Unwell patients in the intensive care unit (ICU) often need supplementary fluids to be given into the bloodstream through a drip in a vein (venous cannula), however too much fluid can be harmful. It can sometimes be difficult to tell whether or not a patient will benefit from extra fluids so they are given a "fluid challenge", whereby a small volume of fluid is given quickly into the cannula and the change in their status is noted. If the patient's condition improves, this suggests that the patient is "fluid responsive" and needs more fluid.

A LiDCOplus haemodynamic monitor is a device used in the ICU to estimate the amount of blood ejected from the heart on each heartbeat using pressure readings obtained from a tube placed in one of the patient's arteries (arterial line).

The investigators aim to determine whether or not the use of this device called makes a difference to the judgement of "fluid responsiveness" when the patient is given a fluid challenge when compared to simply using measurements of pulse and blood pressure and assessing the circulation in the patient's limbs. This will allow the investigators to determine whether or not the LiDCOplus alters the decisions made by doctors and nurses about how much fluid to give their patients and hence if it is of any benefit.

Inclusion Criteria:

• Adult patients
• Admitted to Intensive Care Unit (ICU)
• Undergoing fluid resuscitation guided by fluid challenges
• Monitored by a LiDCOplus haemodynamic monitor attached to an arterial line

Exclusion Criteria:

• Poor arterial line trace
• Moribund state
• Patient / relative refusal
• Contra-indication to fluid bolus

Adult patients in the Intensive Care Unit (ICU) undergoing fluid resuscitation guided by fluid challenges monitored by a LiDCOplus haemodynamic monitor.