Postoperative fluid retention after heart surgery is accompanied by a strongly positive sodium balance and a negative potassium balance

A system theory based digital model for predicting the cumulative fluid balance course in intensive care patients

Background: Surgical interventions can cause severe fluid imbalances in patients undergoing cardiac surgery, affecting length of hospital stay and survival. Therefore, appropriate management of daily fluid goals is a key element of postoperative intensive care in these patients. Because fluid balance is influenced by a complex interplay of patient-, surgery- and intensive care unit (ICU)-specific factors, fluid prediction is difficult and often inaccurate. Methods: A novel system theory based digital model for cumulative fluid balance (CFB) prediction is presented using recorded patient fluid data as the sole parameter source by applying the concept of a transfer function. Using a retrospective dataset of n = 618 cardiac intensive care patients, patient-individual models were created and evaluated. RMSE analyses and error calculations were performed for reasonable combinations of model estimation periods and clinically relevant prediction horizons for CFB. Results: Our models have shown that a clinically relevant time horizon for CFB prediction with the combination of 48 h estimation time and 8-16 h prediction time achieves high accuracy. With an 8-h prediction time, nearly 50% of CFB predictions are within ±0.5 L, and 77% are still within the clinically acceptable range of ±1.0 L. Conclusion: Our study has provided a promising proof of principle and may form the basis for further efforts in the development of computational models for fluid prediction that do not require large datasets for training and validation, as is the case with machine learning or AI-based models. The adaptive transfer function approach allows estimation of CFB course on a dynamically changing patient fluid balance system by simulating the response to the current fluid management regime, providing a useful digital tool for clinicians in daily intensive care.

Electrolytes, albumin and acid base equilibrium during laparoscopic surgery

BACKGROUND

Surgery, causing inflammation, disrupts endothelial permeability leading to movement of fluids and albumin across the vascular barrier. Fluid therapy for restoring circulatory homeostasis may lead to positive fluid balance which has been shown to increase morbidity and mortality in surgical patients. The current investigation aims to describe physio-pathological changes in circulating albumin, fluid and electrolyte balance, and acidbase equilibrium in a cohort of patients undergoing laparoscopic surgery under general anesthesia.

METHODS

Single-center prospective observational study. Patients undergoing laparoscopic colorectal surgery were screened for eligibility.. Before surgery the baseline fasting conditions were homogenized. Hemoglobin, urinary and plasmatic were collected before surgery and then at pre-defined timepoints. Albumin/Creatinine ratio was measured before and after surgery. Expected and actual circulating Sodium concentrations were compared according to a physiological theoretical model. Assessment and quantification of changes in major electrolytes, albumin and acid-base balance was defined as the primary outcome of the study.

RESULTS

38 patients were enrolled in the protocol. Patients had a positive electrolytes (Na+ 295 [244-375] mmol, Cl- 234 [195-295] mmol, K+ 16.8 [12.0-21.4] mmol) and fluid balance (2165 [1727-2728] ml). The positive fluid balance was associated with stable chloride (105 [103-107], end study vs 103 [102-106] mmol/L, baseline, p=ns) and potassium (4.2 [3.8-4.4], end study vs 4.1 [3.6-4.4] mmol/L, baseline, p=ns) levels, but sodium concentrations decreased over time (138 [137-140], end study vs 139 [138-141] mmol/L, baseline, p<0.05). The Albumin/Creatinine ratio was higher at the end of surgery 134 [61-267] vs 7 [4-14], p<0.001).

CONCLUSIONS

Data from patients undergoing colorectal laparoscopic surgery showed a positive fluid balance, decreased circulating albumin and increased albuminuria. A positive sodium balance was not always associated with an increase in sodium plasma concentration.

Estimation of sodium and chloride storage in critically ill patients: a balance study

BACKGROUND

Nonosmotic sodium storage has been reported in animals, healthy individuals and patients with hypertension, hyperaldosteronism and end-stage kidney disease. Sodium storage has not been studied in ICU patients, who frequently receive large amounts of sodium chloride-containing fluids. The objective of our study was to estimate sodium that cannot be accounted for by balance studies in critically ill patients. Chloride was also studied. We used multiple scenarios and assumptions for estimating sodium and chloride balances.

METHODS

We retrospectively analyzed patients admitted to the ICU after cardiothoracic surgery with complete fluid, sodium and chloride balance data for the first 4 days of ICU treatment. Balances were obtained from meticulously recorded data on intake and output. Missing extracellular osmotically active sodium (MES) was calculated by subtracting the expected change in plasma sodium from the observed change in plasma sodium derived from balance data. The same method was used to calculate missing chloride (MEC). To address considerable uncertainties on the estimated extracellular volume (ECV) and perspiration rate, various scenarios were used in which the size of the ECV and perspiration were varied.

RESULTS

A total of 38 patients with 152 consecutive ICU days were analyzed. In our default scenario, we could not account for 296 ± 35 mmol of MES in the first four ICU days. The range of observed MES in the five scenarios varied from 111 ± 27 to 566 ± 41 mmol (P < 0.001). A cumulative value of 243 ± 46 mmol was calculated for MEC in the default scenario. The range of cumulative MEC was between 62 ± 27 and 471 ± 56 mmol (P = 0.001 and P = 0.003). MES minus MEC varied from 1 ± 51 to 123 ± 33 mmol in the five scenarios.

CONCLUSIONS

Our study suggests considerable disappearance of osmotically active sodium in critically ill patients and is the first to also suggest rather similar disappearance of chloride from the extracellular space. Various scenarios for insensible water loss and estimated size for the ECV resulted in considerable MES and MEC, although these estimates showed a large variation. The mechanisms and the tissue compartments responsible for this phenomenon require further investigation.

Postoperative fluid retention after heart surgery is accompanied by a strongly positive sodium balance and a negative potassium balance

The conventional model on the distribution of electrolyte infusions states that water will distribute proportionally over both the intracellular (ICV) and extracellular (ECV) volumes, while potassium homes to the ICV and sodium to the ECV. Therefore, total body potassium is the most accurate measure of ICV and thus potassium balances can be used to quantify changes in ICV. In cardiothoracic patients admitted to the ICU we performed complementary balance studies to measure changes in ICV and ECV. In 39 patients, fluid, sodium, potassium, and electrolyte‐free water (EFW) balances were determined to detect changes in ICV and ECV. Cumulatively over 4 days, these patients received a mean ± SE infusion of 14.0 ± 0.6 L containing 1465 ± 79 mmol sodium, 196 ± 11 mmol potassium and 2.1 ± 0.1 L EFW. This resulted in strongly positive fluid (4.0 ± 0.6 L) and sodium (814 ± 75 mmol) balances but in negative potassium (−101 ± 14 mmol) and EFW (−1.1 ± 0.2 L) balances. We subsequently compared potassium balances (528 patients) and fluid balances (117 patients) between patients who were assigned to either a 4.0 or 4.5 mmol/L blood potassium target. Although fluid balances were similar in both groups, the additionally administered potassium (76 ± 23 mmol) in the higher target group was fully excreted by the kidneys (70 ± 23 mmol). These findings indicate that even in the context of rapid and profound volume expansion neither water nor potassium moves into the ICV.