Fluid therapy in the emergency room (Proceedings)

The use of intravenous fluids in the emergency room is essential to the proper resuscitation and support of the critically ill patient.  Knowledge of the different types and qualities of the available intravenous fluids will allow clinicians to tailor choices to individual patients.  Close monitoring of resuscitation end points will allow clinicians to tailor fluid doses for specific circumstances.

Fluid types, crystalloid

Crystalloids are solutions composed of water and ions.  The ions are osmotically active and can move freely between the vascular space and intracellular and interstitial spaces.  The redistribution of crystalloid solutions is important to consider when choosing fluid types; after one hour, only ? of the initially infused volume remains in the vascular space. In patients with vasculitis or in other conditions that may predispose to edema formation (e.g. hypoalbuminemia), this redistribution may occur even faster, and result in worsening of edema.

Crystalloid fluids may be further classified according to their osmolality relative to that of normal plasma, which is approximately 300 mOsm/L.  Fluids that have similar osmolality to plasma are characterized as isoösmolar or isotonic, while those with lower osmolality are classified as hypotonic, and those with higher osmolality are hypertonic.  The osmolality of a solution is calculated using the equation: 2 x Na+(mEq/L) + Glucose (mg/dL)/18 + BUN (mg/dL)/2.8. 

Isotonic fluids that are used in the emergency room can be 0.9% NaCl (308 mOsm/L), Normosol-R (296 mOsm/L), or lactated Ringer's solution (272 mOsm/L).  Isotonic fluids are primarily used for resuscitation.  Because they share similar tonicity to the plasma, rapid infusion of isotonic fluids will mimic plasma electrolyte concentrations, and should not cause significant perturbations to the patient's electrolytes (the exception to this rule is 0.9% NaCl, which has significantly more chloride than is found in plasma, and which may cause a hyperchloremic metabolic acidosis).  All of the isotonic resuscitative fluids have very low concentrations of potassium, which allows them to be administered rapidly.  Some isotonic fluids (Normosol-R, lactated Ringer's solution, acetated Ringer's solution, Plasmalyte-148) contain buffer compounds.  These compounds (lactate or acetate) will be converted to bicarbonate and represent the addition of a buffer to the fluids.  0.9% NaCl does not contain a buffer.

5% Dextrose in water (D5W) is technically an isotonic fluid (252 mOsm/L), by virtue of the dextrose concentration.  It is not generally thought of as a crystalloid, however, as it essentially provides the body with free water.  Almost immediately following infusion, the dextrose is metabolized, and the free water rapidly relocates to intracellular and interstitial spaces.  This fluid is used most frequently in patients with free water losses (indicated by hypernatremia).

Hypertonic fluids are crystalloids that have a higher osmolality to plasma, and are almost exclusively used for resuscitation from shock.  The most common hypertonic fluid is NaCl.  NaCl may come in a 3%, 7%, or 23% solution.  The 23% solution must be diluted prior to administration.  It may be diluted using hetastarch or other colloids, or crystalloid fluids.  The 7% NaCl (2599 mOsm/L) is most frequently used in the emergency room.  The high osmolarily pulls (via osmosis) large amounts of fluid from the interstitial spaces into the vascular space.  This strong pull of water into the vasculature rapidly expands intravascular volume, for a relatively small volume of injected solution.  Despite this strong pull, hypertonic solutions are crystalloids, and as such, will redistribute to the interstitial space.  The use of hypertonic solutions for resuscitation buys the clinician time to replenish vascular volume using other fluids while maintaining stable hemodynamics.

The other use of hypertonic solutions is in patients with cerebral edema.  Just as hypertonic crystalloids are able to pull fluid out of the interstitial spaces into the vasculature, they may also act to pull fluid out of the brain when cerebral edema is present.  Used in this manner, hypertonic saline can be an effective medication to treat patients with elevated intracranial pressure, and is indicated for resuscitation of patients with head trauma and shock.  Mannitol acts to lower intracranial pressure via a similar mechanism; it is a hyperosmolar solution and rapidly pulls fluid from interstitial spaces and areas of cerebral edema.

Hypotonic fluids are not commonly used in the emergency room; by definition, they are not designed to match the plasma osmolality, and consequently are not particularly good resuscitation fluids.  Because the possibility exists that hypotonic fluids may cause cell swelling or lysis if directly administered as such, many commercially available hypotonic crystalloid solutions will have the addition of 2.5% dextrose, which helps to normalize the osmolality of the solution, and decreases the risk for hypotonic damage to cells.  Commonly used hypotonic fluids include 0.45% NaCl (154 mOsm/L) and half-strength lactated Ringer's solution (usually supplied already mixed with 2.5% Dextrose).  Even though D5W is technically an isotonic fluid, it is commonly thought of as a hypotonic fluid, as it supplies free water after the dextrose is metabolized.

Fluid types, colloids

Colloid solutions contain large organic molecules that result in a hyperoncotic solution. The large molecules are usually dissolved in normal saline or a balanced electrolyte solution, resulting in an isotonic solution for infusion.  The presence of large molecules in the solution results in a fluid pull into the vasculature due to an increased oncotic pressure.  In other words, the presence of larger molecules in solution will tend to pull water into this solution.  The negative charge of the colloid particles also attracts sodium and water (the Gibbs-Donnan effect) which serves to increase fluid flux into the vasculature. 

Oncotic pressure in blood is ordinarily maintained by albumin at a colloid osmotic pressure (COP) of about 20 mm Hg.  The oncotic pressure of a particular solution is dependent on the number of particles in solution (not size).  Colloid solutions may be synthetic or natural in origin.  The benefit of using intravenous colloid solutions is that the large molecules are restricted to the plasma compartment, and do not redistribute in the same mechanism as crystalloid fluids.   This can result in a longer lasting volume expansion from a given dose of fluids.  Colloids are frequently co-administered with crystalloids as part of a resuscitation plan. 

Synthetic colloids in general are derived from natural products, and include hetastarch (HES, derived from amylopectin, COP = 32 mm Hg) and dextran 70 (derived from sucrose after bacterial fermentation, COP = 61mm Hg).  Oxyglobin, a solution of polymerized bovine hemoglobin, is also considered a synthetic colloid (COP = 43 mm Hg).  Biologic or natural colloids include plasma (COP = 20 mm Hg), canine albumin (COP (16% soln) = 100 mm Hg), or human albumin (COP (25% soln) > 200 mm Hg).  Concentrated human albumin is the most potent colloid available; however, due to the fact that it is a foreign protein and may cause severe allergic or anaphylactic reactions, its use in veterinary medicine is controversial.

Administration of colloid solutions will thus result in an expansion of plasma volume.  HES expands plasma volume to a degree equal to the infused volume. Because dextran-70 has a higher COP, it can expand plasma volume 20–50% more than the infused volume. HES is well tolerated by dogs and cats in the clinical setting, and can be used for treatment of animals with low COP secondary to hypoalbuminemia, as well as used for rapid volume expansion during treatment of shock.  It is generally recommended not to exceed 20 mL/kg/day of HES in the dog and 10 mL/kg/day in the cat, based on theoretical interference with platelet function.  In all likelihood, total volumes approaching 40 mL/kg are tolerable under certain circumstances and resuscitation.  As part of a resuscitation protocol, bolus administration of 5 mL/kg of HES or dextran-70 over 15-20 minutes can help to attenuate the loss of plasma COP from excessive crystalloid administration, and can help to support vascular volume even after resuscitation is complete.

Fluid types, mixed

Hypertonic colloid is a combination of hypertonic saline and a colloid, and can be used in the emergency setting for potent, rapid intravascular volume expansion. This is the most appropriate use for the 23% hypertonic NaCl solution; 43 mL of colloid (Dextran 70 or HES) is added to 17 mL of 23% hypertonic saline. This dilutes the saline down to approximately 7%, making it safe for intravenous administration. When using this combination, the recommended dose for resuscitation is 3-5 mL/kg, administered over 10-15 minutes.   The rapid volume expansion from the hypertonic saline is balanced by the fluid retention from the colloid, and the combination is of benefit for any resuscitation plan involving the administration of volume.  This combination can also be quite useful in the resuscitation of large or giant breed dogs, as it may be more rapidly administered than large volumes of crystalloid fluids.  In an experimental study evaluating hypertonic saline mixed with dextran 70 in a canine model of hemorrhagic shock, 4 mL/kg of the hypertonic colloid achieved the same resuscitation endpoints as 32 mL/kg of LRS.

Resuscitation endpoints

The approach to resuscitation from shock in the emergency room should be directed towards restoration of physiologic homeostasis in the patient.  Shock is associated with decreased oxygen delivery to tissues, and is frequently accompanied by tachycardia, tachypnea, delayed capillary refill time (CRT), elevated serum lactate concentrations, and potentially hypotension.  All of these parameters should be monitored at regular intervals during the resuscitation period.  Although there is a theoretical “shock dose” of crystalloid fluids (90 mL/kg in the dog, 60 mL/kg in the cat), the administration of a full dose of crystalloid is rarely advisable or necessary.  In fact, the property of crystalloid fluids that causes redistribution may lead to significant edema (pulmonary or otherwise) if given in excess. 

The most appropriate way to approach resuscitation is to administer 1/3 of the theoretical shock dose (usually 20-30 mL/kg) over 15-20 min. and reassess; if perfusion is restored, heart rate should decrease, blood pressure should increase, and lactate should decrease.  If this does not happen, the fluid bolus may be repeated (or interspersed with a 5 mL/kg bolus of colloid), and these parameters reassessed.  Resuscitation should continue until oxygen delivery is normalized.  During the resuscitation, additional data may be gathered that will also help guide resuscitation; for instance, if anemia is diagnosed, a transfusion of packed red blood cells or whole blood may complement the fluid resuscitation.  Once resuscitation endpoints have been achieved, additional fluid therapy should be directed towards maintenance of intravascular volume and tissue perfusion, while addressing any ongoing fluid losses (e.g. diarrhea).