Our Fluid Day study shows that crystalloids are the main intravenous fluid used in the perioperative period, and that balanced solutions are used more often for this purpose, while normal saline is still frequently administered to surgical patients. We know that there are differences between these types of crystalloid solutions, and that this has generated controversy regarding their management in the surgical patient. Firstly, 0.9% chloride-rich saline causes a higher degree of acidosis and dose-dependent hyperchloremia, which can favour smooth muscle vascular contraction that can reduce renal perfusion [15,16,17]. In a study carried out in healthy volunteers who received 2 litres of normal saline 0.9% vs. balanced crystalloid - Plasma- Lyte ®, perfusion in the renal artery and total urine output significantly decreased, with an increase in extravascular fluid compared to Plasma- Lyte ® . These findings support the notion that hyperchloremia can reduce renal cortical perfusion . Similarly, a large observational study showed that the use of Plasma - Lyte ® versus normal saline 0.9% in patients undergoing major abdominal surgery produced less acute kidney failure and need for renal replacement therapy .
Two recent studies also compared the administration of balanced crystalloids and normal saline 0.9% in critical and non-critical patients [16, 17]. Both studies showed a lower incidence of acute kidney damage with balanced solutions, and a lower incidence of death and new-onset renal replacement therapy in critically ill patients. However, the SOLAR study  showed no clinically significant differences in postoperative complications with Ringer lactate ® (balanced crystalloid) or normal saline 0.9% in elective orthopaedic surgery patients and colorectal cancer patients .
Despite this, prior to conducting the Fluid Day study, normal saline 0.9% was not recommended during major surgery , since its administration was associated with hyperchloremia, metabolic acidosis and acute renal injury in the postoperative period [5, 23,24,25]. However, normal saline 0.9% was used in 45% of patients included in the Fluid Day study, specifically in 50% of high-risk patients and high-risk surgeries.
Moreover, the Fluid Day study has shown that increased patient and/or surgery risk was associated with an increase in the total amount of fluid administered within 24 hours, while the total volume/kg/hr administered in the intraoperative period was lower in patients with high risk, and patients with high anaesthesia and surgical risk received the lowest volume. The optimal amount of perioperative maintenance fluid is a highly controversial issue, and this inevitably leads to variability in its administration and total volume contribution [3, 8], although in recent years volume overload avoidance has been recommended, as it increases postoperative complications [1, 9, 26,27,28]. Recently, a cohort study conducted in 500 US hospitals in adult patients undergoing colorectal surgery and primary hip or knee arthroplasty  found a significant association between liberal fluid administration and worse outcomes (increased cost and total hospital stay), as well as increased presence of postoperative ileus, especially in patients undergoing colorectal surgery. Interestingly, the authors also found that restrictive fluid administration (25% lower volume administered with respect to the liberal approach) was also associated with worse outcomes, particularly acute kidney injury in high-risk patients undergoing high-risk surgery [9, 29].
In general, the literature suggests that fluid management during the perioperative period should be based on algorithms and protocols, because they provide better outcomes, especially in terms of volume or total amount administered [8, 9, 13, 26]. Currently, restrictive fluid maintenance therapy is recommended for enhanced recovery after surgery pathways [7, 30]. The RELIEF study  showed that the restrictive approach led to a median of 1.7 litres of intraoperatively fluid administered compared to 3 L with the liberal approach . Patients in the restrictive group had proportionally greater acute renal injury than patients in the liberal group (8.6% vs. 5%, p <0.001). These authors recommended a fluid system to provide a positive fluid balance of 10 to 12 ml/Kg/h during major abdominal surgery, and 1.5 ml/Kg/h in the first 24 h postoperative hours.
In the Fluid Day study, we administered around 6.35 ml/Kg of volume in any group during the intraoperative period, and 3-4 ml/Kg in the first 24 postoperative hours, affirming the aforementioned hypothesis. However, overall fluid administration including the intraoperative period and the first 24 postoperative hours, as well as the administration of fluid as drug excipients, placing the total balance at 15.4 ml/Kg.
Other studies  used a crystalloid maintenance infusion of 10 ml/Kg/h, while the OPTIMISE study  administered 1 ml/Kg/h crystalloid as maintenance fluid. Both studies were performed in the UK and in similar surgeries. Therefore, variability in dosage persists.
Other types of surgery not associated with significant losses do not usually require high intraoperative administration of fluids to achieve a moderate positive fluid balance at the end of surgery, for example, in low-risk patients undergoing low / intermediate risk surgery. For this reason, the total volume administered in our study was 13ml/Kg [9.52; 20.5]. For these patients, a maintenance balanced crystalloid fluid ratio of 1 to 3 ml/Kg/h would be recommended as an early transition to oral fluid therapy after surgery .
Regarding colloid administration, 7.3% of patients received at least one colloid during surgery (mostly HEA), while 2.2% received some type of colloid in the first 24 postoperative hours (mostly gelatine). The mean volume administered was 500 mL. High-risk patients and those who underwent high-risk surgery received colloids in a higher percentage. These results appear to be consistent with the clinical context and show that the administration of colloids was performed in a restricted manner during the study period. Clinical studies comparing colloid and crystalloid administration for goal-directed volume replacement in major abdominal surgery showed that the latter did not reduce the serious complications or hospital stay, but also did not cause long-term or acute renal toxicity . The recent Fluid Loading in Abdominal Surgery: Saline vs Hydroxyethyl Starch (FLASH) trial  compared administration of hydroxyethyl-starch (HEA) vs unbalanced solution in 775 patients at high risk of postoperative renal injury in major abdominal surgery. HEA administration was associated with lower fluid balance, better hemodynamic parameters, and lower vasopressors during surgery; however, they found no differences in their composite outcome of mortality and postoperative complications or postoperative AKI .
In our study, 24.4% of patients (1541) underwent high-risk surgery, 23.7% were high-risk patients (1497) and only 8.7% (554) were high-risk patients undergoing high-risk surgeries. Very few (19%) were admitted to a critical care unit in the postoperative period, giving an idea of the risk of liberal or restrictive fluid therapy in this group. Despite recommendations , only 15% of patients undergoing high-risk surgeries included in the Fluid Day study underwent invasive haemodynamic monitoring, and less than 10% received goal-directed therapy. A small proportion of patients received fluid according to a goal-directed strategy, such as fluid challenge with monitoring strategy (per protocol) or fluid load without monitoring strategy. The use of goal-directed fluid therapy is gradually being incorporated into clinical practice for high-risk patients undergoing high-risk surgery [8, 36]. Cannesson et al.  studied the impact of the systematic implementation of a goal-directed perioperative hemodynamic strategy in patients undergoing high-risk abdominal surgery, finding an 18% decrease in the length of hospital stay and a significant decrease in postoperative complications from 39% to 25%.
In a recent meta-analysis including 45 randomized controlled trials (N = 6344 participants), Sun et al.  reported that a goal-directed therapy was associated with a significant reduction in short- and long-term mortality in patients undergoing major abdominal surgery.
The Fluid Day study is the first observational study on the management of perioperative fluid therapy carried out in different Spanish hospitals. It provides a general description of routine clinical practice of anaesthesiologists in terms of fluid management in patients with different surgical and anaesthesia risks.
The variability shown in Fluid Day highlights the need to improve the management of perioperative fluid therapy. The forthcoming publication of the ideal fluid pattern according to anaesthetic and surgical risk, together with the spread of this knowledge and its implementation and subsequent follow-up will show whether the Fluid Day study has benefited patients.
First, this is an observational study with a minimum follow-up time of 24 hours, a limitation that has not allowed us to analyse safety aspects in the short and long term. This cross-sectional study, unfortunately, did not collect clinical or patient-reported outcomes.
There was also no comparison with a standard of care, because the main objective of the study was to observe the clinical management of perioperative fluid therapy with no intention of making comparisons with an established standard practice.
We did not associate the variability observed in our study with any postoperative outcome. While this is of interest in future studies, we believe that the existence of such variability is in itself an important issue to address to improve the quality of fluid therapy management.
The results of this cross-sectional observational study suggest that balanced solutions are the most widely used crystalloids in any type of surgery and type of patient in our surgical setting. Fluid management is performed without the use of monitoring or goal-directed protocols in most patients and surgeries, and in a small proportion of high-risk patients undergoing high-risk surgeries. This suggests that advanced monitoring is only valued in high-risk patients undergoing high-risk surgeries but does not specifically target fluid therapy management.
We believe that the results obtained in this study are important because they show that there is currently great variability in clinical practice and in the management of perioperative fluid therapy. It appears that more seriously ill patients tend to receive less fluids but often with little advanced monitoring.