Volume expansion is one of the most common therapeutic procedures in intensive care units (ICUs). There is no doubt that in some cases (e.g. hemorrhage or severe diarrhea) care-givers can reasonably rely on clinical examination to identify patients who will benefit from fluid loading. However, in more complex—but not uncommon—situations (e.g. septic shock), both clinical examination and indicators of cardiac pre-load have been shown to be of minimal value in answering the question: “Can we improve cardiac output and hence hemodynamics by administering fluid?”1
Over the last decade, many clinical studies have demonstrated the value of stroke volume variation (SVV) induced by mechanical ventilation to predict fluid responsiveness, i.e. an increase in cardiac output as a result of fluid infusion.2–6 SVV is now automatically calculated and displayed on minimally invasive cardiac output monitors. This should greatly facilitate the volume management of critically ill patients.
In the following interview, Frédéric Michard, MD, PhD, discusses some of the important matters relating to the volume management of critically ill patients using SVV and new cardiac output monitoring technologies.
Q. Fluid therapy is often used to increase cardiac pre-load and improve the hemodynamic status of patients with circulatory shock. However, cardiac output increases after a fluid challenge in only approximately 50% of such patients. What is the clinical significance of this observation?
A: Only 50% of patients with shock experience a significant increase in cardiac output in response to fluid administration when the decision to give fluid is based on the clinical examination or on the measurement of cardiac filling pressures.1 This observation means that, until recently, clinicians were unable to accurately identify patients who could benefit from fluid expansion—in other words, fluid-responsive patients.
Over the last few years, the concept of fluid responsiveness has become popular in Europe and South America, likely because it is a pragmatic approach to fluid therapy. Indeed, we have a clear idea of the normal total blood volume (800–1,000ml/m2), and of normal right and left ventricular end-diastolic volumes (90–110ml/m2 and 60–80ml/m2, respectively) in healthy subjects.