Rethinking your approach to perioperative fluid therapy

Few people would argue that perioperative fluid therapy is an integral part of anesthesia and surgery; however, perioperative fluid therapy in dogs and cats is in dire need of focused investigation and reappraisal. Dehydration, oliguria, and hypotension prompt bolus fluid administration (fluid challenge) in surgical candidates to improve cardiac output and tissue perfusion and to promote diuresis. Yet very little, if any, data on the safety and efficacy of this fluid challenge have been generated from awake or anesthetized animals with naturally occurring disease.

PHOTO BY GREGORY KINDRED

Perioperative fluid therapy in people is known to have a direct bearing on outcome measures, including length of hospitalization, morbidity, and mortality.1-4 Errors in fluid management (usually fluid excess, or overload) in human surgical candidates have been identified as the most common cause of perioperative morbidity and mortality.1-4 Studies conducted in people concluded that a gain of 5% to 10% body weight from fluid therapy is associated with worsening organ function in critically ill ICU patients and worse postoperative outcomes after routine surgery, with no evidence of beneficial effects on renal function.4 These conclusions have important implications for current fluid replacement procedures and are likely to be equally important in animals.

Traditional approach

Most contemporary authors recommend administering a "balanced" electrolyte solution (lactated Ringer's solution) or a 0.9% sodium chloride solution at rates ranging from 10 to 15 ml/kg/hr for the duration of a surgical procedure in anesthetized dogs and cats.5-9 Interestingly, the term balanced is never defined, and the rationale for administering fluids at 10 to 15 ml/kg/hr is never justified or defended. Furthermore, specific details are often not included about how to administer fluids to treat perioperative blood loss and hypotension (systolic pressure < 80 to 90 mm Hg; mean pressure < 60 mm Hg), even though blood loss is integral to surgery, and hypotension occurs in more than 25% of anesthetized dogs and more than 30% of anesthetized cats.6-9-14

This traditional "recipe-based" approach to perioperative fluid therapy is outdated. Although arguably founded on a concern for maintaining physiologic functions and replacing insensible fluid losses, it does not consider hemoglobin concentration, the duration of surgery, fluid loss from fasting or into traumatized tissues or surgical wounds, and, most important, anesthetic-imposed problems associated with monitoring and treating intravascular volume deficits.

Fluid balance basics: Then and now

An understanding of fluid balance is based on the distribution of water into various fluid compartments (intravascular, interstitial, intracellular) and the concentration of salts (electrolytes including sodium, chloride, and potassium) and protein within these fluid compartments. The complexities of neuroendocrine, renal, and interstitial fluid dynamics including lymph flow, although appreciated, are often underemphasized.15

For example, the classic Starling equation (Starling's law of the capillary), which emphasizes hydrostatic and osmotic forces, is generally used to explain fluid balance in the vascular system, Although instructive, this equation is quantitatively inconsistent with experimental and clinical observations of transvascular fluid flux and edema formation.16

New knowledge about this basic physiologic concept and the mechanisms responsible for intravenous fluid distribution and elimination continues to emerge and has helped to clarify and redefine best practices for perioperative fluid therapy.17,18

The outdated Starling's law of the capillary has been replaced by the double barrier concept, wherein fluids exit over the entire length of the capillary.17-19 An endothelial surface layer (ESL), the glycocalyx, is now known to control vascular permeability, thereby serving as the principal determinant of fluid flux from the vasculature and the development of interstitial edema (Figure 1).19

1. An endothelial surface layer (glycocalyx) is a key factor in determining vascular permeability. Fluid leaves the capillary throughout its entire length.

The ESL can be degraded by trauma, inflammation (sepsis), and fluid overload, all of which predispose the animal to interstitial fluid accumulation and edema.19 Importantly, albumin helps to maintain the ESL and vascular fluid homeostasis. Hypoalbuminemia is strongly associated with a poor outcome, implying that the serum albumin concentration should be maintained within normal limits (> 1.5 g/dl).20

Unfortunately, and as a result of decades of liberal and inadequately monitored fluid administration, the consequences of fluid overload are well-known (Figure 2).17,18,21,22 They include tissue edema, coagulopathy, pulmonary and renal failure, ileus, delayed wound healing, hypothermia, nosocomial infections, and abdominal compartment syndrome.

2. Some of the many consequences of fluid overload.

Improved protocols

Traditional methods for monitoring perioperative intravenous fluid administration—measuring heart rate, arterial blood pressure, and central venous pressure—are poor, generally unpredictable, and late indicators of blood volume changes or of the animal's response to fluid administration (fluid responsiveness).23-26 Furthermore, differences in intravenous fluid electrolyte concentrations (strong ion difference), colloid composition, and viscosity are now recognized as producing important acid-base, immunologic, and rheological effects that have important implications for their therapeutic value.27-34

Anesthetic drugs and anesthetic depth are rarely considered in perioperative fluid protocols, but they are now known to have a significant influence on factors that determine fluid distribution, elimination, and efficacy.35-37 Isoflurane anesthesia, for example, increases vascular capacitance, decreases urine production, promotes interstitial fluid accumulation, and inhibits transcapillary refill after either normotensive or hypotensive hemorrhage.35

These findings, coupled with concerns over the safety of crystalloid and colloid fluids used as replacement fluid therapy, have led to impassioned debates about optimal perioperative fluid therapy practices.38-41 These debates highlight critical dilemmas (see sidebar "Four perioperative fluid therapy dilemmas") and have prompted the adoption of more "restrictive," " fast-track," "zero balance" (minimal or no weight gain), and "goal-directed" fluid therapy protocols.3,26,42-46

Four perioperative fluid therapy dilemmas

Conclusion

Perioperative fluid therapy is equivalent to intravenous drug therapy and should be individualized and goal-directed.47-49 As discussed in the next article, each fluid's unique characteristics in relation to the anesthetic and surgical circumstances for which it is prescribed should be considered.26,50 And, as discussed in the final article, fluid administration and monitoring should be unique to each animal's requirements.

ACKNOWLEDGMENTS

The author thanks Dava Cazzolli, DVM, DACVECC; Jaime Chandler, DVM, DACVECC; Michelle Albino, LVT, CVT (Anesth); and Yukie Ueyama, DVM, for their assistance in abstracting data.

William W. Muir, DVM, MS, PhD, DACVA, DACVECC

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For this article's reference citations, go to dvm360.com/FluidTherapy1Refs.