Prediction of fluid challenge effect: filling pressure when left ventricular function is abnormal, diastolic volume when left ventricular function is normal

Abstract

Fluid resuscitation is a cornerstone of intensive care unit patient care, but prediction of the cardiovascular response remains difficult, despite many efforts in clinical research. The concept of responders and nonresponders illustrates such a difficulty. Many techniques have been tested, from strictly non-invasive to invasive, delivering various parameters related to the fluid challenge response. Considering the physical parameters available, such as pressure, volume and flow generated by right and left pumps circulating in elastic or compliant tubes, it sounds useful to go back to the basic knowledge to discuss the results of the present article. This published study tested in the postoperative period of cardiovascular surgery the prediction obtained with filling pressures and the diastolic volume. When left ventricular function (global ejection fraction) is adequate, the volume before fluid administration seems to predict well the fluid challenge response; whereas when the global ejection fraction is poor, the filling pressure seems more suitable. The present commentary discusses the main physiological issues related to these findings, with some methodological aspects.

Figure 1

Simplification of the right and left stroke volume variations during mechanical ventilation. (a) During insufflation, the right ventricular afterload increases with venous return impairment leading to a reduced stroke volume (SV). Simultaneously, lung squeezing pushes blood towards the left ventricle, creating a better left ventricular preload and then a better SV. As a consequence, the pulmonary blood volume decreases. (b) During exsufflation, the right ventricle works with a reduced afterload and a better preload, ejecting a higher SV. In addition, the lung is engorged according to pulmonary vessel compliance, storing blood and reducing preload of the left ventricle and its SV. As a consequence, the pulmonary blood volume increases. Globally, insufflation reduces the pulmonary blood volume when exsufflation is increasing it. MV, mechanical ventilation.

Figure 2

Ventricular compliance. When the change in volume/pressure (ΔV/ΔP) is normal, the increase in volume is larger than the increase in pressure when the preload is reduced during hypovolemia. Conversely, when compliance is reduced during hypovolemia, the increase in volume generates a larger change in pressure than in volume. P, pressure; V, volume.