Volume Responsiveness in Critically Ill Patients

odern resuscitation has changed since the advent of goaldirected therapy. Today, practitioners providing fluid resuscitation are cognizant of the danger associated with volume depletion while being aware of the morbidity of volume overload.1 Thus, fluid resuscitation must be rapid, precise, and individually tailored to each patient based on reliable data.2 Critically ill patients have a mixture of intravascular volume depletion, low systemic vascular resistance, and decreased cardiac output, which makes responses to attempts at volume resuscitation difficult to predict. As a result, despite initial attempts at fluid resuscitation, persistent hypotension is common and poses the dilemma of whether the patient should receive additional fluid boluses, a vasopressor, or a positive inotropic agent. Traditionally, resuscitation was guided by static measurements, such as central venous pressure, which was thought to determine the patient’s “intravascular volume.”1 These static measurements have been shown to be unreliable predictors of a patient’s ability to positively respond to volume expansion.3 Clinicians have increasingly relied on fluid responsiveness, defined as an increase in cardiac output by 15% following a 500-mL fluid bolus given over 10 minutes, to guide the resuscitation process. Several minimally invasive methods have been used to determine whether a patient is fluid responsive, including pulse counter analysis,4 transpulmonary thermodilution,5 and reactance.6 All of these methods have shown promise in evaluation of the volume status of septic patients; however, bedside sonography has also emerged as a useful tool for evaluating cardiac function in critically ill patients.7 The echocardiographic methods described below are entirely noninvasive, provide real-time data, can be taught reasonably quickly, and can be repeated frequently until desirable clinical outcomes are achieved. This article will review 3 methods aimed at predicting volume responsiveness in critical ill patients: (1) measurement of the caval index; (2) measurement of cardiac output with passive leg raising; and (3) measurement of common carotid artery (CCA) blood flow with passive leg raising.8–10 These latter techniques enable bedside clinicians to determine changes in blood flow in the left ventricular outflow tract (LVOT) and aorta in response to manipulations of right ventricular preload by passive leg raising, thus predicting the response to a fluid bolus without exposing the patient to potentially harmful hypervolemia.11 David Evans, MD, Giovanna Ferraioli, MD, John Snellings, MD, Alexander Levitov, MD