Volume Kinetic Analysis of Fluid Shifts Accompanying Intravenous Infusions of Glucose Solution

Understanding Volume Kinetics: The Role of Pharmacokinetic Modeling and Analysis in Fluid Therapy

Fluid therapy is a rapidly evolving yet imprecise clinical practice based upon broad assumptions, species-to-species extrapolations, obsolete experimental evidence, and individual preferences. Although widely recognized as a mainstay therapy in human and veterinary medicine, fluid therapy is not always benign and can cause significant harm through fluid overload, which increases patient morbidity and mortality. As with other pharmaceutical substances, fluids exert physiological effects when introduced into the body and therefore should be considered as "drugs." In human medicine, an innovative adaptation of pharmacokinetic analysis for intravenous fluids known as volume kinetics using serial hemoglobin dilution and urine output has been developed, refined, and investigated extensively for over two decades. Intravenous fluids can now be studied like pharmaceutical drugs, leading to improved understanding of their distribution, elimination, volume effect, efficacy, and half-life (duration of effect) under various physiologic conditions, making evidence-based approaches to fluid therapy possible. This review article introduces the basic concepts of volume kinetics, its current use in human and animal research, as well as its potential and limitations as a research tool for fluid therapy research in veterinary medicine. With limited evidence to support our current fluid administration practices in veterinary medicine, a greater understanding of volume kinetics and body water physiology in veterinary species would ideally provide some evidence-based support for safer and more effective intravenous fluid prescriptions in veterinary patients.

Osmotic and Hemodynamic Effects of Hypertonic Glucose During Hemodialysis

It was the purpose to quantify the hemodynamic effects of a bolus of hypertonic glucose injected into the extracorporeal system in a group of stable and nondiabetic patients during hemodialysis (HD). Glucose and electrolytes were measured in frequent intervals. Arterial blood pressures and heart rates were continuously recorded by noninvasive vascular unloading technique. Beat-to-beat stroke volume, cardiac output, and total peripheral resistance were determined by Modelflow method. Relative blood volumes were continuously measured by ultrasonic and optical means. Eight patients were studied in two treatments. Although arterial pressures and heart rates remained stable, stroke volume and cardiac output transiently increased above (19.2 ± 12.3%) and total peripheral resistance dropped below baseline (18.2 ± 8.6%) by a comparable magnitude. Relative blood volume transiently increased above baseline at 100% (104.9 ± 1.0%). Glucose concentrations were significantly related to relative blood volumes (r = 0.86, p < 0.001). In spite of a substantial increase in blood volume, a bolus of hypertonic glucose does not increase arterial pressures in nondiabetic patients because of concomitant vasodilatation. The relative increase in blood volume quantified by noninvasive HD technology follows the course of glucose and could be used as a surrogate to characterize patients with regard to their glucose metabolism during HD.

The osmotic link between hypoglycaemia and hypovolaemia

OBJECTIVE

Hypoglycaemia is regularly accompanied by hypovolaemia. To suggest a mechanism for this phenomenon, we reviewed data from eight studies conducted by our group and examined the circumstances under which rebound hypoglycaemia develops after intravenous infusion of glucose solutions.

MATERIAL AND METHODS

Forty healthy volunteers and 40 patients received a total of 122 infusions of glucose solutions at different rates, volumes and concentrations. Plasma glucose and the haemodilution were measured repeatedly during and for at least 2 h after the infusions ended. Glucose kinetics was calculated using a one-compartment turnover model and the plasma volume expansion was estimated from changes in Hb.

RESULTS

A strong linear correlation was found between the glucose level and the plasma volume expansion in all series of experiments (p<0.001). After infusion, there was a risk of hypoglycaemia and hypovolaemia developing in healthy volunteers with a high glucose clearance and when infusing glucose solutions of higher concentrations than 2.5 %. Few and mild hypoglycaemic events occurred in patients with insulin resistance, such as in diabetics and in those undergoing surgery. The immediate linear relationship between hypoglycaemia and hypovolaemia suggests an osmotic link between the two parameters. More specifically, infused fluid accompanies glucose during uptake into the cells, while volume expansion by the same fluid has already elicited an effective diuretic response.

CONCLUSION

Hypovolaemia is a consequence of hypoglycaemia after intravenous infusion of glucose solution and is caused by the osmotic translocation of fluid from the extracellular to the intracellular fluid space that occurs despite effective renal elimination.

Comparison of effects of rapid colloid loading before and after spinal anesthesia on maternal hemodynamics and neonatal outcomes in cesarean section

BACKGROUND

The effects of colloid loading after spinal anesthesia on hemodynamics in parturients during cesarean section have not been fully understood. This study tested the hypothesis that colloid loading after spinal blockade can reduce hypotension compared with preloading, and affect neonatal outcomes.

METHODS

A prospective, randomized, double-blinded study was performed in 54 healthy parturients (ASA I or II) undergoing elective cesarean section. Patients were randomly allocated into one of three groups to receive rapid infusion of 6% hydroxyethylstarch (HES) (70 kDa/0.5) before spinal anesthesia (15 ml x kg(-1), HES preload group, n = 18), or rapid infusion of HES after induction of spinal anesthesia (15 ml x kg(-1), HES coload group, n = 18), or no rapid infusion (control, n = 18). The incidence of hypotension, and the amount of ephedrine used to treat hypotension was compared. Neonatal outcomes were also assessed by pH, base excess, lactate concentration, and Apgar scores.

RESULTS

The incidence of hypotension was significantly lower in HES preload and HES coload groups than control group (P < 0.01). Although systolic blood pressure decreased after spinal blockade in all groups, the lowest SBP after spinal blockade until delivery was significantly higher in fluid loading groups than control (P < 0.001). Similarly, total dose of ephedrine to treat hypotension was lower in fluid loading groups (P < 0.001). Umbilical cord pH, umbilical lactate concentration, and the incidence of neonates with Apgar score <7 were similar.

CONCLUSION

Colloid loading after induction of spinal anesthesia was similarly effective in reducing hypo- tension compared with preloading in cesarean section.

Intravenous hydration with a 2.5% glucose solution in Type II diabetes

Physicians are often unclear about how fast intravenous glucose solutions should be administered to adequately hydrate patients with Type II diabetes while avoiding hyperglycaemia and excessive plasma volume expansion. The aim of the present study was to analyse the disposition of a 2.5% glucose solution and create a nomogram which could serve as a guide to fluid therapy in these patients. Twelve males (mean body mass index, 29 kg/m2) with Type II diabetes due to insulin resistance, as quantified by an euglycaemic hyperinsulinaemic glucose clamp, received an infusion of iso-osmotic 2.5% glucose solution with electrolytes (70 mmol/l sodium, 45 mmol/l chloride and 25 mmol/l acetate) at individual rates over 30 and 60 min respectively. Blood glucose and haemoglobin levels were measured repeatedly over 3.5 h to estimate the kinetics of glucose and fluid volume. Mean insulin sensitivity was 4.2×10−4 dl·kg−1·min−1·(μ-units/ml)−1. The individualized infusion rates reached the predetermined blood glucose level of 12 mmol/l with a mean difference of 0.2 mmol/l. The disposition of glucose was an important factor governing fluid distribution. The volume of distribution of exogenous glucose averaged 19.8 litres, but for the fluid volume it was only 3.7 litres. The clearance was 0.37 litre/min for glucose and 0.10 litre/min for the fluid volume, and the results of the 30-min and 60-min infusions agreed reasonably well. It is concluded that kinetic analysis can be used to guide the infusion time and infusion rate of 2.5% glucose to reach any predetermined glucose level and volume expansion.

Short-term hyperglycemia increases endothelial glycocalyx permeability and acutely decreases lineal density of capillaries with flowing red blood cells

Hyperglycemia is becoming recognized as an important risk factor for microvascular dysfunction. We hypothesized that short-term hyperglycemia, either on the scale of hours or weeks, alters the barrier function and the volume of the endothelial glycocalyx and decreases functional capillary density and deformability of the red blood cells (RBCs). All experiments were performed in anesthetized, mechanically ventilated, C57BL/6 mice that were either normoglycemic, acutely hyperglycemic (25 mM) for 60 min due to infusion of glucose, or hyperglycemic (25 mM) for 2–4 wk (db/db mice). The glycocalyx was probed using 40-kDa Texas red dextran, which is known to permeate the glycocalyx, and 70-kDa FITC dextran, which has impaired access to the glycocalyx in healthy animals. Clearance of the dye from the blood was measured. An orthogonal polarization spectral imaging technique was used to visualize the number of capillaries with flowing RBCs of the dorsal flexor muscle. The data indicate that short-term hyperglycemia causes a rapid decrease of the ability of the glycocalyx to exclude 70-kDa dextran. No change in the vascular permeation of 40-kDa dextran was observed. Glycocalyx volume was not affected by short-term hyperglycemia. In addition, 1 h of hyperglycemia resulted in a 38% decrease of the lineal density of capillaries with flowing RBCs. This decreased lineal density was not observed in the 2- to 4-wk hyperglycemia model. Short-term hyperglycemia was without any effect on the deformablity of the RBCs. The data indicate that the described increased vascular permeability with hyperglycemia can be ascribed to an increased permeability of the glycocalyx, identifying the glycocalyx as a potential early target of hyperglycemia.

The Volume Kinetics of Acetated Ringer???s Solution During Laparoscopic Cholecystectomy

We studied the distribution and elimination of an IV infusion of 20 mL/kg of acetated Ringer's solution (approximately 1500 mL) over 60 min in 12 women undergoing laparoscopic cholecystectomy. A plasma dilution of 4.2% developed during the induction of general anesthesia, even though fluid was withheld. The additional plasma dilution induced by the subsequent volume expansion was slightly larger than expected from previous volunteer experiments and averaged 18%. The diuretic response to intravascular fluid administration was small, and only 20% of the infused fluid had been excreted 4 h later. Volume kinetic analysis showed that the IV fluid expanded a central body fluid space by 3.2 L. The clearance constants for distribution and elimination averaged 115 mL/min and 6.8 mL/min, respectively. These data represent a half-life of the fluid in the patients that is 17 times longer (median, 4.5 h) than the half-life of the plasma dilution (16 min), indicating a strong tendency to the formation of peripheral edema. A nomogram based on the kinetic variables suggests that infusion rates should be relatively rapid early on during surgery but slower later. This strategy creates a constant plasma dilution at any desired level without causing undue peripheral accumulation of fluid.