Understanding important hemodynamic concepts in 2024: an integrated approach to resuscitation
In medicine, resuscitation practices have evolved significantly. We've moved from generalised approaches to more tailored strategies that incorporate individual patient physiology and the latest evidence. This shift has led to the popularization of several critical concepts, especially for clinicians involved in hemodynamic assessments of our sickest patients.
Let’s explore some essential hemodynamic concepts for any acute care practitioner or resuscitationist.
Fluid Responsiveness: Fluid responsiveness refers to whether a ‘preload challenge’ (such as a 500mL IV bolus or passive leg raise) will increase stroke volume or cardiac output by 10-15%. Fluid responsiveness is a normal physiological state and should not trigger a reflex action of giving IV fluids. For patients who are hypoperfused and fluid responsive, a small fluid challenge may help improve organ perfusion, but this should be assessed through a surrogate of end organ perfusion like capillary refill whenever possible. The key take home is that being fluid responsive is a normal state —right now, you are fluid responsive, and that’s fine.
Fluid Tolerance: Fluid tolerance is an emerging concept that helps quantify the potential harms of IV fluids for an individual patient. A patient can be fluid intolerant due to factors like acute illness (e.g., ARDS), pre-existing conditions (e.g., low EF), or physiological signs such as RV failure or venous congestion. Patients who are fluid intolerant are at a higher risk of harm from excessive IV fluids.
Sonographic indicators of fluid intolerance include venous congestion Doppler patterns, B-lines on lung ultrasound, elevated E/e’ ratios, RV failure, significantly reduced LVEF, and signs of pulmonary hypertension. While this doesn’t rule out giving fluids, it does mean that any fluid administration must carefully consider the risks versus the potential benefits.
Microcirculation: Modern resuscitation protocols increasingly focus on microcirculation, or the perfusion of vital organs, rather than relying solely on macrohemodynamic metrics like MAP, cardiac output, or heart rate. This shift recognizes that shock leads to organ failure by disrupting microcirculatory function. Current approaches emphasize monitoring capillary refill, mottling, and lactate as proxies for microcirculatory health and using these to guide hemodynamic interventions. The landmark trial(s) in this field come from the Andromeda-Shock network, which focus on capillary refill guided resuscitation in septic shock.
Venous Congestion: Venous congestion occurs when high venous pressures are transmitted backward from the right atrium to vital organs such as the kidneys, bowel, and brain, leading to dysfunction. It's now recognized as a key contributor to organ failure in various critically ill patients. The VEXUS scoring system has gained popularity as a multi-organ assessment tool for evaluating venous congestion by examining the IVC, hepatic vein, portal vein, and intrarenal vein. Patients with high VEXUS scores are at greater risk of developing AKI, needing renal replacement therapy, or even dying. It's important to remember that venous congestion isn't just due to excess fluids; it can also result from cardiac (e.g., RV failure), pericardial (e.g., effusion), valvular (e.g., tricuspid regurgitation), or intra-abdominal causes (e.g., abdominal hypertension). Treatment must address the underlying cause.
Bringing It All Together:
So how do these concepts inform resuscitation decisions? I start by assessing the patient’s microcirculation. Is there evidence of organ failure or hypoperfusion? Indicators like capillary refill time, lactate levels, mottling, and urine output can help answer this question. If hypoperfusion is present, I move to the next step.
The second step is determining whether the patient is fluid responsive. If their cardiac output will improve with a fluid bolus, there may be a benefit to giving fluids.
Finally, I assess fluid tolerance. Will the patient be harmed by additional fluids? If the patient is hypoperfused, fluid responsive, and fluid tolerant, a small trial of IV fluids may be warranted, with close monitoring of the microcirculation. If they are not fluid tolerant, alternative treatments such as vasopressors or inotropes might be required.
About the Author
Dr Ross Prager is an Intensivist in Canada, and co-host of HCP Rounds. Follow him (@ross_prager) on X for ultrasound cases, hemodynamics, and discussions around performing impactful research
For anyone interested in more content focused on on improving research productivity and impact, check out my blog on Resub, where I share PEARLS on academic writing, navigating journal selection, and improving research efficiency.
