Fluid Therapy: The Who, What, When, Why & How

VCS Milford
The use of fluid therapy is one of the basic treatments for many of our hospitalized patients (and some of our outpatients). The proper use of fluids can be very beneficial and even life saving. However, just as with any other medication, the inappropriate or injudicious use of fluids can lead to detrimental, and even tragic, outcomes. This presentation discusses the proper use of fluid therapy for the treatment of many medical and surgical conditions. Since fluid therapy is a vast and complex topic, this review is not meant to be all encompassing but to provide general guidelines for effective fluid administration and there are many exceptions to the basic tenants discussed here.

Types of Fluids
There are 5 basic categories of fluids that are available to the veterinarian for intravenous use: crystalloids, colloids, hemoglobin-based oxygen carrying solutions, blood products, and intravenous nutrition. Each one of these fluid types has specific indications for their use.

The crystalloid fluids are the mainstay of veterinary practice. Crystalloid fluids consist primarily of water with a sodium or glucose base, plus the addition of other electrolytes and/or buffers. The concentration of these different solutes dictates their indication in various clinical situations. Within the crystalloid group are 4 different types of fluids: replacement solutions, maintenance solutions, hypertonic solutions, and dextrose in water.

Replacement crystalloid solutions contain dissolved solutes that approximate the solute concentration found in plasma water. These solutions are indicated for the rapid replacement of intravascular volume and electrolytes as seen with shock and hemorrhage or severe volume depletion secondary to the losses associated with vomiting, diarrhea, third body spacing, or excessive diuresis.

With replacement crystalloid fluids, only 20-25% of the infused volume of fluid remains within the intravascular space 1 hour after infusion. Therefore, large volumes of replacement crystalloids need to be administered to replace intravascular volume. The commonly available replacement solutions include normal saline (0.9% NaCl), Ringer’s solution (lactate or acetate), Normosol R, and Plasmalyte A. Each type of replacement fluid listed above has a specific composition that makes its use in certain situations preferential (beyond the scope of this review).

Maintenance solutions are also composed of dissolved solutes that approximate the solute concentration found in intravascular and extracellular fluid. The difference in these solutions compared to replacement solutions is maintenance solutions are designed to fulfill the electrolyte requirements of patients with normal daily electrolyte losses that are unable to maintain adequate fluid or electrolyte intake.

Since these solutions are used to meet maintenance fluid requirements, they are rarely infused at rates greater that the patient’s maintenance needs. Commercially available maintenance fluid solutions include half strength saline (0.45% NaCl) + dextrose, lactated Ringer’s + dextrose, and Plasmalyte 56. Commonly, potassium supplementation is added to maintenance solutions to further balance them for the patient’s electrolyte requirements.

Hypertonic saline (7.2% NaCl) is commonly used to rapidly increase intravascular volume. Most commonly, this fluid is used where severe hypovolemia is present that may lead to impending death, when low-volume resuscitation is appropriate such as with head trauma, or when large volumes of crystalloids cannot be infused fast enough to have the rapid desired effect (GDV). Doses of 4-7ml/kg (dogs) or 2-4ml/kg (cats) infused at 1ml/kg/minute of hypertonic saline produce a hemodynamic effect similar to infusion 60-90ml/kg of a replacement crystalloid solution.

Too rapid administration of hypertonic saline has been associated with vagally-mediated hypotension and bradycardia. Due to the sodium rapidly diffusing out of the vasculature, the effect of hypertonic saline is very transient (lasts up to 30 minutes). To prolong this effect, hypertonic saline is often combined with a colloid fluid to help keep the fluid that has shifted from the extravascular space in the vascular space. Replacement crystalloid fluids should be administered after hypertonic saline infusion to replace the fluid that was translocated into the vasculature.

Contra-indications for hypertonic saline include patients that are dehydrated (inadequate fluid to draw into intravascular space), hyperosmolar, hypokalemic, that may develop problems with hypervolemia (pre-existing heart or lung disease), or that have uncontrolled hemorrhage (intra-abdominal hemorrhage or pulmonary contusion).

5% Dextrose in Water (D5W) is not commonly used in veterinary medicine. This fluid contains no active solute; therefore it readily redistributes throughout the body. The most common indications for D5W are as a vehicle for infusion of other medications and to provide free water in severely hypernatremic states. Infusion of large volumes of this fluid can lead to dilution of serum electrolytes and/ or the development of edema.

Colloids are high molecular weight compounds that do not readily leave the intravascular space. They exert their effect of expanding intravascular volume by holding and potentially drawing water into the vasculature. Colloid fluid solutions are indicated for the treatment of hypovolemia, sepsis, other inflammatory conditions such as pancreatitis. They are also used to improve colloid oncotic pressure in patients with low albumin from protein loss secondary to renal or gastro-enteric disease, vasculitis, or burns.

Colloid solutions include plasma, and synthetic compounds such as hetastarch, dextrans, modified gelatin solutions, and Oxyglobin TM. Plasma use for strictly colloidal effects is relatively ineffective- a patient requires approximately 100ml/kg of plasma to raise a patient’s albumin by 1g/dl. Therefore, synthetic compounds are a better choice for colloidal support.

Contra-indications for the use of colloidal fluids include coagulopathies, potential for volume overload (heart disease, pulmonary disease, oliguric renal failure), and anaphylactic reactions.

OxyglobinTM is a synthetic compound made of cross-linked bovine hemoglobin molecules. Indications for its use include volume resuscitation in shock or hypovolemic states and in the treatment of anemia. The greatest advantage of Oxyglobin is its ability to carry oxygen to the tissues and offload oxygen in the tissues more effectively since it is not limited by red blood cell flow.

Other advantages of Oxyglobin are that it is a colloid- as such, it will provide intravascular volume support, no blood typing or cross-matching is required prior to infusion, and it has also been shown to improve microvascular perfusion, improving oxygen tension in injured tissues. Also, because it is a synthetic compound, it has a long shelf life and does not require special storage procedures.

Disadvantages of Oxyglobin are that certain clinical chemistry parameters are invalidated by its presence, that laboratory monitoring of its effect requires a hemoglobinometer, and that its colloidal effect can lead to fluid overload and pulmonary edema if not monitored carefully, especially in cats. At this time, Oxyglobin is not approved for use in cats. Oxyglobin has a flexible dosing range (10-30 ml/kg) that allows the clinician to tailor therapy to the approximate duration of needed effect.

Blood products are indicated to replace red blood cells, plasma proteins, platelets, and/or coagulation factors. Blood products are available as whole blood and component therapy. The type of product needed is based on the component of patient blood that needs to be replaced. For more specific information regarding the proper selection and administration of blood products, please consult one of the many excellent review articles on the subject.

Parenteral nutrition is considered for short or long-term treatment in patients whose clinical condition dictates that enteral nutrition is not feasible or when enteral nutrition may not provide sufficient nutrient intake to assist recovery. Some of these conditions include severe pancreatitis, protracted vomiting or regurgitation, severely painful conditions, burns, sepsis, multiple trauma, or intestinal malabsorption.

Parenteral nutrition can be provided through a peripheral or central vein. Elemental formulations of amino acids, lipids, carbohydrate sources, vitamins, and trace minerals are formulated for each individual patient based on their body weight, type of injury or illness, duration of parenteral nutritional supplementation, and type of nutrition being administered.

There are several different formulations of parenteral nutrition available. The most basic is a supplement consisting of a solution of amino acids and electrolytes in glycerol. This may be delivered through a peripheral vein and can supply up to 25% of basal metabolic needs. Partial or peripheral parenteral nutrition (PPN) is delivered through a peripheral vein and can supply up to 50% of metabolic needs. Total parenteral nutrition (TPN) is delivered through a central vein and provides 100% of a patient’s nutritional needs.

Reasons for Fluid Therapy
When choosing a fluid type for your patient, an important question to think about is What am I trying to accomplish?.

The most common reasons for administration of supplemental fluid therapy include replacing deficits of intravascular volume (relative or absolute) to improve tissue perfusion, replacing deficits of tissue volume (dehydration), meeting the maintenance fluid needs of patients that are not consuming sufficient quantities of fluid, and replacing ongoing losses due to vomiting, diarrhea, pneumonia, and 3rd body spacing. In most of these situations, crystalloid fluid therapy is all that is required, although colloids or Oxyglobin can be very beneficial in providing intravascular volume.

At certain times, a fluid diuresis is required, such as with renal disease or to hasten the elimination of toxins that are excreted by the kidneys. In these situations, replacement crystalloid fluids are the fluid of choice. Rates required to induce a sufficient diuresis can be as high as 2.5 to 4 times a patient’s maintenance requirements.

Fluid therapy is also indicated for patients undergoing anesthesia and surgery. Anesthetics commonly decrease vascular tone and/or cardiac output. When this occurs, blood flow to the tissues is decreased (poor perfusion). Fluid therapy increases vascular volume, thereby improving perfusion. Also, many surgical procedures lead to loss of blood, which further reduces intravascular volume.

Commonly suggested replacement crystalloid fluid administration rates for normovolemic patients undergoing anesthesia are 5ml/kg/hr for procedures where minimal blood loss is anticipated (orthopedic and uncomplicated soft tissue surgeries) and 10ml/kg/hr for procedures where moderate blood loss is anticipated (many soft tissue procedures such as liver and splenic surgery). In surgical procedures where the patient is unstable or may lose large volumes of blood, crystalloid fluids, colloid fluids, blood products, and/or Oxyglobin may be required in volumes higher than those listed above.

The other time that fluid therapy is appropriate is when there is need for a specialized fluid. In situations where the patient is anemic, whole blood (fresh or stored) or Oxyglobin can be administered to provide oxygen-carrying molecules. In situations where the patient is coagulopathic, fresh frozen or frozen plasma can provide certain clotting factors. Fresh frozen plasma provides all clotting factors and frozen plasma provides all clotting factors except the most labile (Factors V, VIII).

When a patient has low plasma proteins such as protein-losing disease, prolonged starvation, or vasculitis, fluids with osmotically active particles such as colloids, Oxyglobin, or plasma (fresh frozen or frozen) should be administered for colloidal oncotic support.

Finally, intravenous nutritional solutions are indicated when the patient is not able to consume sufficient food for a prolonged period of time.

How to Administer Fluid Therapy
Fluid therapy is administered as subcutaneous therapy, a rapid intravenous bolus, or as an intravenous constant rate infusion.

Subcutaneous fluid therapy is indicated for the replacement of deficits, for maintenance needs, or to counteract ongoing losses. It is usually used on an out-patient basis, as intravenous fluid therapy has proven more effective in the hospital setting. Because only a limited amount of fluid can be administered subcutaneously, it has the limitation of possibly not being able to fully account for the patients needs.

A contra-indication for subcutaneous fluid therapy is a severely volume depleted patient. In this situation, blood is shunted away from the cutaneous vasculature leading to poor and inconsistent absorption of the fluids.

In the hospitalized setting, intravenous is the preferred route of fluid therapy administration. Although convenient, the use of fluid therapy via a multiple of maintenance needs is inappropriate as most patients do not lose or require fluid therapy in these multiples.

Fluids may be administered as a bolus or a constant rate infusion. Bolus fluid therapy is indicated in the severely volume depleted patient and the dehydrated patient. In the intravascular volume depleted patient, the volume of the bolus is determined based on resolution of clinical signs (slower heart rate and respirations, improved pulse quality, improved mucus membrane color). Therefore, frequent reassessment of the volume depleted patient is required.

Fluid rates as high as 90ml/kg/hr (canine) or 60ml/kg/hr (cats) of crystalloid fluids may be required. Colloids can also be administered as boluses. Colloids are administered in boluses of 5ml/kg up to 20ml/kg (dogs) and 10ml/kg (cats). The patient should be re-assessed after each bolus to determine if the bolus has been effective in resolving the volume depleted state or if additional boluses are needed.

Crystalloid fluids are often administered along with colloids to augment their vascular volume expanding effect. Smaller doses of crystalloids than those listed above are necessary with the concomitant use of colloid. In the dehydrated patient, the quantity of a fluid bolus is based on the estimated degree of dehydration. The dehydrational deficit can be calculated as body weight (kg) x estimated degree of dehydration (%) x 100 = ml of fluid required. It is recommended that one-half of the dehydrational deficit be given as a bolus and the remainder replaced as a constant rate infusion over 12-24 hours.

Constant rate fluid administration is indicated in several situations. As described above, it is used to replace dehydrational deficits. It is also necessary to account for a patient’s maintenance fluid requirements if the patient is not consuming sufficient quantities of fluid on its own.

Daily maintenance needs for patients varies on the age and size of the patient. Most of us were taught that the maintenance crystalloid fluid needs of a patient are approximately 54-66ml/kg/day (lower end in large dogs and higher end in small dogs and cats). In the pediatric patient, the requirements may be as high as 90ml/kg/day. With the use of colloid fluid therapy, infusions at rates of up to 20ml/kg/day (dog) or up to 10ml/kg/day (cat) have been used to provide continuous intravascular volume support.

With the use of infusions of colloid fluids, lower infusion rates of crystalloid fluids are necessary. If a patient has ongoing losses due to vomiting, diarrhea, third spacing (ascites, pleural effusion), or diuresis (post-obstructive diuresis, glucosuria, diseases that produce polyuria such as hyperadrenocorticism or renal failure), these losses should be replaced on a 2 milliliter replaced per estimated 1 milliliter lost.

The administration of specialized fluids such as blood products and intravenous nutrition is extensively covered in other excellent review articles. Since blood products and nutrition are provided intravenously and there is a higher potential for complications compared to other forms of fluid therapy, patients receiving blood products or parenteral nutrition require hospitalization and extensive monitoring.

Monitoring Fluid Therapy
Just as with any other medication that is administered, monitoring for the desired effect as well as potential adverse effects is necessary for successful fluid therapy. Proper monitoring of the patient receiving fluid therapy is a hands-on endeavor. Much of the information we need is gained through serial examinations. No single parameter evaluated will necessarily provide all the information required to guide fluid therapy.

Physical examination parameters that should be evaluated include a patient’s weight, mentation, skin turgor, pulse rate and quality, respiratory rate and effort, serial lung auscultation for rales, mucus membrane color, and capillary refill time. Skin turgor allows for gross assessment of hydration status- prolonged skin turgor will be noted with 6-8% dehydration, markedly prolonged skin turgor will be noted with > 8% dehydration. Limitations of this method are that obesity may mask decreases in skin turgor and emaciation may artifactually increase skin turgor.

If an indwelling urinary catheter is in place, serial evaluation of fluid input and urine output can provide significant information regarding whether too little or too much fluid therapy is being administered. Renal chemistry parameters (BUN, creatinine) in conjunction with urine specific gravity measurement provide additional information. Increased renal parameters and/or increasing urine specific gravity may provide clues that a patient is receiving too little fluid therapy.

Serum lactate measurement can provide information regarding tissue perfusion. Increased serum lactate concentrations indicate that tissues are not receiving adequate oxygen for metabolism and are instead relying on anaerobic metabolism to provide energy.

The gold standard in veterinary medicine for evaluating adequacy of fluid therapy is serial central venous pressure (CVP) measurement. This method allows the clinician to make adjustments to fluid type and rate based on the patient’s ability for his heart to handle the infused volume. Performing CVP measurement is well covered in many review articles on fluid therapy monitoring.

Contraindications for Fluid Therapy
There are several situations when fluid therapy is not necessarily in the best interest of the patient. The most common situation is when a patient has congestive heart disease. In this situation, the patient develops congestion because the heart is not able to pump the volume it currently has. However, heart disease is not an absolute contra-indication. In some situations, there may be concomitant dehydration or the need for medications infused at a constant rate infusion to treat the heart disease and judicious use of fluid therapy may be warranted.

In these patients, hypotonic fluids such as the maintenance crystalloids or D5W are indicated. Meticulous monitoring of vital parameters, urine output, and central venous pressure is necessary for successful use of fluid therapy in this patient population.

The other situation when fluid therapy may not be necessary is when the patient is consuming adequate volumes of water and is adequately hydrated. In this situation, fluid therapy may lead to dilution of blood cell or plasma volume, medullary washout, or development of pulmonary edema if occult heart disease is present. However, even in this situation, fluid therapy may be warranted if fluid diuresis is necessary due to toxic ingestion or if a medication needs to be administered via constant rate infusion.

Discontinuation of Fluid Therapy
Discontinuing fluid therapy is as important as initiating fluid therapy. In most instances, fluid therapy should not be abruptly discontinued, especially if the patient is receiving high flow rates.

During fluid therapy, the solute gradient in the kidneys may be changed due to the fluid therapy (medullary washout). If the fluid therapy is abruptly discontinued, the patient may not be able to concentrate urine well and may continue to lose excessive fluid in the urine for several days. This can be a serious problem if the patient is not ingesting adequate amounts of water, leading to dehydration.

The patient should be gradually weaned off fluid therapy. In the ideal situation, the fluid therapy should be tapered to a sub-maintenance rate for at least 24 hours prior to the discontinuation of fluid therapy. However, this is not always possible. In this situation, the patient should be consuming adequate quantities of water and the owner informed of the patient’s increased water requirements over the next several days.

As stated in the introduction, this is not meant to be all encompassing and there are likely exceptions to many of the rules of thumb stated within this paper. The most important caveat is that there is no set formula for fluid administration and thus we need to tailor fluid type(s) and rate(s) to a patient’s needs as required by its clinical condition. Judicious monitoring of our patients is required to ensure a successful outcome.

Posted on July 14, 2014
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