Fluid responsiveness

Pathophysiology

Body water is divided into several compartments – circulatory, interstitial and intracellular
Changes in circulatory volume can result in changes in haemo-dynamic parameters such as pulse rate and blood pressure
Nonetheless, haemo-dynamic parameters can be also altered by other factors
The water is also in dynamic flux between compartments and governed by two main forces – Starling forces (between circulation and interstitium) and osmolality (between the interstitial and intracellular space
Pathological changes in capillary permeability from disease processes such as sepsis can alter this relationship further
This can be exacerbated by further damage to the glycocalyx layer at this interface by overzealous fluid administration
Therefore, fluid entering the circulation is not a static event
Volumes in the other compartments are also much harder to quantify
There are important impacts of excess fluid in the interstitium such as pulmonary oedema.

Clinical implications

In hypovolaemic shock, volume resuscitation is required because it addresses the pathophysiology, but in other forms of shock where there is capillary damage as in SIRS or sepsis, the clinician may need to limit the amount of fluid given
It is difficult to precisely determine at what point further fluid will actually result in deleterious effects.
It is also uncertain if it has any sustained haemo-dynamic benefit

Controversies and Challenges

The bedside use of JVP height or central venous pressure may not correlate with degree of fluid balance or fluid responsiveness
Invasive monitoring with Pulmonary Arterial Wedge Pressure also has potential limitations
Some proprietary systems using sophisticated analysis of invasive haemodynamic parameters to measure ‘Lung Water’ have been used
The most promising non-invasive advances are bedside echocardiography and electrical cardiometry (bio-impedance) methods of evaluating cardiac function in response to a fluid challenge

Further reading