Perioperative fluid management remains controversial. Until recently, fluid management was guided by targets such as urine output, static pressures, blood pressure, and other physiologic variables. Such physiologic signs, however, are inadequate for detecting subclinical hypovolemia. This has prompted the emergence of an approach to fluid administration based on stroke volume and cardiac output—a “flow-guided” approach—designed to overcome the inadequacies of conventional physiologic signs and improve outcomes. Recent technological advances are permitting noninvasive guidance of intravenous fluid therapy to optimize intravascular volume status.
This article reviews the rationale for perioperative fluid management, strategies for perioperative fluid therapy and their associated outcomes, the types of volume expanders used, and considerations for improving perioperative fluid administration.
WHY FLUID MANAGEMENT MATTERS
Postoperative complications predict survival
In 2005, Khuri et al published a study of survival after major surgery that starkly illustrated the prognostic importance of postoperative complications.1 In an effort to identify predictors of long-term survival, they analyzed a National Surgical Quality Improvement Program database of 105,951 patients who underwent eight common operations at Veterans Administration facilities. They found that the most important determinant of reduced postoperative survival over 8 years of follow-up was the occurrence of a complication within 30 days after surgery. The presence of a postoperative complication was a stronger predictor of death than any intraoperative or preoperative risk factor.
Fluid management is key to preventing complications
Optimizing perioperative fluid management is essential to reducing the risk of postoperative complications and mortality. Surgical patients are more likely to have serious complications and die if they have limited physiologic reserve. Adequate fluid administration may reduce the stress response to surgical trauma and support recovery.
Building on early work showing that survivors of major surgery have consistently higher postoperative cardiac output and oxygen delivery (DO2) than do nonsurvivors,2,3 a seminal study by Shoemaker et al showed that these types of blood flow–related parameters are predictive of both survival and complication-free survival.4 Specifically, Shoemaker and his team showed that a protocol designed to achieve DO2 of at least 600 mL/min/m2 was associated with reductions in both postoperative complications and death.4
PROBLEMS WITH PERIOPERATIVE FLUID THERAPY―AND EFFORTS TO OVERCOME THEM
Despite the utility of fluid management in reducing postoperative complications, perioperative fluid therapy is fraught with several fundamental problems:
- Blood volume cannot be evaluated accurately.
- Fluid overload cannot be identified accurately, apart from tissue edema as a result of gross fluid overload.
- Hypovolemia cannot be identified accurately. Commonly measured variables (heart rate, blood pressure, base excess, lactate) are late markers, and the patient’s status upon admission to the operating room is often unknown.
- Tissue perfusion cannot be evaluated accurately. Although lactate and venous oxygen saturation are surrogate markers, genuinely accurate markers for tissue perfusion are lacking.
For these reasons, fluids are commonly administered without the guidance of direct markers of fluid status.
Assessing flow-guided fluid therapy
These shortcomings prompted me and several other researchers to assess the evidence regarding a flow-guided approach to fluid administration, which aims to achieve maximal cardiac output and stroke volume while avoiding excess fluid administration. We conducted a systematic literature search for randomized controlled trials evaluating the postsurgical effects of perioperative fluid therapy to increase global blood flow to explicitly defined goals, after which we performed a meta-analysis of the 22 qualifying studies.5 The trials collectively included 4,546 patients undergoing relatively high-risk elective or emergency surgery, consisting of general, vascular, cardiac, orthopedic, and urologic procedures. Overall mortality in these trials was 10.6% (481 deaths). The primary outcome assessed was mortality; secondary outcomes included morbidity and length of stay in the hospital and in the intensive care unit. Outcomes were assessed according to the timing of the intervention, the fluid type, and explicit measured goals. Fluids were given to all patients, usually as a dynamic bolus, using a flow-guided approach above and beyond that of the control group.
Our analysis found that a flow-guided protocol was associated with a significant reduction in mortality compared with control protocols (odds ratio = 0.82 [95% CI, 0.67–0.99]; P = .04).5 However, sensitivity analysis showed that the largest and best-designed studies tended to yield no significant differences in mortality between the groups, which highlights the remaining need for larger studies to more definitively clarify the effect on mortality.
Timing of administration (ie, whether fluid was given pre-, intra-, or postoperatively) influenced the primary outcome: compared with control, flow-guided fluid therapy was associated with a significant reduction in mortality only when administered intraoperatively, but not when given preoperatively or postoperatively.5
Length of hospital stay was reduced by approximately 1.6 days with flow-guided therapy compared with control (P < .00001), but there was no significant difference between approaches in terms of intensive care unit stay.5
Postoperative complication rates are difficult to compare, given the lack of a uniform definition of a complication and the relative importance of different complications. Nevertheless, when grouped as a whole, the rate of complications was 48% lower (P < .00001) with flow-guided therapy compared with control. Of all outcomes assessed, the effect on complications was the most consistent among all the studies in the analysis. To provide an example using one easily defined complication, the incidence of renal failure was reduced by 35% with flow-guided therapy compared with control (P = .002).5
COLLOID OR CRYSTALLOID?
Two pharmacologically distinct classes
Intravenous fluids can be broadly classified into colloid and crystalloid solutions, and the relative merits of these two fluid classes are at the center of an enduring debate that predates the advent of flow-based fluid administration. Despite fundamental differences in their pharmacokinetics and other characteristics, colloids and crystalloids are often not sufficiently distinguished from one another in discussions of perioperative fluid therapy.
The effect of a colloid depends on its molecular weight. Ninety minutes following administration, a significant proportion of a colloid with a high molecular weight (eg, hydroxyethyl starch) will be retained in the circulation. In contrast, crystalloid solutions (eg, 0.9% saline) readily disappear from the circulation, owing to the ease with which they travel across the cell membrane.6
No evidence of outcome differences
A systematic literature review by Choi et al reflects the current state of knowledge on the relative effects of colloids and crystalloids for fluid resuscitation.7 It concluded that there are no apparent differences between these fluid classes in their effects on pulmonary edema, mortality, or length of stay. The authors noted that methodologic limitations of the available comparative studies prevent meaningful conclusions and that larger randomized controlled trials are needed to detect any differences in outcomes between the two classes.
Although using a crystalloid for fluid resuscitation probably results in a greater volume of fluid given, a study known as SAFE (Saline versus Albumin Fluid Evaluation),8 published after the Choi analysis, showed no differences in 28-day all-cause mortality or other significant outcomes between patients randomized to the colloid (4% albumin) and those assigned to the crystalloid (0.9% saline). Patients receiving the colloid had a higher central venous pressure at all time points, a lower heart rate at the end of the first day, and less overall volume on days 1 and 2 compared with patients receiving the crystalloid. While SAFE was conducted in critically ill patients, these physiologic advantages of the colloid may have implications for results in the perioperative arena, although this remains speculative.