Fluid escapes to the "third space" during anesthesia, a commentary

Abstract

Background: The "third fluid space" is a concept that has caused much confusion for more than half a century, dividing anesthesiologists into believers and non-believers.

Aim: To challenge the existence of the "third fluid space" based on analysis of crystalloid fluid kinetics.

Methods: Data on hemodilution patterns from 157 infusion experiments performed in volunteers and from 85 patients undergoing surgery under general anesthesia were studied by population volume kinetic analysis. Elimination of infused crystalloid fluid from the kinetic model could occur either as urine or "third space" accumulation. The latter fluid volume remained in the body, but without equilibrating with the plasma within the 3-4 h of the experiment.

Results: The rate constant for "third space" loss of fluid accounted for 20% of the elimination in conscious volunteers and for 75% during general anesthesia and surgery. The two elimination constants showed a reciprocal relationship, resulting in that "third-space" losses increase when urinary excretion is restricted. The effect on the plasma volume was smaller than indicated by these figures because fluid distributed to the extravascular space continuously redistributed to the plasma. Worked-out examples show that one-third of an infused crystalloid volume has been confined to the "third space" after 3 h of surgery. When equilibration with the plasma eventually occurs, which is necessary for excretion of the fluid, is not known.

Conclusion: During anesthesia and surgery one third of the infused crystalloid fluid is at least temporarily unavailable for excretion, which probably contributes to postoperative weight increase and edema.

FIGURE 1

The volume kinetics model, where “third‐spacing” is represented by the rate constant k _b [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

Relationships between k ₁₀ and k _b in 157 infusion experiments in conscious volunteers and 85 patients receiving crystalloid fluid during general anesthesia. Extensive overlapping. The sum of these two rate constants is the total elimination of fluid volume from the kinetic model and k ₁₀ the fraction accounted for by urine

FIGURE 3

The effect of "thirds‐spacing" of infused fluid in conscious vs anesthetized humans. Simulations based on mean kinetic data from conscious volunteers and anesthetized patients. If elimination of 1 L of Ringers infused over 30 min was determined only by urinary excretion the plasma volume expansion would follow the top curve (blue). In reality, the modeled plasma volume expansion follows the lower curve (red). The difference represents the elimination of fluid that is not in balance with the circulating blood. A similar but larger deficit is present in the interstitial fluid space