This refers to the rapid administration of IV fluids. E.g. 200 ml of colloid over 10 min. or 500 ml of crystalloid over the same period. The volume should be reduced in patients with heart failure. A fluid challenge is normally given in states of hypoperfusion with the aim of detecting hypovolaemia.
A 10% or more rise in cardiac output after a fluid challenge indicates relative hypovolaemia and normally indicates the need for a further fluid challenge.
In the absence of a cardiac output monitor, the central venous pressure (CVP) might be used. If the CVP does not change after a fluid challenge, this indicates the presence of hypovolaemia and a further fluid challenge can be given. If the CVP increase by 3 cm H20 or more, this indicates the absence of hypovolaemia and further fluid challenges should not be given.
This is obviously, very simplistic. In practice, other factors have to be taken into consideration, such as, history, presence of cardiovascular disease, perfusion of skin and peripheries, urine output, chest signs ..etc.