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Summary
Fluid challenges are used to improve cardiac output
and oxygen delivery. This is done in the presence of hypovolaemia. Hypovolaemia is generally diagnosed on static
haemodynamic parameters, such as Central Venous Pressure, Pulmonary Capillary Wedge Pressure etc. Only about half of patients administered fluid in this manner, have benefitted. Recently interest has been directed at
functional haemodynamic parameters. These are based on the relationship between mechanical ventilation
and venous return due to the cyclical changes in intra thoracic pressure. These cause variation in the stroke volume, systolic blood pressure
and pulse pressure during the respiratory cycle. There are factors which can affect the accuracy
and the interpretation of these parameters. Spontaneous respiration, the tidal volumes used to ventilate, PEEP, lung
and chest compliance, heart rhythm, right ventricular function are these factors. Passive leg raising can be used in the presence of these or in doubt.
This article attempts to set out, how to determine whether a fluid challenge will improve the cardiac output, and also to identify the problems in arriving at that decision.