What is the optimal approach to perioperative fluid management?

Intravenous fluid therapy (IVT) is integral to the perioperative management of surgical patients. Fluid volume and composition must be considered, along with operation-specific and goal-directed therapeutic regimens. There are many factors that affect perioperative fluid status and without due diligence, significant morbidity may result. Failure to counteract net fluid deficits can result in end-organ dysfunction such as acute kidney injury (AKI). Poorly conceived IVT orders can result in fluid overload states such as pulmonary oedema and surgical complications such as failure of anastomoses and intention. However, despite extensive research, the most effective IVT regimen remains unclear.

Surgery creates intravascular volume derangements. Patients are required to fast, undergo bowel preparation and in emergency settings may present in acute pathological states. The type of anaesthesia and choice of agent may contribute to hypotension, dose-dependent vasodilation and myocardial depression (Royse et al 2008). Prolonged operating time increases the risk of insensible losses (Jacob et al 2009), and a timely transition from IV to oral fluid has been associated with improved gastrointestinal motility and a reduction of fluid loss into the bowel (Miller & Myles 2019). Intraoperatively, a sympathetic ‘stress’ response induces metabolic changes that are often proportional to severity of surgical trauma (Burton et al 2004). Hormonal mediators such as antidiuretic hormone, aldosterone, and cortisol result in increased water reabsorption in the kidney, reduced diuresis and potassium wasting (Holte et al 2002). Surgical insult produces an inflammatory response and triggers cytokine release that propagates vascular, tissue and organ dysfunction (Holte et al 2002). This pathological process derived the concept of ‘third spacing’. Initially conceptualised as a non-anatomical extracellular compartment, the classical third-space is better characterised by a fluid shift from the intravascular to interstitial space secondary to endothelial damage from surgical manipulation, reperfusion injury and iatrogenic hypervolemia (Jacob et al 2009, Strunden et al 2011).

Perioperative IVT may be administered using crystalloid or colloid solutions. Crystalloids are inexpensive solutions of sterile water, containing salt and minerals of small molecular size (Lewis et al 2018), and vary in tonicity. Alternatively, colloids are semi-synthetic or naturally occurring larger molecular substances suspended in a crystalloid solution and include albumin, starches and gelatins (Lewis et al 2018). Crystalloids are cheap, safe, easy to administer and closely emulate the physiological contents of the plasma. However, they are subject to volume redistribution with only 20% remaining intravascular (Chappell & Jacob 2014). Comparatively, colloids offer a fourfold increase in plasma volume expansion and require lower volumes to maintain haemodynamic stability, but are more expensive and have potential adverse effects (Yi et al 2019). A recent Cochrane review comparing colloids to crystalloids in critically ill patients concluded that using starches may increase the need for blood transfusions and renal replacement therapy (RRT), however, found no convincing evidence to suggest additional adverse effects in the remaining colloids when compared to crystalloids (Lewis et al 2018).

Despite a theoretical superiority of colloids, recent literature has failed to identify a superior fluid composition for use within the perioperative setting. Furthermore, the postoperative complication rates for each type have not been statistically different (Futier et al 2010). The recent PLUS study aimed to further discriminate between crystalloids, comparing plasma-lyte 148 to normal saline, however, identified no difference in the risk of death or AKI in the critically ill (Finfer et al 2022). With little strong evidence supporting the use of crystalloid or colloid over the other, clinician-dependent decision-making without clear evidence remains the mainstay of care in this area.

Despite ongoing research, there is no unified definition for restrictive and liberal perioperative fluid regimens. The rate of infusion and the frequency and volume of fluid boluses are vastly different across studies and individual study groups. Both regimens, however, exhibit diametrically opposing risk profiles. The liberal approach increases the risk of fluid overload, third space loss and postoperative weight gain (Myles et al 2018). Contrarily, the restrictive approach increases the risk of AKI, surgical site infections and vasopressor requirements (Myles et al 2018).

The success of IVT centres on effective perioperative monitoring. Depending on the setting, a combination of reported symptoms, traditional non-invasive parameters, blood markers and intravascular monitoring may be used. Recent advances in monitoring have created a third potentially superior goal-directed approach to IVT (Messina et al 2021). However, goal-directed fluid therapy (GDFT) has yet to be universally defined. In practice, it centres on dynamic monitoring options such as an oesophageal Doppler to estimate stroke volume and titrate IVT in real-time (Calvo-Vecino et al 2018). Encouraging results from the FEDORA trial, a multicentre randomised control trial exploring the effect of GDFT on postoperative complications in low to moderate-risk surgical patients supported this approach, demonstrating a reduction in postoperative complications and hospital length of stay (Calvo-Vecino et al 2018).

There continues to be a paucity of evidence to support the prescription of perioperative IVT. A major contributing factor to this is the lack of standardisation and agreed definitions for IVT regimens and the significant heterogeneity in outcomes. Abdominal surgery is the largest category used to analyse perioperative IVT in the literature (Myles et al 2018). However, further investigation into specifically high-risk surgical patients may be beneficial. Broadening the scope of research to include under-represented high-risk specialties may allow for more targeted approaches to IVT.

As surgical advances have allowed for operations on increasingly comorbid patients, further research into identifying the optimal approach to perioperative IVT should be a high priority. Avenues for further research include utilising standardised definitions and outcomes to investigate the effects of fluid composition, operation-specific fluid regimens and the utility of goal-direct fluid therapy and harmonisation with oral intake. Using such evidence, we can work towards more clearly defined parameters to provide clarity when assessing the effectiveness of different approaches to perioperative IVT.