Blood viscosity and the overnight fast.

A 12-hour overnight fast in 17 subjects did not result in a significant increase in blood viscosity, as occurs after a 24-hour fast. The usual preoperative fast does not therefore appear to have any adverse rheological effect. However, when it is necessary to prolong the fast, intravenous fluids should be given to prevent the expected increase in blood viscosity which then might have clinical effects as a result of decreased organ perfusion.