Monitoring for fluid management: dynamic guides and fluid responsiveness

ABSTRACT

In the heart, ventricular end diastolic volume (EDV) before ejection (preload) is directly related to the amount of stroke volume. Generally, the filling pressures such as central venous pressure or pulmonary artery occlusion pressure are used as an indirect indicator of preload. Since cardiac compliance dose change, however, the filling pressure may not be an accurate indicator of the cardiac preload. As substitutes, volumetric parameters like right ventricular EDV or global end diastolic volume were developed and reported to be superior to the filling pressure in the assessment of preload. Preload responding volume resuscitation, however, is different according to the patient's condition. Whether any improvement is to be expected from volume resuscitation depends on whether the heart operates on the steep portion in its function curve. Under mechanical ventilation, because of the influence of positive pressure on vena caval and pulmonary venous return, arterial blood pressure and pulse pressure are maximum during inspiration and minimum a few heart-beats later, i.e., during the expiratory period. These periodic changes become prominent under the hypovolemic condition. Recently, various monitors continuously measuring pulse pressure variation (PPV) or stroke volume variation (SVV) using analysis of arterial wave is widely used as a dynamic guidance for volume resuscitation in mechanically ventilated patients. The ability of those variables to predict fluid responsiveness is better than those of filling pressure or EDV. Thus, PPV and SVV could be beneficially used to guide fluid therapy, while the safety limit of fluid therapy should be based on filling pressure.