The patterns in urine excretion and transvascular fluid exchange in human subjects during intravenous fluid infusion: A quantitative analysis.

Introduction: Investigations of responses of animals and humans to changes of plasma volume are usually reported as average responses of groups of individuals. This ignores considerable quantitative variation between individuals. We examined the hypothesis that individual responses follow a common temporal pattern with variations reflecting different parameters describing that pattern.

Methods: We illustrate this approach using data of Hahn, Lindahl and Drobin (Acta Anaesthesiol Scand.2011, 55:987-94) who measured urine volume and haemoglobin dilution of 10 female subjects during intravenous Ringer infusions for 30 min and subsequent 3.5 h. The published time courses were digitised and analysed to determine if a family of mathematical functions accounted for the variation in individual responses.

Results: Urine excretion was characterised by a time delay (Td) before urine flow increased and a time course of cumulative urine excretion described by a logarithmic function. This logarithmic relation forms the theoretical basis of a family of linear relations describing urine excretion as a function of Td. Measurement of Td enables estimation of subsequent values of urine excretion and thereby the fraction of infused fluid retained in the body.

Conclusion: The approach might be useful for physiologists and clinical investigators to compare the response to infusion protocols when both test and control responses can be described by linear relations between cumulative urine volume at specific times and Td. The approach may also be useful for clinicians by complementing strategies to guide fluid therapy by enabling the later responses of an individual to be predicted from their earlier response.