A primer on current veterinary fluid therapy techniques

(Evdoha/adobestock)One of the first things I think about when assessing a critical patient is whether that patient will require fluid therapy. Intravenous fluid therapy is vital not only to managing shock but also to correcting interstitial dehydration and addressing daily maintenance fluid needs in hospitalized patients. While many of the lessons about fluid therapy learned in veterinary school remain true, some newer lessons can be lifesaving for some patients.

Because critically ill animals often have fluid and electrolyte balance derangements, overall recovery often depends on recognizing and appropriately treating these derangements in addition to diagnosing and treating the primary disease process. Fluid therapy has three primary clinical indications: resuscitation, rehydration and meeting maintenance needs. To determine why a patient may require fluids, three questions should be considered: What fluid space is deficient? If resuscitation is required, what type of shock is present? Are there any contraindications to giving fluids?

Identifying the deficiency

Total body water content, which is about 60% of body weight in a nonobese adult dog or cat, is distributed between two main compartments, intracellular fluid (ICF) and extracellular fluid (ECF). Each compartment consists of solutes, mostly electrolytes, dissolved in water. The ICF compartment is the larger of the two and comprises two-thirds of the total body water (40% of body weight). It is separated from the ECF compartment by the cell membrane, which is very permeable to water but impermeable to most solutes. The ECF comprises the remaining one-third of the total body water (20% of body weight). It is subdivided into plasma (25% of ECF) and interstitial (75% of ECF) fluid compartments. The interstitial fluid bathes all cells and includes lymph.

Water moves freely within most compartments in the body. Small particles such as electrolytes move freely between the intravascular and interstitial compartments but cannot enter or leave the cellular compartment without a transport system. Larger molecules (>20,000 Da) do not cross the vascular endothelial membrane easily and may attract small, charged particles, thus creating the colloid osmotic pressure (COP).

There are three primary natural colloid particles: albumin, globulins and fibrinogen. An increase in the pressure of fluid within a compartment that pushes against a membrane is known as hydrostatic pressure. In health, fluid distribution within the ECF is determined by the balance between forces that favor reabsorption of fluid into the vascular compartment (increased COP or decreased hydrostatic pressure) and those that favor filtration out of the vascular space (decreased COP or increased hydrostatic pressure). Changes in the osmolality between any of the fluid compartments within the body will cause free water movement across the respective membrane.

Understanding the difference between the intravascular and interstitial compartments is the key to recognizing shock or dehydration. In patients with shock, the intravascular compartment is depleted, leading to pale mucous membranes, prolonged capillary refill time, tachycardia, poor pulse quality, tachypnea, weakness and mental dullness. In patients with dehydration, the interstitial compartment is depleted, leading to dry mucous membranes and prolonged skin tenting. This article focuses mostly on shock because recognizing shock is paramount and without rapid treatment it can be life-threatening.

Fluid types used for resuscitation from shock

Before choosing a fluid for resuscitation, consider the type of shock at hand. By definition, shock involves impaired tissue perfusion resulting from inadequate delivery or utilization of oxygen. In all cases, a major goal of resuscitation is to improve oxygen delivery to or oxygen use by the tissues. While it is clear that cardiogenic shock should be identified early, as fluids are often contraindicated, differentiating hypovolemic and distributive shock is also very important.

Hypovolemic shock involves an absolute or relative reduction in blood volume, whereas in distributive shock inappropriate vasodilation leads to poor perfusion despite an often-normal blood volume. In hypovolemic shock, isotonic crystalloids, hypertonic crystalloids, synthetic colloids and blood products can all be considered. In distributive shock, arguments may exist for specific types of fluid or for conservative fluid resuscitation and early pressor therapy.1

Isotonic crystalloids. Isotonic crystalloids are electrolyte-containing fluids that are similar in composition to the ECF and have a similar osmolality to that of plasma. Commonly used for resuscitation, isotonic crystalloids cause extracellular expansion but redistribute quickly to the interstitial space. While they are often well-tolerated for resuscitation in hypovolemic patients, these fluids can cause damage when they redistribute to the interstitial space.

In critically ill patients (including those with distributive shock and a low COP, increased capillary permeability, etc.), increases in interstitial fluid can cause tissue edema, including cerebral and pulmonary edema. The standard “shock dose” of fluids is a total of 90 ml/kg in dogs and 50 to 60 ml/kg in cats, keeping in mind that fluids should be given incrementally (one-quarter to one-third of a shock dose at a time) while monitoring perfusion parameters and stopping or adjusting fluids once those parameters return to normal. Remember that an entire shock dose is often not necessary to achieve stabilization.

Hypertonic crystalloids. Hypertonic crystalloids (most often 7.2%-7.5% hypertonic saline) have an osmolality approximately eight times that of plasma (2,400 mOsm/L) and can be used for rapid intravascular volume expansion, up to 3.5 times greater than the volume infused. By creating an osmotic gradient from the intracellular to extracellular space, hypertonic saline decreases intracellular volume and increases vascular volume. As a crystalloid, the effects are short lived, but rapid volume expansion can be helpful in cases of hypovolemic shock, especially for small-volume resuscitation or in large patients.

Hypertonic saline is used commonly for resuscitation in patients with head trauma or intracranial hypertension. It has also been shown to decrease endothelial swelling, improve cardiac contractility and improve cardiac output and tissue perfusion by decreasing afterload and increasing preload. Immunomodulatory effects (inhibition of neutrophil respiratory burst activity and cytotoxic effects) have also been reported. Because hypertonic saline is usually provided as a 23.4% solution, it must be diluted before use. This can be done by mixing it with an isotonic crystalloid in a ratio of one part hypertonic saline to two parts crystalloid. Typical doses are 3 to 5 ml/kg over five to 10 minutes. Administering hypertonic saline too rapidly (>1 ml/kg/min) can result in hypotension and bradycardia.

Synthetic colloids. Synthetic colloids are isotonic crystalloids to which large molecules have been added. Administration results in increases in COP and a pull of fluid into the vascular space from the interstitial space. Colloids also increase the intravascular volume greater than the volume infused (about 1.4-1.5 times). In addition to volume expansion, synthetic colloids remain in the intravascular space longer than crystalloids and provide a more sustained effect. The synthetic colloids most commonly used in the United States include VetStarch (Zoetis), a 6% tetrastarch veterinary product, and 6% hetastarch.

Colloids can adversely affect coagulation via dilutional effects and interference with platelet function, von Willebrand factor and factor VIII, with prolongation of bleeding times seen at doses higher than 20 ml/kg/day. In human patients, concern regarding the occurrence of acute kidney injury has led to the removal of synthetic colloids from the market in some countries.2 In veterinary medicine, recent studies have yielded mixed results correlating the use of synthetic colloids to acute kidney injury.3,4 Based on these concerns and a number of large human trials whose results have shown no clear benefit to using synthetic colloids,5 caution is recommended when using these products.

Other factors to consider

Regardless of the type of fluid chosen for resuscitation, two main points should be remembered. Many years of research and clinical studies in human patients and animal models (and some veterinary patients) have failed to find an optimal fluid type overall. Thus, it is of utmost importance to choose a fluid that one is comfortable using and base resuscitation needs and success on monitoring the end points of resuscitation. A combination of variables, including physical exam parameters, lactate level, base deficit, arterial blood pressure, cardiac output, mixed venous oxygen saturation and central venous pressure, can be helpful in guiding therapy.

Fluid therapy is not benign; potential complications include volume overload (organ edema and cavitary effusion), electrolyte changes and dilutional effects on hematocrit, albumin and coagulation factors. More recent evidence from human studies has highlighted the importance of conservative fluid therapy, especially in patients with septic shock.6 Remember that fluids can save your patient's life, but they can also have adverse effects.

References

1. Rhodes A, Evans LE, Alhazzani W, et al. Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock: 2016. Intensive Care Med 2017;43(3):304-377.

2. Wiedermann CJ, Eisendle K. Comparison of hydroxyethyl starch regulatory summaries from the Food and Drug Administration and the European Medicines Agency. J Pharm Policy Pract 2017;10:12.

3. Sigrist NE, Kälin N, Dreyfus A. Changes in serum creatinine concentration and acute kidney injury (AKI) grade in dogs treated with hydroxyethyl starch 130/0.4 from 2013 to 2015. J Vet Intern Med 2017;31(2):434-441.

4. Hayes G, Benedicenti L, Mathews K. Retrospective cohort study on the incidence of acute kidney injury and death following hydroxyethyl starch (HES 10% 250/0.5/5:1) administration in dogs (2007-2010). J Vet Emerg Crit Care (San Antonio) 2016; 26(1):35-40.

5. American Thoracic Society. Evidence-based colloid use in the critically ill: American Thoracic Society Consensus Statement. Am J Resp Crit Care Med 2004;170(11):1247-1259.

6. de Oliveira FS et al. Positive fluid balance as a prognostic factor for mortality and acute kidney injury in severe sepsis and septic shock. J Crit Care 2015;30(1):97-101.

Dr. Beer obtained her DVM from Michigan State University and completed her internship and residency at the University of Pennsylvania. After spending several years in academia, she made the jump to private practice and now works as a critical care specialist in Michigan and Pennsylvania. Her interests include trauma, resuscitation and coagulopathy in critical illness.