Understanding volume kinetics

Diuretic responses to Ringer's solution and 20% albumin at different arterial pressures

Intravenous volume loading is a common treatment when hypovolemia is a potential cause of oliguria. We studied whether the effectiveness of Ringer's solution and 20% albumin in inducing diuresis differs depending on the mean arterial pressure (MAP). For this purpose, volume kinetic analysis was performed based on urine output and hemoglobin-derived plasma dilution obtained during and after 136 infusions of Ringer and 85 infusions of 20% albumin. Covariance analysis quantified the diuretic response at different arterial pressures. The results show that the diuretic response to a known plasma volume expansion was greater for Ringer's solution above a MAP of 70 mmHg, while 20% albumin was significantly more effective at lower pressures (p < 0.03). Simulations of the urinary output in response to infusion of a predefined fluid volume yielded superior efficacy for 20% albumin when the MAP was low, while Ringer's was similarly effective when the MAP averaged 100 mmHg. In conclusion, urine output in response to plasma volume expansion with 20% albumin was similar to, or even stronger, than that of Ringer's solution when the MAP was below 70 mmHg.

Fluid distribution during surgery in the flat recumbent, Trendelenburg, and the reverse Trendelenburg body positions

BACKGROUND

The distribution and elimination of infused crystalloid fluid is known to be affected by general anesthesia, but it is unclear whether changes differ depending on whether the patient is operated in the flat recumbent position, the Trendelenburg ("legs up") position, or the reverse Trendelenburg ("head up") position.

METHODS

Retrospective data on hemodilution and urine output obtained during and after infusion of 1-2 L of Ringer's solution over 30-60 min were collected from 61 patients undergoing surgery under general anesthesia and 106 volunteers matched with respect to the infusion volume and infusion time. Parameters describing fluid distribution in the anesthetized and awake subjects were compared by population volume kinetic analysis.

RESULTS

General anesthesia decreased the rate constant for urine output by 79% (flat recumbent), 91% (legs up) and 91% (head up), suggesting that laparoscopic surgery per se intensified the already strong anesthesia-induced fluid retention. General anesthesia also decreased the rate constant governing the return of the distributed fluid to the plasma by 32%, 15%, and 70%, respectively. These results agree with laboratory data showing a depressive effect of anesthetic drugs on lymphatic pumping, and further suggest that the "legs up" position facilitates lymphatic flow, whereas the "head up" position slows this flow. Both Trendelenburg positions increased swelling of the "third fluid space".

CONCLUSIONS

General anesthesia caused retention of infused fluid with preferential distribution to the extravascular space. Both Trendelenburg positions had a modifying influence on the kinetic adaptations that agreed with the gravitational forces inflicted by tilting to body.

The patterns in urine excretion and transvascular fluid exchange in human subjects during intravenous fluid infusion: A quantitative analysis

INTRODUCTION

Investigations of responses of animals and humans to changes of plasma volume are usually reported as average responses of groups of individuals. This ignores considerable quantitative variation between individuals. We examined the hypothesis that individual responses follow a common temporal pattern with variations reflecting different parameters describing that pattern.

METHODS

We illustrate this approach using data of Hahn, Lindahl and Drobin (Acta Anaesthesiol Scand.2011, 55:987-94) who measured urine volume and haemoglobin dilution of 10 female subjects during intravenous Ringer infusions for 30 min and subsequent 3.5 h. The published time courses were digitised and analysed to determine if a family of mathematical functions accounted for the variation in individual responses.

RESULTS

Urine excretion was characterised by a time delay (Td) before urine flow increased and a time course of cumulative urine excretion described by a logarithmic function. This logarithmic relation forms the theoretical basis of a family of linear relations describing urine excretion as a function of Td. Measurement of Td enables estimation of subsequent values of urine excretion and thereby the fraction of infused fluid retained in the body.

CONCLUSION

The approach might be useful for physiologists and clinical investigators to compare the response to infusion protocols when both test and control responses can be described by linear relations between cumulative urine volume at specific times and Td. The approach may also be useful for clinicians by complementing strategies to guide fluid therapy by enabling the later responses of an individual to be predicted from their earlier response.

Sequential recruitment of body fluid spaces for increasing volumes of crystalloid fluid

Introduction

The interstitial space harbours two fluid compartments linked serially to the plasma. This study explores conditions that lead to fluid accumulation in the most secluded compartment, termed the "third space".

Methods

Retrospective data was collected from 326 experiments in which intravenous crystalloid fluid was administered to conscious volunteers as well as a small group of anaesthetized patients. The urinary excretion and plasma dilution derived from haemoglobin served as input variables in nine population volume kinetic analyses representing subtly different settings.

Results

An infusion of 250-500 mL of Ringer's solution expanded only the central fluid space (plasma), whereas the infusion of 500-1,000 mL extended into a rapidly exchanging interstitial fluid space. When more than 1 L was infused over 30 min, it was distributed across plasma and both interstitial fluid compartments. The remote space, characterized by slow turnover, abruptly accommodated fluid upon accumulation of 700-800 mL in the rapidly exchanging space, equivalent to an 11%-13% volume increase. However, larger expansion was necessary to trigger this event in a perioperative setting. The plasma half-life of crystalloid fluid was 25 times longer when 2,000-2,700 mL expanded all three fluid compartments compared to when only 250-500 mL expanded the central space (14 h versus 30 min).

Conclusion

As the volume of crystalloid fluid increases, it apparently occupies a larger proportion of the interstitial space. When more than 1 L is administered at a high rate, there is expansion of a remote "third space", which considerably extends the intravascular half-life.

A Slow-Exchange Interstitial Fluid Compartment in Volunteers and Anesthetized Patients: Kinetic Analysis and Physiology

BACKGROUND

Physiological studies suggest that the interstitial space contains 2 fluid compartments, but no analysis has been performed to quantify their sizes and turnover rates.

METHODS

Retrospective data were retrieved from 270 experiments where Ringer's solution of between 238 and 2750 mL (mean, 1487 mL) had been administered by intravenous infusion to awake and anesthetized humans (mean age 39 years, 47% females). Urinary excretion and hemoglobin-derived plasma dilution served as input variables in a volume kinetic analysis using mixed-models software.

RESULTS

The kinetic analysis successfully separated 2 interstitial fluid compartments. One equilibrated rapidly with the plasma and the other equilibrated slowly. General anesthesia doubled the rate constants for fluid entering these 2 compartments (from 0.072 to 0.155 and from 0.026 to 0.080 min -1 , respectively). The return flows to the plasma were impeded by intensive fluid therapy; the rate constant for the fast-exchange compartment decreased from 0.251 to 0.050 when the infusion time increased from 15 to 60 minutes, and the rate constant for the slow-exchange compartment decreased from 0.019 to 0.005 when the infused volume increased from 500 to 1500 mL. The slow-exchange compartment became disproportionately expanded when larger fluid volumes were infused and even attained an unphysiologically large size when general anesthesia was added, suggesting that the flow of fluid was restrained and not solely determined by hydrostatic and oncotic forces. The dependence of the slow-exchange compartment on general anesthesia, crystalloid infusion rate, and infusion volume all suggest a causal physiological process.

CONCLUSIONS

Kinetic analysis supported that Ringer's solution distributes in 2 interstitial compartments with different turnover times. The slow compartment became dominant when large amounts of fluid were infused and during general anesthesia. These findings may explain why fluid accumulates in peripheral tissues during surgery and why infused fluid can remain in the body for several days after general anesthesia.

The effect of human albumin administration on postoperative renal function following major surgery: a systematic review and meta-analysis

Optimal fluid management during major surgery is of considerable concern to anesthesiologists. Although crystalloids are the first choice for fluid management, the administration of large volumes of crystalloids is associated with poor postoperative outcomes. Albumin can be used for fluid management and may protect renal function. However, data regarding the effects of albumin administration on kidney function are conflicting. As such, the present study aimed to investigate the effect of albumin administration on renal function in patients undergoing major surgery and compare its effects with those of crystalloid fluid. The Embase, Medline, Web of Science, Cochrane Library, and KoreaMed databases were searched for relevant studies. The primary endpoint of the meta-analysis was the incidence of postoperative kidney injury, including acute kidney injury and renal replacement therapy. Twelve studies comprising 2311 patients were included; the primary endpoint was analyzed in four studies comprising 1749 patients. Perioperative albumin levels in patients undergoing major surgery did not significantly influence kidney dysfunction (p = 0.98). Postoperative fluid balance was less positive in patients who underwent major surgery and received albumin than in those who received crystalloids. Owing to the limitations of this meta-analysis, it remains unclear whether albumin administration during major surgery is better than crystalloid fluid for improving postoperative renal function.

Fluid management for sepsis-induced hypotension in patients with advanced chronic kidney disease: a secondary analysis of the CLOVERS trial

BACKGROUND

Early fluid management in patients with advanced chronic kidney disease (CKD) and sepsis-induced hypotension is challenging with limited evidence to support treatment recommendations. We aimed to compare an early restrictive versus liberal fluid management for sepsis-induced hypotension in patients with advanced CKD.

METHODS

This post-hoc analysis included patients with advanced CKD (eGFR of less than 30 mL/min/1.73 m2 or history of end-stage renal disease on chronic dialysis) from the crystalloid liberal or vasopressor early resuscitation in sepsis (CLOVERS) trial. The primary endpoint was death from any cause before discharge home by day 90.

RESULTS

Of 1563 participants enrolled in the CLOVERS trial, 196 participants had advanced CKD (45% on chronic dialysis), with 92 participants randomly assigned to the restrictive treatment group and 104 assigned to the liberal fluid group. Death from any cause before discharge home by day 90 occurred significantly less often in the restrictive fluid group compared with the liberal fluid group (20 [21.7%] vs. 41 [39.4%], HR 0.5, 95% CI 0.29-0.85). Participants in the restrictive fluid group had more vasopressor-free days (19.7 ± 10.4 days vs. 15.4 ± 12.6 days; mean difference 4.3 days, 95% CI, 1.0-7.5) and ventilator-free days by day 28 (21.0 ± 11.8 vs. 16.5 ± 13.6 days; mean difference 4.5 days, 95% CI, 0.9-8.1).

CONCLUSIONS

In patients with advanced CKD and sepsis-induced hypotension, an early restrictive fluid strategy, prioritizing vasopressor use, was associated with a lower risk of death from any cause before discharge home by day 90 as compared with an early liberal fluid strategy.

TRIAL REGISTRATION

NCT03434028 (2018-02-09), BioLINCC 14149.

European Society of Intensive Care Medicine clinical practice guideline on fluid therapy in adult critically ill patients. Part 1: the choice of resuscitation fluids

PURPOSE

This is the first of three parts of the clinical practice guideline from the European Society of Intensive Care Medicine (ESICM) on resuscitation fluids in adult critically ill patients. This part addresses fluid choice and the other two will separately address fluid amount and fluid removal.

METHODS

This guideline was formulated by an international panel of clinical experts and methodologists. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) methodology was applied to evaluate the certainty of evidence and to move from evidence to decision.

RESULTS

For volume expansion, the guideline provides conditional recommendations for using crystalloids rather than albumin in critically ill patients in general (moderate certainty of evidence), in patients with sepsis (moderate certainty of evidence), in patients with acute respiratory failure (very low certainty of evidence) and in patients in the perioperative period and patients at risk for bleeding (very low certainty of evidence). There is a conditional recommendation for using isotonic saline rather than albumin in patients with traumatic brain injury (very low certainty of evidence). There is a conditional recommendation for using albumin rather than crystalloids in patients with cirrhosis (very low certainty of evidence). The guideline provides conditional recommendations for using balanced crystalloids rather than isotonic saline in critically ill patients in general (low certainty of evidence), in patients with sepsis (low certainty of evidence) and in patients with kidney injury (very low certainty of evidence). There is a conditional recommendation for using isotonic saline rather than balanced crystalloids in patients with traumatic brain injury (very low certainty of evidence). There is a conditional recommendation for using isotonic crystalloids rather than small-volume hypertonic crystalloids in critically ill patients in general (very low certainty of evidence).

CONCLUSIONS

This guideline provides eleven recommendations to inform clinicians on resuscitation fluid choice in critically ill patients.

Evidence of serial connection between the plasma volume and two interstitial fluid compartments

BACKGROUND

Kinetic analysis of fluid volume shifts can identify two interstitial fluid compartments with different turnover rates, but how they are connected to the bloodstream is unknown.

METHODS

Retrospective data were retrieved from 217 experiments where 1.5 L of Ringer's solution (mean) had been administered by intravenous infusion over 30 min to awake and anesthetized humans (mean age 40 years). Urinary excretion and hemoglobin-derived plasma dilution served as input variables in a volume kinetic analysis using mixed models software. Possible modes of connection between the two interstitial fluid compartments and the bloodstream were judged by covariance analysis between kinetic rate constants, physiological variables, and time factors.

RESULTS

The return flow of already distributed fluid to the plasma via a fast-exchange interstitial compartment was inhibited ongoing infusion of fluid (-38 %), which was probably due to increase of the venous pressure during volume loading. Ongoing infusion also greatly retarded the entrance of fluid to the slow-exchange compartment (-85 %), which suggests that infused Ringer's first had to enter the fast-exchange compartment. A high mean arterial pressure markedly increased the urine output and, to a lesser degree, also the rate of entrance of fluid to the fast-exchange compartment. Moreover, a high blood hemoglobin concentration retarded the rate of entrance of fluid to the fast-exchange compartment.

CONCLUSIONS

The fast-exchange but not the slow-exchange interstitial fluid compartment was affected by intravascular events, which suggests that only the fast-exchange compartment is directly connected to the circulating blood.

[Efficacy of intensive therapy for massive intraoperative blood loss in children: experience of the Morozov Hospital]

OBJECTIVE

To evaluate the effectiveness of intensive therapy for massive intraoperative blood loss in children.

MATERIAL AND METHODS

A retrospective analysis of primary medical records of 39 children with massive intraoperative blood loss was performed. Patients were divided into two groups (younger 1 year (n=18) and older 1 year (n=21)). Each group was divided into two subgroups (blood loss <10% and >100% of total blood volume). We analyzed total intraoperative infusion, qualitative composition of transfusions, reinfusion of washed autologous erythrocytes and vasopressor support. In postoperative period, we assessed hemoglobin, platelets, albumin, fibrinogen, lactate, prothrombin index, duration of mechanical ventilation, severity of organ dysfunction (pSOFA score) after 1 and 3 days, ICU stay and incidence of repeated blood transfusions.

RESULTS

With regard to transfusion volume, we found a general pattern (3 parts of crystalloids, 2 parts of erythrocyte-containing components and 1 part of fresh frozen plasma in all groups with the exception of children older 1 year with blood loss >100% of total blood volume. The last ones had ratio 3:5:1 due to large volume of reinfusion of washed autologous erythrocytes. In all groups, target levels of hemoglobin, platelets, albumin and prothrombin index were achieved. Serum fibrinogen was slightly lower in the group with blood loss >100% of total blood volume. There was a direct relationship between blood loss and ICU stay (Spearman's test rs=0.421, p<0.05), as well as duration of mechanical ventilation (Spearman's test rs=0.509, p<0.05). Mean pSOFA score upon admission to intensive care unit was 3-4 points in both groups with blood loss <100% of total blood volume. In patients with blood loss >100% of total blood volume, this indicator averaged 9 points and regressed to 3-4 points over the next 72 hours.

CONCLUSION

Intraoperative intensive therapy contribute to minimal severity of postoperative organ dysfunction in children with blood loss < 100% of total blood volume and rapid regression of multiple organ failure in patients with blood loss exceeding this indicator.

Isotonic saline causes greater volume overload than electrolyte-free irrigating fluids

OBJECTIVES

Systemic absorption of the irrigating fluid used to flush the operating site is a potentially serious complication in several types of endoscopic operations. To increase safety, many surgeons have changed from a monopolar to a bipolar resection technique because 0.9% saline can then be used instead of electrolyte-free fluid for irrigation. The present study examines whether the tendency for excessive plasma volume expansion is greater with saline than with electrolyte-free fluid.

METHODS

Pooled data were analyzed from four studies in which a mean of 1.25 L of either 0.9% saline or an electrolyte-free irrigating fluid containing glycine, mannitol, and sorbitol was given by intravenous infusion on 80 occasions to male volunteers and patients scheduled for transurethral prostatic surgery. The distribution of the infused fluid was analyzed with a population volume kinetic model based on frequently measured hemodilution and the urinary excretion.

RESULTS

Electrolyte-free fluid distributed almost twice as fast and was excreted four times faster than 0.9% saline. The distribution half-life was 6.5 and 10.6 min for the electrolyte-free fluid and saline, respectively, and the elimination half-lives (by urinary excretion) from the plasma volume were 21 and 87 min. Simulation showed that the plasma volume expansion was twice as great from 0.9% saline than from electrolyte-free fluid.

CONCLUSIONS

Isotonic (0.9%) saline expands the plasma volume by twice as much as occurs with electrolyte-free irrigating fluids. This difference might explain why signs of cardiovascular overload are the most commonly observed adverse effects when saline is absorbed during endoscopic surgery.

Role of Crystalloids in the Perioperative Setting: From Basics to Clinical Applications and Enhanced Recovery Protocols

Perioperative fluid management, a critical aspect of major surgeries, is characterized by pronounced stress responses, altered capillary permeability, and significant fluid shifts. Recognized as a cornerstone of enhanced recovery protocols, effective perioperative fluid management is crucial for optimizing patient recovery and preventing postoperative complications, especially in high-risk patients. The scientific literature has extensively investigated various fluid infusion regimens, but recent publications indicate that not only the volume but also the type of fluid infused significantly influences surgical outcomes. Adequate fluid therapy prescription requires a thorough understanding of the physiological and biochemical principles that govern the body's internal environment and the potential perioperative alterations that may arise. Recently published clinical trials have questioned the safety of synthetic colloids, widely used in the surgical field. A new clinical scenario has arisen in which crystalloids could play a pivotal role in perioperative fluid therapy. This review aims to offer evidence-based clinical principles for prescribing fluid therapy tailored to the patient's physiology during the perioperative period. The approach combines these principles with current recommendations for enhanced recovery programs for surgical patients, grounded in physiological and biochemical principles.

ENGINEERED INTRAVENOUS THERAPIES FOR TRAUMA

Trauma leading to severe hemorrhage and shock on average kills patients within 3 to 6 hours after injury. With average prehospital transport times reaching 1-6 hours in low- to middle-income countries, stopping the bleeding and reversing hemorrhagic shock is vital. First-generation intravenous hemostats rely on traditional drug delivery platforms, such as self-assembling systems, fabricated nanoparticles, and soluble polymers due to their active targeting, biodistribution, and safety. We discuss some challenges translating these therapies to patients, as very few have successfully made it through preclinical evaluation in large-animals, and none have translated to the clinic. Finally, we discuss the physiology of hemorrhagic shock, highlight a new low volume resuscitant (LVR) PEG-20k, and end with considerations for the rational design of LVRs.

Perioperative Fluid Management

The medical complexity of the geriatric patients has been steadily rising. Still, as outcomes of surgical procedures in the elderly are improving, centers are pushing boundaries. There is also a growing appreciation of the importance of perioperative fluid management on postoperative outcomes, especially in the elderly. Optimal fluid management in this cohort is challenging due to the combination of age-related physiological changes in organ function, increased comorbid burden, and larger fluid shifts during more complex surgical procedures. The current state-of-the-art approach to fluid management in the perioperative period is outlined.

Advances in the Physiology of Transvascular Exchange and A New Look At Rational Fluid Prescription

The Starling principle is a model that explains the transvascular distribution of fluids essentially governed by hydrostatic and oncotic forces, which dynamically allow vascular refilling according to the characteristics of the blood vessel. However, careful analysis of fluid physiology has shown that the principle, while correct, is not complete. The revised Starling principle (Michel-Weinbaum model) provides relevant information on fluid kinetics. Special emphasis has been placed on the endothelial glycocalyx, whose subendothelial area allows a restricted oncotic pressure that limits the reabsorption of fluid from the interstitial space, so that transvascular refilling occurs mainly from the lymphatic vessels. The close correlation between pathological states of the endothelium (eg: sepsis, acute inflammation, or chronic kidney disease) and the prescription of fluids forces the physician to understand the dynamics of fluids in the organism; this will allow rational fluid prescriptions. A theory that integrates the physiology of exchange and transvascular refilling is the "microconstant model", whose variables include dynamic mechanisms that can explain edematous states, management of acute resuscitation, and type of fluids for common clinical conditions. The clinical-physiological integration of the concepts will be the hinges that allow a rational and dynamic prescription of fluids.

Hypovolemia with peripheral edema: What is wrong?

Fluid normally exchanges freely between the plasma and interstitial space and is returned primarily via the lymphatic system. This balance can be disturbed by diseases and medications. In inflammatory disease states, such as sepsis, the return flow of fluid from the interstitial space to the plasma seems to be very slow, which promotes the well-known triad of hypovolemia, hypoalbuminemia, and peripheral edema. Similarly, general anesthesia, for example, even without mechanical ventilation, increases accumulation of infused crystalloid fluid in a slowly equilibrating fraction of the extravascular compartment. Herein, we have combined data from fluid kinetic trials with previously unconnected mechanisms of inflammation, interstitial fluid physiology and lymphatic pathology to synthesize a novel explanation for common and clinically relevant examples of circulatory dysregulation. Experimental studies suggest that two key mechanisms contribute to the combination of hypovolemia, hypoalbuminemia and edema; (1) acute lowering of the interstitial pressure by inflammatory mediators such as TNFα, IL-1β, and IL-6 and, (2) nitric oxide-induced inhibition of intrinsic lymphatic pumping.

The kinetics of isotonic and hypertonic resuscitation fluids is dependent on the sizes of the body fluid volumes

Background and Aims

The extracellular and intracellular fluid volumes (ECV and ICV) vary not only with age, gender, and body weight but also with the habitual intake of water. The present study examines whether the baseline variations in the ECV and ICV change the distribution and elimination of subsequently given infusion fluids.

Material and Methods

Twenty healthy male volunteers underwent 50 infusion experiments with crystalloid fluid for which the fluid volume kinetics was calculated based on frequent measurements of the hemodilution using mixed-effects modeling software. The results were compared with the ECV and ICV measured with multifrequency bioimpedance analysis before each infusion started. The fluids were given over 30 minutes and comprised 25 mL/kg Ringer's acetate (N = 20), Ringer's lactate, 5 mL/kg 7.5% saline, and 3 mL/kg 7.5% saline in 6% dextran 70 (these fluids, N = 10).

Results

A large ICV was associated with a small extravascular accumulation of infused fluid, which increased the plasma volume expansion and the urinary excretion. With hypertonic fluid, a large ECV greatly accelerated urinary excretion. The body weight did not serve as a covariate in the kinetic models. Albumin was recruited to the plasma during infusion of both types of fluid. The hypertonic fluids served as diuretics. The infused excess sodium and osmolality were distributed over a 35% larger space than the sum of the ECV and ICV.

Conclusion

A large ICV reduced the rate of distribution of Ringer's solution, whereas a large ECV accelerated the excretion of hypertonic saline.

Interstitial washdown during crystalloid fluid loading in graded hypovolemia -A retrospective analysis in volunteers

BACKGROUND

"Interstitial washdown" is an edema-preventing mechanism that implies a greater redistribution of interstitial albumin occurs whenever the capillary filtration is increased.

OBJECTIVE

To study the effect of interstitial washdown on fluid distribution in normovolemic and hypovolemic volunteers.

METHODS

Capillary filtration was increased by infusing 25 mL/kg Ringer's acetate intravenously over 30 min 10 male just after withdrawal of 0, 450, and 900 mL of blood. Population volume kinetic analysis was used to assess the effects of washdown and hemorrhage on fluid distribution, using the difference in plasma dilution based on hemoglobin and albumin as biomarker of washdown.

RESULTS

Blood withdrawal during 10-15 min recruited 100-150 mL of fluid of high albumin content to the plasma, which was probably lymph. The albumin recruitment was temporarily reduced during the fluid loading but increased from 40 min post-infusion and was then greater when preceded by hemorrhage. Simulations suggested that interstitial washdown decreased the extravascular fluid volume by 200 mL over 3 h. The plasma volume and urinary excretion both increased by approximately half this amount.

CONCLUSIONS

Blood loss without hypotension probably recruited lymph to the plasma, but interstitial washdown played no major role in determining the distribution of crystalloid fluid after hemorrhage.

Semimechanistic modeling of copeptin and aldosterone kinetics and dynamics in response to rehydration treatment for diabetic ketoacidosis in children

Diabetic ketoacidosis (DKA), a frequent complication of type 1 diabetes (T1D), is characterized by hyperosmolar hypovolemia. The response of water-regulating hormones arginine vasopressin (AVP; antidiuretic hormone) and aldosterone to DKA treatment in children is not well understood, although they may have potential as future diagnostic, prognostic, and/or treatment monitoring markers in diabetic patients. We aimed to characterize the dynamics of the response in copeptin (marker for AVP) and aldosterone secretion to rehydration treatment in pediatric patients with DKA. Data originated from a prospective, observational, multicenter study including 28 pediatric T1D patients treated for DKA (median age, 11.5 years; weight, 35 kg). Serial measurements of hormone levels were obtained during 72 h following rehydration start. Semimechanistic pharmacometric modeling was used to analyze the kinetic/dynamic relationship of copeptin and aldosterone secretion in response to the correction of hyperosmolality and hypovolemia, respectively. Modeling revealed different sensitivities for osmolality-dependent copeptin secretion during the first 72 h of rehydration, possibly explained by an osmotic shift introduced by hypovolemia. Response in aldosterone secretion to the correction of hypovolemia seemed to be delayed, which was well described by an extra upstream turnover compartment, possibly representing chronic upregulation of aldosterone synthase (cytochrome P450 11B2). In conclusion, semimechanistic modeling provided novel physiological insights in hormonal water regulation in pediatric patients during DKA treatment, providing rationale to further evaluate the potential of monitoring copeptin, but not aldosterone due to its delayed response, for future optimization of rehydration treatment to reduce the risk of acute complications such as cerebral edema.

Low-Fouling Zwitterionic Polymeric Colloids as Resuscitation Fluids for Hemorrhagic Shock

Colloids, known as volume expanders, have been used as resuscitation fluids for hypovolemic shock for decades, as they increase plasma oncotic pressure and expand intravascular volume. However, recent studies show that commonly used synthetic colloids have adverse interactions with human biological systems. In this work, a low-fouling amine(N)-oxide-based zwitterionic polymer as an alternative volume expander with improved biocompatibility and efficacy is designed. It is demonstrated that the polymer possesses antifouling ability, resisting cell interaction and deposition in major organs, and is rapidly cleared via renal filtration and hepatic circulation, reducing the risk of long-term side effects. Furthermore, in vitro and in vivo studies show an absence of adverse effects on hemostasis or any acute safety risks. Finally, it is shown that, in a head-to-head comparison with existing colloids and plasma, the zwitterionic polymer serves as a more potent oncotic agent for restoring intravascular volume in a hemorrhagic shock model. The design of N-oxide-based zwitterionic polymers may lead to the development of alternative fluid therapies to treat hypovolemic shock and to improve fluid management in general.

Fast versus slow infusion of 20% albumin: a randomized controlled cross-over trial in volunteers

BACKGROUND

We investigated whether plasma volume (PV) expansion of 20% albumin is larger when the fluid is administered rapidly compared with a slow infusion.

METHODS

In this open-labeled randomized interventional controlled trial, 12 volunteers (mean age, 28 years) received 3 mL/kg of 20% albumin (approximately 225 mL) over 30 min (fast) and 120 min (slow) in a cross-over fashion. Blood hemoglobin and plasma albumin were measured on 15 occasions during 6 h to estimate the PV expansion and the capillary leakage of albumin and fluid.

RESULTS

The largest PV expansion was 16.1% ± 6.5% (mean ± SD) for fast infusion and 12.8% ± 4.0% for slow infusion (p = 0.52). The median area under the curve for the PV expansion was 69% larger for the fast infusion during the first 2 h (p = 0.034), but was then similar for both infusions. The half-life of the PV expansion did not differ significantly (median, 5.6 h versus 5.4 h, p = 0.345), whereas the intravascular half-life of the excess albumin was 8.0 h for fast infusion and 6.3 h for slow infusion (p = 0.028). The measured urine output was almost three times larger than the infused volume. The plasma concentration of atrial natriuretic peptide (MR-proANP) accelerated the capillary leakage of albumin and the urine flow.

CONCLUSIONS

The intravascular persistence of albumin was longer, but the fluid kinetics was the same, when 20% albumin was infused over 30 min compared with 120 min. We found no disadvantages of administering the albumin at the higher rate. Trial registration EU Clinical Trials Register, EudraCT2017-003687-12, registered September 22, 2017, https://www.clinicaltrialsregister.eu/ctr-search/trial/2017-003687-12/SE.

Diuretic response to Ringer's solution is normal shortly after awakening from general anaesthesia: a retrospective kinetic analysis

Background

The elimination of Ringer's solution is severely depressed during general anaesthesia, but the degree to which this continues postoperatively is poorly established.

Methods

An intravenous infusion of Ringer's acetate solution 20 ml kg-1 was administered over 60 min in 12 patients undergoing laparoscopic cholecystectomy. Population kinetic analysis was performed based on repeated measurements of blood haemoglobin concentration and urinary excretion over 240 min regardless of when the operations were finished. The analysis contrasted the periods before and after awakening from general anaesthesia and compared them with data from 18 volunteers who received the same fluid at the same rate.

Results

Patients were monitored for approximately 2 h after awakening from general anaesthesia. The rate constant for redistribution of fluid from the extravascular space to the plasma (k21) and the rate constant for urinary excretion (k10) were significantly higher postoperatively than during the surgical period. Computer simulations indicated that urinary excretion after surgery was almost restored to the rate found in the volunteers. In contrast, the redistribution of fluid from the extravascular space to the plasma, which was almost nil during the surgery, showed only limited recovery during the postoperative phase, and was only approximately 10% of the flow rate found in the volunteers. The combination of nearly normalised urinary excretion and lack of adequate return of distributed fluid to the plasma promoted postoperative hypovolaemia.

Conclusion

The kinetic analysis indicates that plasma volume support should be given during the first 2 h after laparoscopic cholecystectomy.

Existing fluid responsiveness studies using the mini-fluid challenge may be misleading: Methodological considerations and simulations

BACKGROUND

The mini-fluid challenge (MFC) is a clinical concept of predicting fluid responsiveness by rapidly infusing a small amount of intravenous fluids, typically 100 ml, and systematically assessing its haemodynamic effect. The MFC method is meant to predict if a patient will respond to a subsequent, larger fluid challenge, typically another 400 ml, with a significant increase in stroke volume.

METHODS

We critically evaluated the general methodology of MFC studies, with statistical considerations, secondary analysis of an existing study and simulations.

RESULTS

Secondary analysis of an existing study showed that the MFC could predict the total fluid response (MFC + 400 ml) with an area under the receiver operator characteristic curve (AUROC) of 0.92, but that the prediction was worse than random for the response to the remaining 400 ml (AUROC = 0.33). In a null simulation with no response to both the MFC and the subsequent fluid challenge, the commonly used analysis could predict fluid responsiveness with an AUROC of 0.73.

CONCLUSION

Many existing MFC studies are likely overestimating the classification accuracy of the MFC. This should be considered before adopting the MFC into clinical practice. A better study design includes a second, independent measurement of stroke volume after the MFC. This measurement serves as reference for the response to the subsequent fluid challenge.

Distribution of crystalloid fluid infused during onset of anesthesia-induced hypotension: a retrospective population kinetic analysis

Background

Induction of anesthesia causes a drop in arterial pressure that might change the kinetics of infused crystalloid fluid. The aim of this report is to provide a mathematical view of how fluid distributes in this setting.

Methods

Data were retrieved from three studies where 76 patients (mean age 63 years, mean body weight 66 kg) had received approximately 1.1 L of Ringer’s solution over 60 min by intravenous infusion before and during induction of spinal, epidural, or general anesthesia. A population kinetic model was used to analyze the fluid distribution and its relationship to individual-specific factors. Frequent measurements of blood hemoglobin and the urinary excretion served as dependent variables.

Results

Before anesthesia induction, distribution to the extravascular space was threefold faster than elimination by urinary excretion. Both distribution and elimination of infused fluid were retarded in an exponential fashion due to the anesthesia-induced decrease in the mean arterial pressure (MAP). A decrease in MAP from 110 to 60 mmHg reduced the rate of distribution by 75% and the rate of elimination by 90%. These adaptations cause most of the infused fluid to remain in the bloodstream. Age, gender, type of anesthesia, and the use of ephedrine had no statistically significant effect on plasma volume expansion, apart from their possible influence on MAP.

Conclusion

The decrease in MAP that accompanies anesthesia induction depresses the blood hemoglobin concentration by inhibiting both the distribution and elimination of infused crystalloid fluid. The report provides mathematical information about the degree of these changes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13741-021-00204-5.

Model-predicted capillary leakage in graded hypotension: Extended analysis of experimental spinal anesthesia

BACKGROUND

Crystalloid fluid infused during the induction of spinal anesthesia is involved in a complex set of physiological responses, including vasodilatation, reactive vasoconstriction, and changes in mean arterial pressure (MAP). The present evaluation compares the modeled capillary leakage in anesthetized versus nonanesthetized body regions.

METHODS

Ten female volunteers (mean age, 29 years) received 25 ml/kg of Ringer's acetate over 60 min during experimental spinal anesthesia. Blood hemoglobin was measured repeatedly in the radial artery (reference), arm (cubital) vein, and leg (femoral) vein for 240 min. Each pattern of data served as a dependent variable in volume kinetic analyses that used mixed models software and MAP as covariate.

RESULTS

The capillary leakage of fluid from the plasma to the extravascular space peaked at 17 ml/min when MAP was 100 mmHg, and the two venous curves were virtually identical. At MAP 60 mmHg, the rate was reduced to 10-12 ml/min when assessed in arterial blood and leg vein blood, but only 5 mmHg in blood collected from the arm vein. The distribution half-life of infused fluid was then 40 min in the leg and 80 min in the arm. These results suggest that vasoconstriction in nonanesthetized body regions halves the capillary leakage that is observed in vasodilated, anesthetized body regions.

CONCLUSION

Graded hypotension during spinal anesthesia reduced the capillary filtration of fluid as determined by volume kinetic analysis. The effect was twice as great when venous blood was sampled from a nonanesthetized body region than from an anesthetized body region.

Evaluation of Evidence, Pharmacology, and Interplay of Fluid Resuscitation and Vasoactive Therapy in Sepsis and Septic Shock

ABSTRACT

We sought to review the pharmacology of vasoactive therapy and fluid administration in sepsis and septic shock, with specific insight into the physiologic interplay of these agents. A PubMed/MEDLINE search was conducted using the following terms (vasopressor OR vasoactive OR inotrope) AND (crystalloid OR colloid OR fluid) AND (sepsis) AND (shock OR septic shock) from 1965 to October 2020. A total of 1,022 citations were reviewed with only relevant clinical data extracted. While physiologic rationale provides a hypothetical foundation for interaction between fluid and vasopressor administration, few studies have sought to evaluate the clinical impact of this synergy. Current guidelines are not in alignment with the data available, which suggests a potential benefit from low-dose fluid administration and early vasopressor exposure. Future data must account for the impact of both of these pharmacotherapies when assessing clinical outcomes and should assess personalization of therapy based on the possible interaction.

The Colloid Osmotic Pressure Across the Glycocalyx: Role of Interstitial Fluid Sub-Compartments in Trans-Vascular Fluid Exchange in Skeletal Muscle

The primary purpose of these investigations is to integrate our growing knowledge about the endothelial glycocalyx as a permeability and osmotic barrier into models of trans-vascular fluid exchange in whole organs. We describe changes in the colloid osmotic pressure (COP) difference for plasma proteins across the glycocalyx after an increase or decrease in capillary pressure. The composition of the fluid under the glycocalyx changes in step with capillary pressure whereas the composition of the interstitial fluid takes many hours to adjust to a change in vascular pressure. We use models where the fluid under the glycocalyx mixes with sub-compartments of the interstitial fluid (ISF) whose volumes are defined from the ultrastructure of the inter-endothelial cleft and the histology of the tissue surrounding the capillaries. The initial protein composition in the sub-compartments is that during steady state filtration in the presence of a large pore pathway in parallel with the "small pore" glycocalyx pathway. Changes in the composition depend on the volume of the sub-compartment and the balance of convective and diffusive transport into and out of each sub-compartment. In skeletal muscle the simplest model assumes that the fluid under the glycocalyx mixes directly with a tissue sub-compartment with a volume less than 20% of the total skeletal muscle interstitial fluid volume. The model places limits on trans-vascular flows during transient filtration and reabsorption over periods of 30-60 min. The key assumption in this model is compromised when the resistance to diffusion between the base of the glycocalyx and the tissue sub-compartment accounts for more than 1% of the total resistance to diffusion across the endothelial barrier. It is well established that, in the steady state, there can be no reabsorption in tissue such as skeletal muscle. Our approach extends this idea to demonstrate that transient changes in vascular pressure favoring initial reabsorption from the interstitial fluid of skeletal muscle result in much less fluid exchange than is commonly assumed. Our approach should enable critical evaluations of the empirical models of trans-vascular fluid exchange being used in the clinic that do not account for the hydrostatic and COPs across the glycocalyx.

Interstitial washdown and vascular albumin refill during fluid infusion: novel kinetic analysis from three clinical trials

BACKGROUND AND AIMS

Increased capillary filtration may paradoxically accelerate vascular refill of both fluid and albumin from the interstitial space, which is claimed to be edema-preventing. We characterized this proposed mechanism, called "interstitial washdown", by kinetic analyses of the hemodilution induced by intravenous infusion of crystalloid fluid during 3 distinct physiological states.

METHODS

Greater plasma dilution of hemoglobin as compared to albumin during fluid therapy indicated recruitment of albumin, which was compared to the flow of interstitial fluid to the plasma as indicated by population volume kinetic analysis. Data for the comparison were derived from 24 infusions of crystalloid fluid in conscious volunteers, 30 in anesthetized patients, and 31 in patients with ketoacidosis from hyperglycemia.

RESULTS

"Interstitial washdown" increased the plasma albumin concentration by between 0.3 and 1.0 g/L in the three series of infusions. The initial albumin concentration in the interstitial fluid returning to the plasma was estimated to between 22 g/L and 29 g/L, which decreased to an average of 50-75% lower during the subsequent 2-3 h. Kinetic simulations show that pronounced washdown was associated with increased capillary filtration (high k₁₂) and, in conscious subjects, with greater plasma and interstitial volume expansion and restricted urine flow. During anesthesia, the main effect was an increase in the non-exchangeable fluid volume ("third-spacing").

CONCLUSIONS

Crystalloid fluid accelerates lymphatic flow that moderately increases plasma albumin, but more clearly helps to maintain the intravascular volume. This "interstitial washdown" mechanism becomes exhausted after a few hours.

Colloids Yes or No? - a "Gretchen Question" Answered

Colloid solutions, both natural and synthetic, had been widely accepted as having superior volume expanding effects than crystalloids. Synthetic colloid solutions were previously considered at least as effective as natural colloids, as well as being cheaper and easily available. As a result, synthetic colloids (and HES in particular) were the preferred resuscitation fluid in many countries. In the past decade, several cascading events have called into question their efficacy and revealed their harmful effects. In 2013, the medicines authorities placed substantial restrictions on HES administration in people which has resulted in an overall decrease in their use. Whether natural colloids (such as albumin-containing solutions) should replace synthetic colloids remains inconclusive based on the current evidence. Albumin seems to be safer than synthetic colloids in people, but clear evidence of a positive effect on survival is still lacking. Furthermore, species-specific albumin is not widely available, while xenotransfusions with human serum albumin have known side effects. Veterinary data on the safety and efficacy of synthetic and natural colloids is limited to mostly retrospective evaluations or experimental studies with small numbers of patients (mainly dogs). Large, prospective, randomized, long-term outcome-oriented studies are lacking. This review focuses on advantages and disadvantages of synthetic and natural colloids in veterinary medicine. Adopting human guidelines is weighed against the particularities of our specific patient populations, including the risk-benefit ratio and lack of alternatives available in human medicine.

Fluid escapes to the "third space" during anesthesia, a commentary

BACKGROUND

The "third fluid space" is a concept that has caused much confusion for more than half a century, dividing anesthesiologists into believers and non-believers.

AIM

To challenge the existence of the "third fluid space" based on analysis of crystalloid fluid kinetics.

METHODS

Data on hemodilution patterns from 157 infusion experiments performed in volunteers and from 85 patients undergoing surgery under general anesthesia were studied by population volume kinetic analysis. Elimination of infused crystalloid fluid from the kinetic model could occur either as urine or "third space" accumulation. The latter fluid volume remained in the body, but without equilibrating with the plasma within the 3-4 h of the experiment.

RESULTS

The rate constant for "third space" loss of fluid accounted for 20% of the elimination in conscious volunteers and for 75% during general anesthesia and surgery. The two elimination constants showed a reciprocal relationship, resulting in that "third-space" losses increase when urinary excretion is restricted. The effect on the plasma volume was smaller than indicated by these figures because fluid distributed to the extravascular space continuously redistributed to the plasma. Worked-out examples show that one-third of an infused crystalloid volume has been confined to the "third space" after 3 h of surgery. When equilibration with the plasma eventually occurs, which is necessary for excretion of the fluid, is not known.

CONCLUSION

During anesthesia and surgery one third of the infused crystalloid fluid is at least temporarily unavailable for excretion, which probably contributes to postoperative weight increase and edema.

Respiratory Variation in Aortic Blood Flow Velocity in Hemodynamically Unstable, Ventilated Neonates: A Pilot Study of Fluid Responsiveness

OBJECTIVES

To assess whether respiratory variation in aortic blood flow peak velocity can predict preload responsiveness in mechanically ventilated and hemodynamically unstable neonates.

DESIGN

Prospective observational diagnostic accuracy study.

SETTING

Third-level neonatal ICU.

PATIENTS

Hemodynamically unstable neonates under mechanical ventilation.

INTERVENTIONS

Fluid challenge with 10 mL/kg of normal saline over 20 minutes.

MEASUREMENTS AND MAIN RESULTS

Respiratory variation in aortic blood flow peak velocity and superior vena cava flow were measured at baseline (T0), immediately upon completion of the fluid infusion (T1), and at 1 hour after fluid administration (T2). Our main outcome was preload responsiveness which was defined as an increase in superior vena cava flow of at least 10% from T0 to T1. Forty-six infants with a median (interquartile range) gestational age of 30.5 weeks (28-36 wk) were included. Twenty-nine infants (63%) were fluid responders, and 17 (37%) were nonresponders Fluid responders had a higher baseline (T0) respiratory variation in aortic blood flow peak velocity than nonresponders (9% [8.2-10.8] vs 5.5% [3.7-6.6]; p < 0.001). Baseline respiratory variation in aortic blood flow peak velocity was correlated with the increase in superior vena cava flow from T0 to T1 (rho = 0.841; p < 0.001). The area under the receiver operating characteristic curve of respiratory variation in aortic blood flow peak velocity to predict preload responsiveness was 0.912 (95% CI, 0.82-1). A respiratory variation in aortic blood flow peak velocity cut-off point of 7.8% provided a 90% sensitivity (95% CI, 71-97), 88% specificity (95% CI, 62-98), 7.6 positive likelihood ratio (95% CI, 2-28), and 0.11 negative likelihood ratio (95% CI, 0.03-0.34) to predict preload responsiveness.

CONCLUSIONS

Respiratory variation in aortic blood flow velocity may be useful to predict the immediate response to a fluid challenge in hemodynamically unstable neonates under mechanical ventilation. If our results are confirmed, this measurement could be used to guide safe and individualized fluid resuscitation in critically ill neonates.

Crystalloid and Colloid Compositions and Their Impact

This manuscript will review crystalloid (hypo-, iso-, and hyper-tonic) and colloid (synthetic and natural) fluids that are available for intravenous administration with a focus on their electrolyte, acid-base, colligative, and rheological effects as they relate to each solution's efficacy and safety. The goal is for the reader to better understand the differences between each fluid and the influence on plasma composition, key organ systems, and their implications when used therapeutically in animals with critical illness.

Interpretation of volume kinetics in terms of pharmacokinetic principles

Volume kinetics is the pharmacokinetics of infusion fluids and describes the distribution and elimination of infused volume. Generally, pharmacokinetic parameters can be estimated by measuring the concentration of a drug. However, it is almost impossible to directly measure the concentration of fluids. Therefore, in volume kinetics, the disposition of fluids is indirectly quantified by measuring the hemoglobin concentration under the premise of no hemoglobin loss. If the hemoglobin concentration is repeatedly measured while administering the fluids, the dilution (relative change of the plasma volume) for each corresponding hemoglobin concentration can be obtained. The dilution is based on the concept of plasma volume expansion. The method of quantifying the drugs disposition with compartmental analysis has been equally applied to volume kinetics. The transfer of fluids between compartments is explained by first-order kinetics, and it is assumed that fluid is only removed from the central compartment. Population analysis can be used to identify covariates that can account for inter-individual variability in volume kinetic parameters. Body weight and mean blood pressure are well-known representative covariates of kinetic volume parameters. Using volume kinetic parameters, the volume expansion effects of crystalloid and colloid solutions can be understood more effectively, thereby facilitating appropriate fluid therapy. Although limitations exist in volume kinetics, its implications are important for clinicians when administering fluids.

Renal water conservation and the volume kinetics of fluid-induced diuresis: A retrospective analysis of two cohorts of elderly men

Renal water conservation after an overnight fast mirrors the habitual intake of liquid. The hypothesis in the present study was that water conservation influences the diuretic response to infusion of two types of crystalloid fluid. Twenty-three elderly male patients (mean age 72 years) underwent a total of 46 intravenous infusions of 1.0 or 1.5 L of either hypotonic non-electrolyte fluid (glycine 1.5%) or isotonic electrolyte fluid (Ringer's acetate or 0.9% saline). Urine osmolality (used to indicate renal water conservation) and plasma creatinine were measured before the infusions started. A two-volume model was fitted to repeated measurements of the blood haemoglobin concentration and the urinary excretion, using mixed-effects modelling software. Urine osmolality was examined as a potential covariate to the fixed kinetic parameters. The results show that distribution and redistribution of infused fluid occurred twice as fast for the non-electrolyte fluids as for the electrolyte-containing fluids, while the urine flow showed less difference. For both types of fluid, high urine osmolality served as a statistically significant covariate to the rate constant describing urinary excretion. Simulations showed that a high pre-infusion urine osmolality doubled the time required for the kidneys to excrete 50% of a 30-minute infusion. High plasma creatinine independently prolonged the elimination of non-electrolyte fluid. The use of 0.9% saline instead of Ringer's prolonged the excretion of electrolyte-containing fluid. In conclusion, renal water conservation is a determinant of the diuretic response to crystalloid fluid, regardless of whether the fluid contains electrolytes, and it should be considered in fluid balance studies.

Terms, Definitions, Nomenclature, and Routes of Fluid Administration

Fluid therapy is administered to veterinary patients in order to improve hemodynamics, replace deficits, and maintain hydration. The gradual expansion of medical knowledge and research in this field has led to a proliferation of terms related to fluid products, fluid delivery and body fluid distribution. Consistency in the use of terminology enables precise and effective communication in clinical and research settings. This article provides an alphabetical glossary of important terms and common definitions in the human and veterinary literature. It also summarizes the common routes of fluid administration in small and large animal species.

Phases of fluid management and the roles of human albumin solution in perioperative and critically ill patients

OBJECTIVE

Positive fluid balance is common among critically ill patients and leads to worse outcomes, particularly in sepsis, acute respiratory distress syndrome, and acute kidney injury. Restrictive fluid infusion and active removal of accumulated fluid are being studied as approaches to prevent and treat fluid overload. Use of human albumin solutions has been investigated in different phases of restrictive fluid resuscitation, and this narrative literature review was undertaken to evaluate hypoalbuminemia and the roles of human serum albumin with respect to hypovolemia and its management.

METHODS

PubMed/EMBASE search terms were: "resuscitation," "fluids," "fluid therapy," "fluid balance," "plasma volume," "colloids," "crystalloids," "albumin," "hypoalbuminemia," "starch," "saline," "balanced salt solution," "gelatin," "goal-directed therapy" (English-language, pre-January 2020). Additional papers were identified by manual searching of reference lists.

RESULTS

Restrictive fluid administration, plus early vasopressor use, may reduce fluid balance, but in some cases fluid overload cannot be entirely avoided. Deresuscitation, with fluid actively removed through diuretics or ultrafiltration, reduces duration of mechanical ventilation and intensive care unit stay. Combining hyperoncotic human albumin solution with diuretics increases hemodynamic stability and diuresis. Hyperoncotic albumin corrects hypoalbuminemia and raises colloid osmotic pressure, limiting edema formation and potentially improving endothelial function. Serum levels of albumin relative to C-reactive protein and lactate may predict which patients will benefit most from albumin therapy.

CONCLUSIONS

Hyperoncotic human albumin solution facilitates restrictive fluid therapy and the effectiveness of deresuscitative measures. Current evidence is mostly from observational studies, and more randomized trials are needed to better establish a personalized approach to fluid management.

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.

Evaluation of the Distribution and Elimination of Balanced Isotonic Crystalloid, 5% Hypertonic Saline, and 6% Tetrastarch 130/0.4 Using Volume Kinetic Modeling and Analysis in Healthy Conscious Cats

This prospective, randomized, blinded, interventional cross-over study investigated the distribution, elimination, plasma volume expansion, half-life, comparative potency, and ideal fluid prescription of three commonly prescribed intravenous (IV) fluids in 10 healthy conscious cats using volume kinetic analysis that is novel to veterinary medicine. Each cat received 20 mL/kg of balanced isotonic crystalloid (PLA), 3.3 mL/kg of 5% hypertonic saline (HS), and 5 mL/kg of 6% tetrastarch 130/0.4 (HES) over 15 min on separate occasions. Hemoglobin concentration, red blood cell count, hematocrit, heart rate, and blood pressure were measured at baseline, 5, 10, 15, 20, 30, 40, 50, 60, and every 15 min until 180 min. Urine output was estimated every 30 min using point-of-care bladder ultrasonography. Plasma dilution derived from serial hemoglobin concentration and red blood cell count served as input variables for group and individual fluid volume kinetic analyses using a non-linear mixed effects model. In general, the distribution of all IV fluids was rapid, while elimination was slow. The half-lives of PLA, HS, and HES were 49, 319, and 104 min, respectively. The prescribed fluid doses for PLA, HS, and HES resulted in similar peak plasma volume expansion of 27–30%. The potency of HS was 6 times higher than PLA and 1.7 times greater than HES, while HES was 3.5 times more potent than PLA. Simulation of ideal fluid prescriptions to achieve and maintain 15 or 30% plasma volume expansion revealed the importance of a substantial reduction in infusion rates following initial IV fluid bolus. In conclusion, volume kinetic analysis is a feasible research tool that can provide data on IV fluid kinetics and body water physiology in cats. The rapid distribution but slow elimination of IV fluids in healthy conscious cats is consistent with anecdotal reports of fluid overload susceptibility in cats and warrants further investigation.

Gelatin and the risk of bleeding after cardiac surgery

BACKGROUND

Gelatins has been used in cardiac surgery because of their ability to preserve intravascular volume better than crystalloids. Unfortunately, gelatin has been associated with impaired coagulation and hemostasis, that may cause increased bleeding. We investigated whether the administration of gelatin increases postoperative bleeding after cardiac surgery.

METHODS

Retrospective, observational single-center cohort study in the intensive care unit of a tertiary teaching hospital. Postoperative bleeding, chest tube drainage volume and consumption of blood products were compared between groups.

RESULTS

Cohort included 3067 consecutive patients who underwent cardiac surgery. First 1698 patients received gelatin (gelatin group), and 1369 patients did not (crystalloid group). The characteristics of the patients in the gelatin and crystalloid groups were comparable. Postoperative chest tube drainage was 18% (95% CI 11%-20%) greater during the first 12 hours (P < .001) and 15% (95% CI 7%-17%) greater during the first 24 hours (P < .001) in the gelatin group compared to the crystalloid group. Severe and massive postoperative bleeding was more common in the gelatin group compared to the crystalloid group (21% vs 16%, P < .001). Patients in the gelatin group received red blood cells (40% vs 20%, P < .001) and platelets (12% vs 8%, P < .001) more frequently than patients in the crystalloid group. However, the number of administered fresh-frozen plasma transfusions did not differ between the groups.

CONCLUSION

Gelatin may increase postoperative bleeding and the need for blood product transfusions after cardiac surgery.

Syndecan-1 and Glypican-1 Knockout Alters Body Water Balance and Urine Response to Fluid Challenge in Mice

Syndecan-1 (Sdc-1) and glypican-1 (Gpc-1) are 2 important proteoglycans found in the glycocalyx and believed to govern transvascular distribution of fluid and protein. In this translational study, we assessed Sdc-1 and Gpc-1 knockout (KO) on whole body water balance after an intravenous volume challenge. Sdc-1 and Gpc-1 KO mice had higher starting blood water content versus strain-matched controls. Sdc-1 KO mice exhibited a significantly higher diuretic response (87%; p < 0.05), higher excreted volume/infusion volume ratio (p < 0.01), higher extravascular/infused ratio, and greater tissue water concentration (60 vs. 52%). Collectively, these suggest differences in kidney response and greater fluid efflux from peripheral vessels. The CD1 strain and Gpc-1 KO had a 2-3-fold larger urine output relative to C57 strain, but Gpc-1 KO reduced the excreted/infused ratio relative to controls (p < 0.01) and they maintained plasma dilution longer. Thus, genetic KO of Sdc-1 and Gpc-1 resulted in markedly different phenotypes. This work establishes the feasibility of performing fluid balance studies in mice.

Kinetics of crystalloid fluid in hyperglycemia; an open-label exploratory clinical trial

BACKGROUND

Infusion with 0.9% saline is a mainstay in the treatment of severe hyperglycemia, but the kinetics of the saline volume in this setting has not been studied.

METHODS

An intravenous infusion of 1 L of 0.9% saline over 30 minutes was given on 31 occasions to 17 patients with hyperglycemia due to poorly controlled diabetes (mean age 51 years). A two-volume kinetic model was fitted to serial data on the hemodilution and urinary excretion, using mixed-effects modeling software.

RESULTS

Plasma glucose was 36 ± 9 mmol/L on arrival to the hospital. The central volume of distribution (the plasma) was only 2.38 L (mean; 95% confidence interval 1.73-3.04) on the day of admission. Uptake into a remote compartment, believed to be the cells, amounted to 300 mL of the first liter of saline, although only small amounts of insulin were given. Plasma glucose, plasma bicarbonate, urine glucose, and plasma creatinine served as covariates in the kinetic model and mathematically affected the urinary excretion. For example, elimination of the infused fluid tripled from an increase in plasma glucose from 5 to 35 mmol/L and doubled from a reduction in plasma bicarbonate from 24 to 5 mmol/L.

CONCLUSIONS

The excretion of 0.9% saline was increased depending on the degree of hyperglycemia. The kinetics was characterized by glucose-accelerated diuresis, and an intracellular uptake that occurred at two thirds the urine flow rate. These data could help to determine appropriate volumes and rates of infusion of crystalloids in hyperglycemia.

The Extended Starling principle needs clinical validation

The Revised (or "Extended") Starling principle is based on highly controlled laboratory-based frog and rodent experiments and remains a hypothesis awaiting clinical validation. A key point is that the endothelial glycocalyx layer moves the oncotic gradient from being between the plasma and the interstitium to between the plasma and a virtually protein-free space between the glycocalyx and the endothelial cell membrane, which dramatically changes the prerequisites for fluid absorption from tissue to plasma. However, many experimental and clinical observations in humans agree poorly with the new microcirculatory proposals. The most troubling aspect of the explanation regarding the role of the glycocalyx in the Revised Starling principle is the effective reabsorption of fluid by skeletal muscle when the capillary filtration pressure is acutely reduced. Other issues include the plasma volume effects of hypertonic saline, iso-oncotic and hyper-oncotic albumin, fluid distribution during cardio-pulmonary bypass, and the virtually identical capillary leakage of plasma and albumin despite marked inflammation found in our fluid therapy studies. The Revised Starling principle deals mainly with steady-state conditions, but the circulatory system is highly dynamic. Second to second vasomotion is always operational and must be considered to understand what we observe in humans.

Water content of the endothelial glycocalyx layer estimated by volume kinetic analysis

BACKGROUND

The water volume of the endothelial glycocalyx layer has been estimated at 0.7 to 1.7 L using tracer methods of unclear value. The present study attempts to measure this fluid volume by analyzing the kinetics of a crystalloid fluid load.

METHODS

An intravenous infusion of approximately 1 L of Ringer's acetate was administered to 35 healthy volunteers, and the central volume of distribution of the water volume was calculated from the urinary excretion and frequent measurements of the fluid-induced hemodilution using mixed-effects modeling software. Comparisons were made with the plasma volume derived from three published anthropometric regression equations based on isotope measurements. In a second analysis, up to 2.5 L of Ringer's was administered to 60 volunteers selected from a cohort of 160 to have as similar hematocrits as possible to the volunteers whose data were used to create the anthropometric equations.

RESULTS

Volume kinetics showed that the infused crystalloid fluid occupied a larger central fluid space than was estimated with the isotope measurements. The first analysis of the 35 subjects indicated a mean difference of 0.51 L in males and 0.49 L in females. The second, larger analysis showed a mean excess volume of 0.43 L, which was approximately 15% of the circulating plasma volume.

CONCLUSIONS

A crystalloid fluid load expands a 0.4-0.5 L larger central fluid space than the circulating plasma volume. The excess volume is probably located in the glycocalyx layer.

Volume kinetic analysis of fluid retention after induction of general anesthesia

Background

Induction of general anesthesia increases the hemodilution resulting from infusion of crystalloid fluid, which is believed to be due to slower distribution caused by arterial hypotension. When normal distribution returns is not known.

Methods

An intravenous infusion of 25 mL kg^− 1 of Ringer’s lactate was infused over 30 min to 25 volunteers just after induction of general anesthesia for open abdominal hysterectomy. A two-volume model was fitted to the repeated measurements of the blood hemoglobin concentration and the urinary excretion using mixed-effects modelling software. Individual-specific covariates were added in sequence.

Results

Distribution of infused fluid was interrupted during the first 20 min of the infusions. During this time 16.6 mL kg^− 1 of lactated Ringer’s had been infused, of which virtually all remained in the circulating blood. Thereafter, the fluid kinetics was similar to that previously been found in awake volunteers except for the elimination rate constant (k₁₀), which remained to be very low (0.86 × 10^− 3 min^− 1). Redistribution of infused fluid from the interstitium to the plasma occurred faster (higher k₂₁) when the arterial pressure was low. No covariance was found between the fixed parameters and preoperatively concentrated urine, the use of sevoflurane or propofol to maintain the anesthesia, or the plasma concentrations of two degradation products of the endothelial glycocalyx, syndecan-1 and heparan sulfate.

Conclusions

Induction of general anesthesia interrupted the distribution of lactated Ringer’s solution up to when 16.6 mL kg^− 1 of crystalloid fluid had been infused. Plasma volume expansion during this period of time was pronounced.

Trial registration

Controlled-trials.com (ISRCTN81005631) on May 17, 2016 (retrospectively registered).