Assessment of fluid requirements: Fluid responsiveness

Abstract

Fluid administration is the first-line therapy in many patients with circulatory failure, though many patients may not respond to it. Accordingly, it is important to determine the chances of the patient responding to fluids. Assessing fluid responsiveness with the dynamic approach is based on the Frank-Starling relationship: using either heart-lung interactions or a postural change, an acute and transient change in preload is obtained, which results in an increase in stroke volume (SV) in preload-dependent patients, while it remains unaltered in preload-independent patients. Static estimates of preload (pressures, surfaces, and volumes) are unfortunately of limited value since each patient is characterized by his or her own Frank-Starling relationship, and so it is difficult to predict the response to fluids from a given value of preload. During mechanical ventilation, cyclic changes in respiratory pressure induce such changes in preload, resulting in cyclic changes in SV in preload-dependent patients. The commonest way to evaluate these is to measure changes in left ventricular stroke volume (SV) at the outflow tract level. Since these measurements only encompass relative changes from inspiration to expiration, measurement of the outflow tract diameter is not required. This technique can be used with both transthoracic and transesophageal echocardiography. Alternatively, one can look at respiratory variations in the superior vena cava diameter, but this can be performed only using the transesophageal approach. In the passive leg-raising test, the change in position induces a transient increase in preload by autotransfusion of the blood in the legs, which may result in an increase in cardiac output in preload-dependent patients. In this maneuver, cardiac output is measured at the left ventricular outflow tract. © 2011 Springer-Verlag Berlin Heidelberg.