An understanding of the half-life (T1/2) of infused fluids can help prevent iatrogenic problems such as volume overload and postoperative interstitial oedema. Simulations show that a prolongation of the T1/2 for crystalloid fluid increases the plasma volume and promotes accumulation of fluid in the interstitial fluid space. The T1/2 for crystalloids is usually 20 to 40 min in conscious humans but might extend to 80 min or longer in the presence of preoperative stress, dehydration, blood loss of <1 l or pregnancy.
The longest T1/2 measured amounts to between 3 and 8 h and occurs during surgery and general anaesthesia with mechanical ventilation. This situation lasts as long as the anaesthesia. The mechanisms for the long T1/2 are only partly understood, but involve adrenergic receptors and increased renin and aldosterone release. In contrast, the T1/2 during the postoperative period is usually short, about 15 to 20 min, at least in response to new fluid.
The commonly used colloid fluids have an intravascular persistence T1/2 of 2 to 3 h, which is shortened by inflammation. The fact that the elimination T1/2 of the infused macromolecules is 2 to 6 times longer shows that they also reside outside the bloodstream. With a colloid, fluid volume is eliminated in line with its intravascular persistence, but there is insufficient data to know if this is the same in the clinical setting.