Robert Hahn on the half life of fluids

Excellent educational contribution from the father of fluid kinetics has appeared in the European Journal of Anaesthesiology. I have drawn freely on Hahn’s kinetic diagrams, but have modified them to include the concept that the glycocalyx can be considered an intravascular part of the extracellular gel phase. This concept is vital to understanding why red cell dilution is exaggerated when a starch solution is infused compared to a preload-equivalent dose of isotonic salt solution.

Some of our current expert Anaesthetists seem unaware of Hahn’s demonstration that infused fluid lingers much longer in its central volume of distribution when the subject is anaesthetised and positive-pressure ventilated. Collar-stiffening starch molecules are unnecessary in the O.R.

Unfortunately the red-cell dilution technique used in Hahn’s studies requires very rapid infusion of a very large volume of the fluid under study, and so is potentially harmful to the subject. The technique is clearly not going to help in the study of restricted fluid therapy approaches, but my modified kinetic diagrams can help us to make rational prescribing choices.

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