart failure with an EF <30%, COPD on home O2, and ESRD on iHD. They are lethargic, hypoxic on a non-rebreather, and hypotensive. This patient will likely need some fluid, but given their multiple risk factors for volume overload, you feel hesitation in ordering that CMS mandated “30cc/kg sepsis bolus.” Your echo savvy resident images the IVC which is small and collapsible and suggests opening the crystalloid, but what can we conclude from this information? Before the groans begin regarding another IVC discussion allow me to set the record straight: IVC size/collapsibility is not a marker of volume status nor volume responsiveness. The simple truth is that IVC size/ collapsibility is merely a rough estimate of CVP (Diagram 1), and we already know that CVP does not predict volume responsiveness. End of story. So why then are we talking about this again?
Diagram 2: Shock phenotype table
In a patient not on positive pressure ventilation, an IVC <2cm and collapsing >50% with tidal respirations predicts an estimated CVP of 3mmHg. At the opposite end of the spectrum, and IVC >2cm and collapsing <50% with tidal respirations predicts and estimated CVP of 15mmHg. Anything in-between those two extremes predicts 8mmHg. That is all. Your options are three, eight, or 15, i.e., a low, normal, or elevated CVP. Before the early 2000s pulmonary artery catheter (i.e. Swan-Ganz catheter) usage in critical care was ubiquitous. PA catheters provide information about right and left ventricular filling pressures (CVPs and wedge pressures respectively) as well as cardiac output (CO). Using this information patients can be classified into a shock phenotype: hypovolemic, distributive, cardiogenic, and obstructive. In general, hypovolemic/distributive patients tend to have low filling pressures, and cardiogenic/obstructive patients tend to have high filling pressures. There are many exceptions, but a more nuanced discussion is beyond the scope of this editorial. Additionally, hypovolemic/cardiogenic/obstructive tend to be in low CO states, whereas distributive patients are in high CO states (Diagram 2). Notice hypovolemic, cardiogenic, and obstructive shock are all low CO states, whereas distributive is a high CO state. Note that septic shock is not listed. Septic shock is usually a mix of hypovolemic and distributive
Diagram 1: IVC and CVP relationship
Diagram 3: LVOT VTI cylinder and doppler envelope diagrams
shock. Initially in a low CO hypovolemic state, but once adequately volume resuscitated, in a high cardiac output distributive state. So yes, I too incorporate an image of the IVC into my assessment of most of my ED patients in shock. The question though is not “does my patient need fluid,” the question is “what category of shock does my patient most likely fall into?” Whether a patient in hypovolemic or distributive shock needs fluid will require a deeper dive. It turns out that patients can be volume responsive despite high filling pressures (large IVCs), and volume non-responsive despite low filing pressures (small collapsible IVCs). What is “volume responsive?” A meaningful increase in cardiac output following a fluid bolus. With a PA catheter in place this is easy to figure out with a “passive leg raise cardiac output challenge.”
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COMMON SENSE SEPTEMBER/OCTOBER 2024
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