How can assessing hemodynamics help to assess volume status?

Abstract

In critically ill patients, fluid infusion is aimed at increasing cardiac output and tissue perfusion. However, it may contribute to fluid overload which may be harmful. Thus, volume status, risks and potential efficacy of fluid administration and/or removal should be carefully evaluated, and monitoring techniques help for this purpose. Central venous pressure is a marker of right ventricular preload. Very low values indicate hypovolemia, while extremely high values suggest fluid harmfulness. The pulmonary artery catheter enables a comprehensive assessment of the hemodynamic profile and is particularly useful for indicating the risk of pulmonary oedema through the pulmonary artery occlusion pressure. Besides cardiac output and preload, transpulmonary thermodilution measures extravascular lung water, which reflects the extent of lung flooding and assesses the risk of fluid infusion. Echocardiography estimates the volume status through intravascular volumes and pressures. Finally, lung ultrasound estimates lung edema. Guided by these variables, the decision to infuse fluid should first consider specific triggers, such as signs of tissue hypoperfusion. Second, benefits and risks of fluid infusion should be weighted. Thereafter, fluid responsiveness should be assessed. Monitoring techniques help for this purpose, especially by providing real time and precise measurements of cardiac output. When decided, fluid resuscitation should be performed through fluid challenges, the effects of which should be assessed through critical endpoints including cardiac output. This comprehensive evaluation of the risk, benefits and efficacy of fluid infusion helps to individualize fluid management, which should be preferred over a fixed restrictive or liberal strategy.

Keywords: Cardiac output; Extravascular lung water; Hypervolemia; Hypovolemia; Tissue edema; Tissue perfusion.

Fig. 1

Interrelation of intravascular pressures, extravascular lung water and venous stasis indices according to volume status. The relationship between intravascular pressures and volume is curvilinear and affected by cardiac function. Occurrence of extravascular lung water (EVLW) or venous stasis is dependent on volume status but may be precipitated at lower volume status in presence of impaired cardiac function, increased permeability or increased intrathoracic pressures

Fig. 2

Integrative interpretation of volume status and extravascular lung water measurements. Volume status can be estimated by volumetric, pressure, or combination of both measurements. Extravascular lung water (EVLW) can be measured either by transpulmonary thermodilution, lung ultrasounds or even estimated by X-rays

Fig. 3

Ultrasonographic evaluation of volume status

Fig. 3

Ultrasonographic evaluation of volume status

Fig. 3

Ultrasonographic evaluation of volume status

Fig. 4

Optimized fluid management. The optimal fluid management is based on defining the indication (trigger), predicting fluid responsiveness and evaluating the response to fluids both in terms of increase in perfusion but also taking into account tolerance to fluids. CRT capillary refill time, CO cardiac output, CVP central venous pressure, EVLW lung edema (estimated by various ways including transpulmonary thermodilution or lung ultrasounds, VS venous stasis