Distribution and elimination of crystalloid fluid in pre-eclampsia

Maldistribution of fluid in preeclampsia: a secondary kinetic analysis

BACKGROUND

Hypovolemia and peripheral edema are frequent components of preeclampsia. The level of the dysregulation of the body fluid distribution is unclear, which complicates the choice of infusion fluid during surgery. The present fluid kinetic study challenges whether the maldistribution of fluid is due to increased capillary leakage or to poor return of already distributed fluid, which occurs via lymphatic pathways.

METHODS

Ringeŕs solution was infused in 10 awake non-pregnant women, eight healthy pregnant women, and in eight women with mild-to-moderately severe preeclampsia. Distribution and redistribution of the infused fluid was calculated with mixed models kinetics based on the excreted urine volumes and 675 measurements of hemodilution. Differences in fluid kinetics between the three groups were studied with covariance analysis.

RESULTS

The return flow of fluid volume to the plasma after distribution (rate parameter k21) was almost zero in women with preeclampsia, while the rate was normal in the other two groups (P< 0.001). By contrast, the capillary leakage rate of fluid in response to the infusion (k12) was normal. The urinary excretion (k10) was moderately accelerated.

CONCLUSION

Decreased flow of extravascular fluid to the plasma was the key disturbance in women with preeclampsia. Such decreased flow alone promotes hypovolemia, peripheral edema, and hypoalbuminemia, and may be explained by inhibition of lymphatic pumping and/or a decreased interstitial hydrostatic pressure due to the presence of vasoactive and inflammatory signal molecules. The moderately accelerated urine flow may be due to "pressure diuresis" in response to hypertension.

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.

The Effects of Acute and Chronic Inflammation on the Dynamics of Fluid Shift of Ringer's Solution and Hemodynamics during Surgery

The aim of this study was to investigate the influences of acute and chronic inflammation on the dynamics of fluid shift of Ringer's solution and hemodynamics in patients during surgery. Thirty-seven patients with the American Society of Anesthesiologists (ASA) grades I-II were enrolled and allocated to two study groups according to the type of disease and operation and inflammation, including patients undergoing emergency appendectomy (Acute group, n = 19) and patients undergoing elective cholecystectomy (Chronic group, n = 18). All of the patients were administered 15 mL/kg of Ringer's lactated (LR) solution at a constant rate over 35 min before the induction of anesthesia. Plasma dilution (PD), volume expansion (VE), volume expansion efficiency (VEE), and extravascular volume (EVV) were calculated based on the concentration of hemoglobin within 2 h post-infusion. Heart rate (HR), arterial blood pressure and urine output were also recorded. PD and VE peaked at the end of infusion, while VEE peaked at the beginning of infusion in all of the patients. After infusion, PD, VE and VEE in the Acute group were all higher than those in the Chronic group (p < 0.05). PD and VE were higher during anesthesia or surgery than during awake or non-surgery (p < 0.001). The mean arterial pressure (MAP) and diastolic pressure (DBP) in the Acute group were significantly lower (p < 0.001) and HR was significantly higher (p < 0.001) than in the Chronic group during the study periods. It was suggested that patients with acute inflammation be treated with individualized fluid therapy during surgery.

Understanding volume kinetics

The distribution and elimination kinetics of the water volume in infusion fluids can be studied by volume kinetics. The approach is a modification of drug pharmacokinetics and uses repeated measurements of blood hemoglobin and urinary excretion as input variables in (usually) a two-compartment model with expandable walls. Study results show that crystalloid fluid has a distribution phase that gives these fluids a plasma volume expansion amounting to 50%-60% of the infused volume as long as the infusion lasts, while the fraction is reduced to 15%-20% within 30 minutes after the infusion ends. Small volumes of crystalloid barely distribute to the interstitium, whereas rapid infusions tend to cause edema. Fluid elimination is very slow during general anesthesia due to the vasodilatation-induced reduction of the arterial pressure, whereas elimination is less affected by hemorrhage. The half-life is twice as long for saline than for Ringer solutions. Elimination is slower in conscious males than conscious females, and high red blood cell and thrombocyte counts retard both distribution and re-distribution. Children have faster turnover than adults. Plasma volume expansions are similar for glucose solutions and Ringer's, but the expansion duration is shorter for glucose. Concentrated urine before and during infusion slows down the elimination of crystalloid fluid. Colloid fluids have no distribution phase, an intravascular persistence half-life of 2-3 hours, and-at least for hydroxyethyl starch-the ability to reduce the effect of subsequently infused crystalloids. Accelerated distribution due to degradation of the endothelial glycocalyx layer has not yet been demonstrated.

The half-life of infusion fluids: An educational review

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An understanding of the half-life (T_1/2) of infused fluids can help prevent iatrogenic problems such as volume overload and postoperative interstitial oedema. Simulations show that a prolongation of the T_1/2 for crystalloid fluid increases the plasma volume and promotes accumulation of fluid in the interstitial fluid space. The T_1/2 for crystalloids is usually 20 to 40 min in conscious humans but might extend to 80 min or longer in the presence of preoperative stress, dehydration, blood loss of <1 l or pregnancy.

The longest T_1/2 measured amounts to between 3 and 8 h and occurs during surgery and general anaesthesia with mechanical ventilation. This situation lasts as long as the anaesthesia. The mechanisms for the long T_1/2 are only partly understood, but involve adrenergic receptors and increased renin and aldosterone release. In contrast, the T_1/2 during the postoperative period is usually short, about 15 to 20 min, at least in response to new fluid.

The commonly used colloid fluids have an intravascular persistence T_1/2 of 2 to 3 h, which is shortened by inflammation. The fact that the elimination T_1/2 of the infused macromolecules is 2 to 6 times longer shows that they also reside outside the bloodstream. With a colloid, fluid volume is eliminated in line with its intravascular persistence, but there is insufficient data to know if this is the same in the clinical setting.

Comparison of the hemodynamics and dynamics of fluid shift of Ringer's solution before surgery in children and adults

The present study investigated the hemodynamics, vascular and extravascular volume expansion induced by infusion of lactated Ringer's solution in children and adults before surgery. This was a prospective randomized double-blind study. A total of 28 patients (14 children and 14 adult patients; American Society of Anesthesiology status I) scheduled for similar minor pelvic, anal rectal or lower limb surgery were recruited for the present study. All patients were administered with 10 ml/kg of lactated Ringer's solution at a constant rate over 20 min. After fluid infusion, plasma dilutions were calculated based on the concentration of hemoglobin. Heart rate (HR), mean arterial pressure (MAP) and urine output were measured before anesthesia was administered for surgery. Results demonstrated that the plasma dilution within 90 min of infusion initiation of lactated Ringer's solution was less pronounced in children compared with adult patients (0.07 vs. 0.16; P<0.001). Children also excreted more of the infused fluid through the kidney within 90 min of infusion initiation than the adults (55% vs. 24%; P=0.01). Following completion of fluid infusion, the volume expansion efficiency was higher in adults [0.82 (0.52-1.00)] than in children [0.46 (0.26-0.68)]. The relative changes in HR were significantly greater in children than in adults 15-60 min after infusion initiation (P<0.01). After 60 min, HRs were comparable between the groups; however, MAP declined significantly from 25-90 min after infusion initiation in children (P<0.05), yet remained nearly constant in adults (P>0.05). Simple regression analysis revealed a positive relationship between the relative changes in MAP and the plasma dilution, and the reduction in MAP in children was able to explain 47% of the variation in plasma dilution (R²=0.47; P=0.007). In conclusion, different hemodynamics and dynamics of fluid shift of Ringer's solution prior to surgery in children and adults may provide anesthesiologists with new information of how to administer fluid treatment for each patient.

[Anesthetic management of severe or worsening postpartum hemorrhage]

INTRODUCTION

Risk factors of maternal morbidity and mortality during postpartum hemorrhage (PPH) include non-optimal anesthetic management. As the anesthetic management of the initial phase is addressed elsewhere, the current chapter is dedicated to the management of severe PPH.

METHODS

A literature search was performed using PubMed and Medline databases, and the Cochrane Library, for articles published from 2003 up to and including 2013. Several keywords related to anesthetic and critical care practice, and obstetrical management were used, in various combinations. Guidelines from several societies and organisations were also read.

RESULTS

When PPH worsens, one should ask for additional team personnel (professional consensus). Patients should be monitored for heart rate, blood pressure, skin and mucosal pallor, bleeding at skin puncture sites, diuresis and the volume of genital bleeding (grade B). Because of the possible rapid worsening of coagulapathy, patients should undergo regular evaluation of coagulation status (professional consensus). Prevention and management of hypothermia should be considered (professional consensus), by warming intravenous fluids and blood products, and by active body warming (grade C). Antibiotics should be given, if not already administered at the initial phase (professional consensus). Vascular fluids must be given (grade B), the choice being left at the physician discretion. Blood products transfusion should be decided based on the clinical severity of PPH (professional consensus). Priority is given to red blood cells (RBC) transfusion, with the aim to maintain Hb concentration>8g/dL. The first round of products could include 3 units of RBC (professional consensus), and the following round 3 units of RBC, and 3 units of fresh frozen plasma (FFP). The FFP:RBC ratio should be kept between 1:2 and 1:1 (professional consensus). Depending on the etiology of PPH, the early administration of FFP is left at the discretion of the physician (professional consensus). Platelet count should be maintained at>50 G/L (professional consensus). During massive PPH, fibrinogen concentration should be maintained at>2g/L (professional consensus). Fibrinogen can be given without prior fibrinogen measurement in case of massive bleeding (professional consensus). General anesthesia should be considered in case of hemodynamic instability, even when an epidural catheter is in place (professional consensus).

CONCLUSION

The anesthetic management aims to restore and maintain optimal respiratory state and circulation, to treat coagulation disorders, and to allow invasive obstetrical and radiologic procedures. Clinical and instrumental monitoring are needed to evaluate the severity of PPH, to guide the choice of therapeutic options, and to assess treatments efficacy.

Dehydration and fluid volume kinetics before major open abdominal surgery

INTRODUCTION

Assessment of dehydration in the preoperative setting is of potential clinical value. The present study uses urine analysis and plasma volume kinetics, which have both been validated against induced changes in body water in volunteers, to study the incidence and severity of dehydration before open abdominal surgery begins.

METHODS

Thirty patients (mean age 64 years) had their urine analysed before major elective open abdominal surgery for colour, specific weight, osmolality and creatinine. The results were scored and the mean taken to represent a 'dehydration index'. Thereafter, the patients received an infusion of 5 ml/kg of Ringer's acetate intravenously for over 15 min. Blood was sampled for 70 min and the blood haemoglobin concentration used to estimate the plasma volume kinetics.

RESULTS

Distribution of fluid occurred more slowly (P < 0.01) and the elimination half-life was twice as long (median 40 min, not significant) in the 11 patients (37%) diagnosed to be moderately dehydrated as compared with euhydrated patients. The dehydration index indicated that the fluid deficit in these patients corresponded to 2.5% of the body weight, whereas the deficit in the others was 1%. In contrast, the 11 patients who later developed postoperative nausea and vomiting had a very short elimination half-life, only 9 min (median, P < 0.01). These patients were usually euhydrated but had microalbuminuria (P < 0.03) and higher natriuresis (P < 0.01).

CONCLUSIONS

The degree of dehydration before major surgery was modest as evidenced both by urine sampling and volume kinetic analysis.

Combining creatinine and volume kinetics identifies missed cases of acute kidney injury following cardiac arrest

Introduction

Fluid resuscitation in the critically ill often results in a positive fluid balance, potentially diluting the serum creatinine concentration and delaying diagnosis of acute kidney injury (AKI).

Methods

Dilution during AKI was quantified by combining creatinine and volume kinetics to account for fluid type, and rates of fluid infusion and urine output. The model was refined using simulated patients receiving crystalloids or colloids under four glomerular filtration rate (GFR) change scenarios and then applied to a cohort of critically ill patients following cardiac arrest.

Results

The creatinine concentration decreased during six hours of fluid infusion at 1 litre-per-hour in simulated patients, irrespective of fluid type or extent of change in GFR (from 0% to 67% reduction). This delayed diagnosis of AKI by 2 to 9 hours. Crystalloids reduced creatinine concentration by 11 to 19% whereas colloids reduced concentration by 36 to 43%. The greatest reduction was at the end of the infusion period. Fluid dilution alone could not explain the rapid reduction of plasma creatinine concentration observed in 39 of 49 patients after cardiac arrest. Additional loss of creatinine production could account for those changes. AKI was suggested in six patients demonstrating little change in creatinine, since a 52 ± 13% reduction in GFR was required after accounting for fluid dilution and reduced creatinine production. Increased injury biomarkers within a few hours of cardiac arrest, including urinary cystatin C and plasma and urinary Neutrophil-Gelatinase-Associated-Lipocalin (biomarker-positive, creatinine-negative patients) also indicated AKI in these patients.

Conclusions

Creatinine and volume kinetics combined to quantify GFR loss, even in the absence of an increase in creatinine. The model improved disease severity estimation, and demonstrated that diagnostic delays due to dilution are minimally affected by fluid type. Creatinine sampling should be delayed at least one hour following a large fluid bolus to avoid dilution. Unchanged plasma creatinine post cardiac arrest signifies renal injury and loss of function.

Trial registration

Australian and New Zealand Clinical Trials Registry ACTRN12610001012066.

A single-model solution for volume kinetic analysis of isotonic fluid infusions

BACKGROUND

Volume kinetics was developed to analyze the distribution and elimination of intravenously given fluid. However, when groups of patients are being compared, the current approach is limited by the need for several models, which yield parameters that cannot be compared. To meet the requirement to handle all patients in a group individually and without pooling, a new all-encompassing model was designed. The aim of this paper was to test whether the new model could be used to analyze all patients in a group.

METHODS

The new model consists of 'rate' and 'amount' parameters instead of 'clearance' and 'dilution' parameters. With this change, a redundant parameter can be taken out, but the biexponential nature is retained. The new parameters are the volume of distribution V1 (ml), the intercompartmental rate constant kt/min and the elimination rate constant kr/min. The success rates of the new and original models in producing results within a set of pre-determined quality requirements were compared using blood dilution data from 10 volunteers challenged with intravenous lactated Ringer's solution.

RESULTS

The new model could be used to analyze all 10 cases within the pre-determined criteria, but the original biexponential model failed in 70% of cases. The residuals improved with the new model. The medians (interquartile ranges) were as follows: V1, 4931 ml (4239-6149 ml); kt), 0.0384/min (0.0024-0.1140/min); kr, 0.0140/min (0.0015-0.0043/min).

CONCLUSION

The new model was suited to the analysis of all cases, and is therefore a better approach to study how clinical conditions change the distribution and elimination of infused fluid.

Adrenergic Drugs Alter Both the Fluid Kinetics and the Hemodynamic Responses to Volume Expansion in Sheep

BACKGROUND

Plasma volume expansion is often performed during adrenergic therapy in the intensive care unit, but little is known about their combined effects.

MATERIALS AND METHODS

The influence of three adrenergic drugs (50 microg/kg/min of dopamine, 0.1 microg/kg/min of isoprenaline, or 3 microg/kg/min of phenylephrine) on the relationship between plasma dilution (an index of volume expansion) and the central hemodynamic responses to volume loading with 24 ml/kg of 0.9% saline were evaluated in 6 adult sheep. Kinetic analysis was also applied to the data on plasma dilution and the urinary excretion measured during and after volume loading.

RESULTS

The adrenergic agents markedly changed the baseline values for all hemodynamic parameters. The kinetic analysis showed that phenylephrine, which is an alpha-adrenergic receptor agonist, promoted renal excretion of infused fluid at the expense of fluid distribution to the periphery (P < 0.05 versus controls). Isoprenaline, which stimulates adrenergic beta-receptors, had the opposite effect. During volume expansion, cardiac atrial pressures increased by 25 to 90%, cardiac output by 13-80% and the arterial pressures by 2 to 22%. Plasma dilution during and after volume loading correlated, in a linear fashion, with these hemodynamic responses. The correlations were strong (r > 0.80) in the control and phenylephrine groups, but weaker in the dopamine and isoprenaline groups. Dopamine was associated with the most variable hemodynamic responses overall.

CONCLUSIONS

Adrenergic drugs altered the hemodynamics at baseline (direct effects), changed the distribution and elimination of infused 0.9% saline (indirect effects) and, finally, modified most hemodynamic responses to plasma dilution (interaction effects).

Bolus injection of Ringer's solution and dextran 1 kDa during induction of spinal anesthesia

BACKGROUND

Arterial hypotension following induction of spinal anesthesia is difficult to prevent with infusion fluids. In a randomized, unblinded and controlled study we evaluated whether a rapid fluid administration planned according to volume kinetic analysis is followed by a more stable blood pressure.

METHODS

Spinal anesthesia was induced in 75 surgical patients, using one of three different fluid regimens: intravenous 'bolus injection' of 5 ml kg(-1) of Ringer's acetate over 3 min, 2 ml kg(-1) of low-molecular weight (1 kDa) dextran over 3 min, or a constant-rate infusion of 15 ml kg(-1) of Ringer's acetate over 40 min (controls). The kinetics of the fluid was studied in five patients in each group and also in eight volunteers.

RESULTS

The decrease in mean arterial pressure averaged 28%, 27% and 26%, respectively, and was fully developed 16 min after the induction. The height of the block, but not the fluid programme, correlated with the hypotension. Nausea or near-fainting associated with marked hypotension or bradycardia was recorded in none, five (20%) and two (8%) of the patients, respectively. Both bolus injections were followed by translocation of fluid from the peripheral tissues to the bloodstream, which maintained the plasma dilution at about 10% for at least 30 min until surgery began.

CONCLUSION

A brisk infusion of Ringer's solution or dextran 1 kDa over 3 min was followed by the same decrease in arterial pressure as a longer and 3-5-times larger infusion of Ringer's solution over 40 min during induction of spinal anesthesia.

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.