Assessment of Crystalloid and Colloid Volume Kinetics During Isoflurane Anesthesia in Cats
Fluid therapy is constantly evolving with new information that challenges current clinical practice. Intravenous fluid therapy in the perioperative and critical care settings can help preserve effective circulating blood volume to maintain systemic tissue perfusion and adequate cellular oxygen delivery. However, inappropriate use of fluid therapy can also result in inadequate resuscitation leading to tissue hypoperfusion or excessive fluid administration leading to tissue edema. General anesthesia can have profound effects on cardiovascular stability and the ideal fluid prescription in the perioperative setting remains a topic of debate. Volume kinetic analysis is an innovative adaptation of traditional pharmacokinetic principles that allows IV fluids to be studied like pharmaceutical drugs, providing new information on their volume of distributions, elimination rates, and half-lives under various physiological conditions. This research project investigated the volume kinetics of a balanced crystalloid solution (20 mL/kg Plasma-Lyte A) and a colloid solution (5 mL/kg 6% tetrastarch 130/0.4) administered over 15 minutes on separate occasions in healthy cats under general anesthesia. Overall fluid elimination under anesthesia is markedly reduced for both fluid types, leading to prolonged elimination half-lives. Volume kinetic analysis suggests that a bolus of Plasma-Lyte A resulted in significant fluid retention and relatively short plasma expansion effects, whereas 6% tetrastarch achieved similar plasma expansion with more sustained effect. In addition, a novel non-invasive 3-dimensional bladder volume estimation method that utilizes bladder circumference tracings obtained from orthogonal bladder ultrasonographic images was assessed and validated for bladder volume estimation. This method may be used to obtain urinary excretion non-invasively for future volume kinetic studies. The agreement between bladder volume estimation using the novel 3-dimensional method was demonstrated against a previously published linear bladder dimension formula and actual measured bladder volumes.