erioperative fluid therapy is one of the most controversial topics in perioperative care.¹ Recent studies suggest that fluid minimization strategies (or at least avoidance of fluid overload) in patients undergoing elective major surgical procedures, particularly abdominal procedures, reduce perioperative complications and improve postoperative outcome.^1-3 Intraoperative fluid overload may be avoided by eliminating the use of predetermined algorithms that emphasize replacement of evaporative losses, third-space losses and losses through diuresis as well as eliminating preloading for neuraxial analgesia.² In addition, blood loss may be replaced with colloid on a volume-to-volume basis rather than by crystalloids.² Furthermore, avoidance of deep general anesthesia may prevent the higher fluid administration required to maintain adequate hemodynamics.

One of the major concerns with intraoperative fluid minimization (or restriction) is potential unrecognized (or subclinical) hypovolemia resulting in organ dysfunction, particularly postoperative acute renal failure, which may increase the length of hospital stay, postoperative mortality and health care costs.^4-6 Intraoperative urine output, which is thought to be the clinical manifestation of renal blood flow, is commonly used as an indicator of intravascular volume status and to guide fluid therapy. Most guidelines recommend that urine output of more than 0.5 ml/kg/h should be maintained, intraoperatively. However, the evidence for this recommendation is questionable. Thus, the relationship between urine output and fluid administration in the intraoperative period remains controversial.⁷

Inhaled anesthetics markedly reduce urine output; therefore, fluid administration based on urine output can lead to fluid overload.^8,9 Surgical stress response and associated sympathetic stimulation; activation of the renin-angiotensin-aldosterone system; renal cortical vasoconstriction; increased antidiuretic hormone; and reduced cardiac output as well as mechanical compression of renal vessels and parenchyma (e.g., during pneumoperitoneum) can also result in oliguria. Overall, oliguria (defined as urine output <0.5mL/kg/h) is not a sensitive marker and does not always indicate impending renal failure.^10,11 There is no evidence that renal function deteriorates with reduced urine output.¹² Furthermore, liberal fluid administration has not been shown to reduce the incidence of acute renal failure.

Kheterpal et al.⁵ recently examined the risk factors for postoperative acute renal failure after major noncardiac surgery in patients with normal preoperative renal function. The authors found that age, body mass index, liver disease, peripheral vascular occlusive disease, chronic obstructive pulmonary disease requiring chronic bronchodilator therapy, high-risk surgery, and emergent surgery were independent predictors of postoperative acute renal failure. In addition, intraoperative factors such as total vasopressor dose, vasopressor infusion, and diuretic administration impact the occurrence of postoperative acute renal failure.⁵

Because of the concerns of fluid minimization strategies, many investigators have proposed goal-directed fluid administration using dynamic (flow-related) hemodynamic variables such as stroke volume.^13-15 It is suggested that goal-directed fluid therapy may allow individualized fluid therapy that adapts to changing patient needs during the perioperative period and prevents subtle hypovolemia or hypervolemia. Goal-directed therapy uses the concept of fluid challenge to optimize the predetermined goal.

A systematic review of the literature found that intraoperative goal-directed fluid therapy with maximization of flow-related hemodynamic variables reduced perioperative complications, including postoperative nausea and vomiting, facilitated gastrointestinal functional recovery and reduced hospital stay.¹³ Another systematic review by Abbas and Hill¹⁴ reported that the use of an esophageal Doppler device for monitoring and optimization of flow-related hemodynamic variables improved short-term outcomes in patients undergoing major abdominal surgery.

A systematic review conducted by the Agency for Healthcare Research and Quality (AHRQ) at the request of the Centers for Medicare & Medicaid Services (CMS) also found that fluid therapy guided by esophageal Doppler monitoring provided clinically significant reduction in minor and major postoperative complications as well as length of hospital stay.¹⁵ Based on this report, CMS has determined that esophageal Doppler monitoring of cardiac output for ventilated patients in the intensive care unit and operative patients with the need for intraoperative fluid optimization is reasonable and necessary, particularly in patients undergoing major surgical procedures with substantial blood loss or fluid shifts; thus, allowing physicians to charge for the appropriate use of the esophageal Doppler device.

Interestingly, in contrast to the fluid minimization studies, goal-directed therapy led to administration of larger amount of fluids compared to the control group.^13,14 With respect to the type of fluids (i.e., crystalloids versus colloids), most studies evaluating fluid minimization or goal-directed therapy used colloid solutions, with most of the volume administered early in the surgical period. Nevertheless, a balanced approach to choice of fluid may be more prudent.

Other functional (dynamic) hemodynamic monitors that have been considered for goal-directed fluid therapy include arterial wave analysis (e.g., pulse contour, pulse power analysis, and non-invasive finger pressure). In addition, variations in pulse-pressure, systolic blood pressure or stroke volume induced by mechanical ventilation can also be considered as predictors of fluid responsiveness. However, these monitors have not been adequately studied and need further investigation before they can be recommended to guide fluid therapy.

In summary, current perioperative fluid therapy is based upon dogma and personal beliefs. Anesthesiologists are desensitized to administration of large fluid volumes, particularly crystalloids, which can result in fluid overload and increase perioperative complications. Similar to any other drug administered in the perioperative period, crystalloids should be administered judiciously. Simple intraoperative maneuvers such as avoidance of deep anesthesia, large tidal volumes and algorithms to replace fluids is critical in avoiding fluid overload. Furthermore, it is imperative that we make every effort to reduce preoperative dehydration by encouraging oral fluids until two hours prior to surgery. It must be emphasized that fluid therapy in the immediate postoperative period is as critical as during the intraoperative period. However, few studies have evaluated this area.
Individualized goal-directed fluid optimization facilitates early recovery and reduces hospital stay. In addition, it is imperative that protocols for accelerated (or fast-track) postoperative rehabilitation programs are also implemented. For example, a fast-track rehabilitation for a major abdominal surgical procedure would consist of avoidance of bowel preparation and postoperative nasogastric tube as well as early aggressive oral therapy and ambulation. It is possible that emphasis on postoperative weight control and fast-track rehabilitation will further influence postoperative outcome. Future studies are necessary to address these issues.

References:
1. Joshi GP. Intraoperative fluid restriction improves outcome after major elective gastrointestinal surgery. Anesth Analg. 2006; 101:601-605.
2. Brandstrup B. Fluid therapy for the surgical patient. Best Pract Res Clin Anaesthesiol. 2006; 20:265-283.
3. Lobo DN, Macafee DAL, Allison SP. How perioperative fluid balance influences postoperative outcome. Best Prac Res Clin Anaesthesiol. 2006; 20:439-455.
4. Chertow GM, Burdick E, Honour M, Bonventre JV, Bates DW. Acute kidney injury, mortality, length of stay, and costs in hospitalized patients. J Am Soc Nephrol. 2005; 16:3365-3370.
5. Kheterpal S, Tremper KK, Englesbe MJ, et al. Predictors of postoperative acute renal failure after noncardiac surgery in patients with previously normal renal function. Anesthesiology. 2007; 107:892-902.
6. Sear JW. Kidney dysfunction in the postoperative period. Br J Anaesth. 2005; 95:20-32.
7. Holte K, Kehlet H. Fluid therapy and surgical outcomes in elective surgery: A need for reassessment in fast-track surgery. J Am Coll Surg. 2006; 202:971-989.
8. Brauer KI, Svensen C, Hahn RG, Traber LD, Prough DS. Volume kinetic analysis of the distribution of 0.9% saline in conscious versus isoflurane-anesthetized sheep. Anesthesiology. 2002; 96:442-449.
9. Connolly CM, Kramer GC, Hahn RG, et al. Isoflurane but not mechanical ventilation promotes extravascular fluid accumulation during crystalloid volume loading. Anesthesiology. 2003; 98:670-681.
10. Bellomo R, Ronco C, Kellum JA, et al. Acute renal failure - definition, outcome measures, animal models, fluid therapy and information technology needs: The Second International Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) Group. Critical Care. 2004; 8:R202-212.
11. Moitra V, Diaz G, Sladen RN. Monitoring hepatic and renal function. Anesthesiol Clin N Am. 2006; 24:847-880.
12. Desborough JP. The stress response to trauma and surgery. Br J Anaesth. 2000; 85:109-117.
13. Bundgaard-Nielsen M, Holte K, Secher NH, Kehlet H. Monitoring of perioperative fluid administration by individualized goal-directed therapy. Acta Anaesthesiol Scand. 2007; 51:331-340.
14. Abbas SM, Hill AG. Systematic review of literature for the use of esophageal Doppler monitor for fluid replacement in major abdominal surgery. Anaesthesia. 2008; 63:44-51.
15. Esophageal Doppler ultrasound-based cardiac output monitoring for real-time therapeutic management of hospitalized patients. A review. Agency for Healthcare Research and Quality Technology Assessment. January 16, 2007. www.cms.hhs.gov/mcd/viewtechassess.asp?from2=viewtechassess.asp&where=index&tid=45&]. Accessed on February 21, 2008.

Ana Crawford, M.D., is a CA-3 Resident in Anesthesiology, University of Texas Southwestern Medical Center, Dallas.

Girish P. Joshi, M.D., F.F.A.R.C.S.I., is Professor of Anesthesiology and Pain Management, and Director, Perioperative Medicine and Ambulatory Anesthesia, University of Texas Southwestern Medical Center, Dallas.