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

Perioperative Fluid Management

The medical complexity of the geriatric patients has been steadily rising. Still, as outcomes of surgical procedures in the older adults 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 older adults. 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.

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.

Fluid balance during acute phase extracorporeal cardiopulmonary resuscitation and outcomes in OHCA patients: a retrospective multicenter cohort study

OBJECTIVE

The association between fluid balance and outcomes in patients who underwent out-of-hospital cardiac arrest (OHCA) and received extracorporeal cardiopulmonary resuscitation (ECPR) remains unknown. We aimed to examine the above relationship during the first 24 h following intensive care unit (ICU) admission.

METHODS

We performed a secondary analysis of the SAVE-J II study, a retrospective multicenter study involving OHCA patients aged ≥ 18 years treated with ECPR between 2013 and 2018 and who received fluid therapy following ICU admission. Fluid balance was calculated based on intravenous fluid administration, blood transfusion, and urine output. The primary outcome was in-hospital mortality. The secondary outcomes included unfavorable outcome (cerebral performance category scores of 3-5 at discharge), acute kidney injury (AKI), and need for renal replacement therapy (RRT).

RESULTS

Overall, 959 patients met our inclusion criteria. In-hospital mortality was 63.6%, and the proportion of unfavorable outcome at discharge was 82.0%. The median fluid balance in the first 24 h following ICU admission was 3673 mL. Multivariable analysis revealed that fluid balance was significantly associated with in-hospital mortality (odds ratio (OR), 1.04; 95% confidence interval (CI), 1.02-1.06; p < 0.001), unfavorable outcome (OR, 1.03; 95% CI, 1.01-1.06; p = 0.005), AKI (OR, 1.04; 95% CI, 1.02-1.05; p < 0.001), and RRT (OR, 1.05; 95% CI, 1.03-1.07; p < 0.001).

CONCLUSIONS

Excessive positive fluid balance in the first day following ICU admission was associated with in-hospital mortality, unfavorable outcome, AKI, and RRT in ECPR patients. Further investigation is warranted.

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.

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.

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.

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.

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.