The state of hydration should be assessed in all cases of fluid loss, including vomiting, diarrhoea, sweating and polyuria. Unless the possibility of dehydration is considered, its existence may be overlooked or its severity underestimated. A detailed history of the nature and the quantity of fluid loss is important bu(, if the patient's usual weight is known, the most satisfactory assessment is
obtained by weighing. A dry tongue is apt to be deceptive as it may be due to mouth breathing, In an adult, a total of 4—6 litres has to be lost before the skin becomes dry. loose and wrinkled. Loss of skin elasticity is a poor index, relating more to collagen damage than to water loss. It can be demonstrated by pinching up a fold, which then remains as a ridge and subsides abnormally slowly. The eyeballs are soft, due to lowering of the intraocular tension but this is difficult to assess manually. The blood pressure may be low and the presence of a postural drop in blood pressure may be a useful index of intravascular volume depletion. Elevation in the haemoglobin concentration, packed cell volume and plasma osmolality provide evidence of the severity of dehydration, and serial readings will indicate when treatment has been adequate.
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