Important hemodynamic concepts and terminology for the echocardiographer in 2024

Like all things in medicine, resuscitation paradigms change. From early one-size-fits-all approaches, medicine has shifted towards personalized resuscitation integrating evidence and physiology for the individual patient. This has led to the popularization of several concepts that need to be clearly understood by the echocardiographer involved in hemodynamically phenotyping critically ill patients. 

Let’s dive into some crucial hemodynamic concepts for the modern sonographer!

Fluid Responsiveness:  Fluid responsiveness refers to whether a ‘preload challenge’ (500mL IV bolus or passive leg raise) will increase the stroke volume (or cardiac output) by 10-15%. Fluid responsiveness is a healthy physiologic state, and is not something that should be sought out and eliminated by IV fluids. Instead, in a hypo-perfused patient that is fluid responsive, a small fluid challenge may improve organ perfusion. Right now, you are fluid responsive and that is OK.  

Fluid Tolerance: Fluid tolerance is an emerging concept that helps quantify the potential harms of IV fluids for a patient. Patients can be fluid intolerant for multiple reasons: their acute illness (e.g. ARDS), comorbidities (e.g. low EF), or physiological markers like RV failure or venous congestion. A fluid intolerant patient is more likely to be harmed by excessive IV fluids. 

Other sonographic markers of fluid intolerance include: venous congestion Doppler signals, B-lines on lung ultrasound, elevated E/e’, RV failure, severely depressed LVEF, and evidence of pulmonary HTN. This doesn’t mean fluids can’t be given, but that their benefit must be carefully weighed against the potential risks.

Microcirculation: There has been a shift in resuscitation paradigms to focus on microcirculation (the perfusion of vital organs), as opposed to macrohemodynamic parameters like MAP, cardiac output, or heart rate. This stems from the fact that shock causes end-organ failure from dysfunction at the capillary (microcirculatory) level. New resuscitation protocols have focused on assessing for surrogates of microcirculation (e.g. capillary refill, mottling, and lactate) to tailor hemodynamic interventions.

Venous Congestion: Venous congestion refers to the harmful effects of high venous pressure being transmitted backwards from the right atrium towards organs like the kidney, bowel, and brain. Venous congestion has increasingly been recognized as an important cause of organ failure for a variety of critically ill patients. The VEXUS scoring system has emerged as a popular multi-organ assessment tool for venous congestion, interrogating the inferior vena cava (IVC), hepatic vein (HV), portal vein (PV), and intrarenal vein (IRV). Patients with high VEXUS scores are more likely to develop AKI, need renal replacement therapy, or die. Of note, venous congestion is not just related to IV fluids, but can be caused by a variety of cardiac (e.g. RV failure), pericardial (e.g. pericardial effusion), valvular (e.g. tricuspid regurgitation), and intrabdominal (e.g. intrabdominal hypertension) causes. The treatment depends on the underlying cause. 

Bringing it Together

So how do you bring these concepts together to resuscitate a critically ill patient? I start at the microcirculatory level and ask “does this patient have evidence of organ failure and hypoperfusion?”. More specifically, do their surrogates of microcirculation like cap refill time, lactate, mottling, and urine output suggest hypoperfusion? If so, you can move on to step 2.  

For step 2, I ask myself is the patient fluid responsive. Will their cardiac output increase in response to an IV fluid bolus? If so, there could be a potential benefit of IV fluids.

Finally (step 3), I ask myself is the patient fluid tolerant. Will they be harmed by IV fluids. If they are 1) Hypo-perfused 2) Fluid Responsive and 3) Fluid tolerant, then a small trial of IV fluids reassessing the microcirculation after the intervention is called for. If not, they may need optimization with other therapy like vasopressors, inotropes etc. 

Hopefully this approach is valuable. If you are also interested in research productivity and impact, I share approaches, tips, and tricks on a blog at resub.app . Our recent post focuses on the tremendous waste of time and effort from formatting scientific manuscripts for publication in peer reviewed journal.  

About the Author

Dr. Ross Prager is an intensivist and clinical researcher interested in hemodynamics, ultrasound, research productivity and impact. 

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