While IV fluids are commonly prescribed to human and veterinary patients; there is a need for better understanding and evidence-based approaches to intravenous (IV) fluid therapy in both human and veterinary medicine. There is ongoing debate and limited scientific literature on ideal fluid choice, dose, and rate of IV fluids, as well as concerns regarding fluid choice and safety. Recommendations are often based on assumptions, anecdotal experiences, or extrapolation from human studies and canine models, which may not account for species differences in fluid dynamics.

A landmark human publication reported increased mortality in human children with severe infection following standard fluid boluses, which challenged previous understanding of fluid pharmacokinetics. This led to a paradigm shift toward fluid stewardship and context-sensitive fluid therapy.

Volume kinetic (VK) modeling is a mathematical research platform used to understand the effects of IV fluid administration on body fluid spaces. VK modeling allows for the description of the distribution and elimination of IV fluids and can provide insights into how fluid therapy should be performed. It enables the study of IV fluids like pharmaceutical drugs and improves our understanding of their effects on plasma and interstitial fluid compartments.

This study aimed to apply VK modeling in cats to investigate IV fluid kinetics and body water physiology. The primary objective was to analyze the distribution and elimination of three commonly prescribed IV fluids (balanced isotonic crystalloid, hypertonic saline, and tetrastarch) in healthy, conscious cats using VK modeling. The secondary objective was to describe the plasma volume expansion, half-lives, potency, and ideal fluid prescriptions for each fluid. The researchers hypothesize that VK modeling can be applied to cats and that the plasma dilution-time curves of the different fluids will exhibit distinct patterns based on their physiological behavior. This research study aimed to understand how three commonly used intravenous (IV) fluids behave in healthy cats.

Ten conscious cats were each given three different IV fluids: a balanced isotonic crystalloid, a hypertonic saline solution, and tetrastarch colloid solution. They measured various parameters such as hemoglobin concentration, red blood cell count, hematocrit, heart rate, and blood pressure at regular intervals after administering the fluids; and estimated urine output using ultrasound.

The authors found that all three IV fluids were distributed quickly throughout the body but eliminated slowly. The half-lives (the time it takes for half of the fluid to be eliminated) were 49 minutes for the balanced crystalloid, 319 minutes for the hypertonic saline, and 104 minutes for the tetrastarch solution.

When comparing the effectiveness of the fluids, they found that hypertonic saline had a potency six times higher than the balanced crystalloid and 1.7 times greater than the tetrastarch solution. The tetrastarch solution was 3.5 times more potent than the balanced crystalloid.

The authors also simulated fluid regimens to achieve and maintain a specific level of plasma volume expansion. They found that it was important to reduce the infusion rates significantly after the initial fluid bolus to avoid fluid overload.

The findings suggest that cats may be susceptible to fluid overload due to the slow elimination of fluids.

Several limitations to this study existed. These included the use of only healthy cats, who may have markedly different fluid handling than cats with systemic illness. The study also used a very small number of cats, which may not reflect the population as a whole.  ~MRK