Volume kinetics of lactated Ringer's solution in adult horses.

Background: Fluid therapy in horses is primarily empirical. Evidence-based quantification of the disposition of intravenous (IV) crystalloids used in clinical practice could enhance the effectiveness of fluid therapy.

Objectives: To determine the pharmacokinetics (i.e., volume kinetics: VK) and associated haemodynamic effects of IV lactated Ringer's solution (LRS) in adult euvolemic horses.

Study design: Unmasked multiple subjects, single baseline design.

Methods: Thirteen healthy, euvolemic adult female horses were administered an IV bolus of LRS and monitored over 4 h. Haemoglobin, albumin and haematocrit concentrations were used to generate VK parameter estimates through a non-linear mixed effects model and stepwise covariate testing. Cardiorespiratory effects, hormonal parameters and urine output were monitored.

Results: Administration of an IV bolus of LRS increased heart rate and systolic arterial pressure. Kinetic analysis was based on plasma albumin, as fluctuations in haemoglobin concentration suggested splenic recruitment of erythrocytes. Fluid disposition was best described by a two-volume fluid space model. Covariate analysis showed that a high systolic arterial pressure is associated with a decrease in urine output, and that haemoglobin recruitment was associated with the transfer of fluid from the central compartment, which was estimated to be 26.2 L, to a peripheral space. Kinetic constants showed rapid fluid distribution to the peripheral compartment and slow return to the central compartment, impeding fluid elimination from the body. Distribution of LRS from the central compartment was rapid but elimination from the body was slow.

Main limitations: Limited sample size and sample collection duration may have influenced model selection and covariate identification.

Conclusions: Volume kinetics provides a method for quantitatively describing the volume expanding effects of administered fluids. Fluid infusion is associated with an increase in heart rate and arterial blood pressure. Volume kinetic analysis offers a context-dependent method for developing and refining more effective fluid infusion protocols.