Water is largest single component to the human body and is requisite for numerous essential metabolic processes. Dehydration refers to deficient body water content and is prevalent in healthcare. It has been repeatedly shown that dehydration is associated with increased mortality and morbidity. Despite its prevalence and deleterious sequelae, there is substantial deficiency in the knowledge, assessment and management of this pathological state: there is no internationally-recognised definition, clinical signs can be subtle and unreliable, and there is no objective marker with everyday clinical utility. As a consequence, diagnosis of dehydration and prompt rehydration strategies are often poorly delivered in healthcare environments. Thirst plays an integral part in body water homeostasis. Plasma osmolality will increase with uncompensated water loss and is considered the most reliable surrogate objective marker of dehydration. Increased osmolality is sensed by hypothalamic osmoreceptors stimulating thirst and pituitary secretion of antidiuretic hormone (ADH). Thirst has been shown to be sensitive to small changes in plasma osmolality and shows little intra-individual variation. In view of this, it is rational to propose tendering control of intravenous rehydration to patients, enabling them to use the finely-honed intrinsic thirst mechanism to guide their own fluid therapy. A recent pilot study demonstrated that healthy subjects, when allowed to regulate their own intravenous fluid therapy in response to thirst intensity, rehydrated themselves more efficiently than subjects receiving a guideline-based, clinician-delivered fluid regimen. What is unclear is the extent of the reliability of thirst in guiding intravenous fluid rehydration therapy. The investigators propose a double-blinded, repeated measures study in which healthy volunteers are dehydrated using exercise-heat stress in a climatic chamber. Once dehydrated by 3-5% of their body weight, subjects will receive intravenous fluid rehydration which they can demand in response to their sensation of thirst. In one arm of the study they will receive low volume fluid bolus on demand, and in the other arm they will receive a higher volume fluid bolus. Thirst scores and surrogate markers of dehydration will be measured throughout this process. The investigators can then assess whether the demand for additional fluid in response to thirst reduces in frequency in proportion to the degree of correction of fluid deficit.
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Total Volume
Timeframe: This will be measured over the 4 hour rehydration infusion