The half-life of infusion fluids: An educational review

Abstract

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.

Fig. 1

Schematic drawing showing the turnover of infused fluids in humans. Kinetic variables discussed in this review are marked in blue and others are indicated in red. Cl_d = distribution clearance, Cl = elimination clearance, T_1/2 = elimination half-life.

Fig. 2

Effects of different elimination half-lives (T_1/2, represented by the different colours) on the distribution of excess volume between plasma (a), interstitial fluid (b), and urine (c) when 2 l of Ringer's acetate is infused over 60 min. The other kinetic variables were set to typical values in conscious adult male volunteers.^–

Fig. 3

(a) The dilution of venous plasma from infusing 10 ml kg⁻¹ of hydroxyethyl starch 130/0.4 over 30 min in 10 male volunteers (thin blue lines). The modelled average (red line) extrapolated to time = 0 demonstrates a maximum peak dilution of 27%, thus the original fluid volume that became expanded was 10/0.27 = 37 ml kg⁻¹, or ∼3 litres if the body weight is 80 kg. The half-life (T_1/2) can be found with reference to the slope of the elimination curve – in this case 110 minutes (See reference 8) (b) Elimination T_1/2 obtained from studies of buffered Ringer's solution. Note the log scale. (c) Distribution clearance (C/_d) falls in relation reductions in MAP during induction of epidural, general, and combined spinal/general anaesthesia. Based on data from References and . MAP, mean arterial pressure.

Fig. 4

(a) The fluid efficiency (plasma volume expansion divided by infused volume) based on kinetic data from thyroid surgery. (b) Comparison between model-predicted and urine-predicted elimination T_1/2 in volunteers receiving various colloid fluids.^, (c) Model-predicted elimination of fluid appears as urine with a lag time of 15–20 min. Median values from 60 laparoscopic operations where 20 ml kg⁻¹ of Ringer's lactate was infused during the first 30 min.