Volume kinetic analysis of 2 crystalloid fluid bolus rates in anesthetized cats.

Objective: To investigate the volume kinetic between 2 crystalloid fluid bolus rates in anesthetized cats.

Design: Prospective, randomized, dose-response study.

Setting: University laboratory.

Animals: Ten convenience-sample, purpose-bred domestic shorthair and medium hair cats.

Interventions: Intravenous 20 mL/kg balanced crystalloid fluid over 10 (G10) or 40 (G40) minutes under anesthesia in a randomized order with at least a 5-day washout period.

Measurements and main results: Serial measurements of hemoglobin (Hb) concentration and PCV were performed up to 60 minutes after conclusion of the fluid bolus. Plasma dilution was calculated with the Hb dilution method and fitted to a 2-compartment microconstant kinetic model using nonlinear mixed-effect models. The apparent central plasma volume (V_c) was similar between the 2 groups (G10: 81.2 ± 23.8 mL/kg and G40: 78.8 ± 10.2 mL/kg). The apparent peripheral volume (V_p) of G10 (4.81E+8 ± 2.66E+8 mL/kg) was twice that of G40 (2.36E+8 ± 6.44E+7 mL/kg). The rate constant from V_c to V_p (K₁₂) of G10 (0.057 ± 0.0196/min) was almost twice that of G40 (0.0302 ± 0.00807/min). The elimination constant of G10 (0.0113 ± 0.00672/min) was almost twice that of G40 (0.00534 ± 0.00279/min). The peak plasma expansion was similar between G10 and G40 (20.7 ± 1.9 and 19.1 ± 5.1 mL/kg). Area under the curve for plasma dilution versus time of the first 90 minutes from the beginning of the boluses was not statistically different between G10 and G40.

Conclusions: The volume expansion over time was not different likely due to the slow elimination. The plasma dilution to crystalloid bolus between subjects is varied in anesthetized cats. Clinicians should consider the slow elimination and return of crystalloid fluid from the V_p to V_c when prescribing fluid therapy in anesthetized cats.