Patient Controlled Fluid Administration

The purpose of the study is to determine whether participants who are dictating their own administration of IV fluid boluses in response to thirst, receive better protection from hypovolaemia and volume overload than those who undergo routine fluid management. Thirst is prominent in critically ill patients and is related to dehydration. In a recent study of the symptoms experience in ITU patients at high risk of dying, the sensation of thirst was reported in 70.8% of assessments made and was considered to be one of the most intense stressors. Thirst and dehydration can be combated in an ITU setting by consuming oral fluids and through administration of intravenous fluids. However, in older adults, frailty and dysphagia reduces patients' capability to access fluid and results in thirst. In addition, the administration of IV fluids is determined by the attending physician and is often only re-evaluated on a daily or twice-daily basis. The Quench machine has been designed to allow the patient to have more control over their oral and IV fluid administration protocol. This may help reduce the sensation of thirst and dehydration in patients. The machinery is an automated fluid delivery system that will administer a given bolus of IV fluid in response to a trigger provided by the patient. The purpose of the current study is to examine the physiological basis for the functional benefit of this Quench system by investigating the effectiveness of the thirst response in healthy humans as a guide for administration of intravenous fluid boluses. To explore this, the investigators have designed a randomised, cross-over study. On one visit IV fluid boluses will be administered as per the participants' request in response to thirst. In the second arm of the study participants will be administered routine IV fluid maintenance as per NICE guidelines. Body mass at the end of a 4 hour fluid administration intervention will be our primary research outcome and will be compared betwee